Evolution

I

Waking from a cold sleep wasn’t at all like a normal waking, in your own bed, with your wife beside you. It was more like surfacing from a deep dunking in a tank of some clinging, deadening fluid.

But now here was a break in the murk, a widening circle of light centered on a blurry face. The face belonged to Ahmed, the splot — the senior pilot — and not to the CO. That was Snowy’s first indication that something was wrong.

Ahmed was repeating, “OK? Are you OK?”

Before submitting to the injections Snowy had rehearsed how he was going to respond to his wake-up call. He smiled and raised the middle finger of his right hand. “Any landing you walk away from is a good one.” His voice was a rasp, and his mouth was desert dry.

“You aren’t walking yet, smart arse,” Ahmed said grimly.

“Where’s Barking?” Robert Madd, blessed with one of the Royal Navy’s less imaginative nicknames, was the unit’s CO.

“Later,” said Ahmed. He withdrew, letting Snowy see the metal walls of the Pit. He threw a ration pack on the bed. “Get out of there. Help me with the others.”

Snowy — Robert Wayne Snow, age 31 — was a lieutenant in the British Royal Navy, which had given him at least an inclination to follow the odd order. So he struggled to sit up.

The Pit was just a cylinder of gunmetal gray, the walls unadorned save for instrument and sensor consoles. The light came from low-energy fluorescents that cast a sickly glow over everything. The instruments were all dead, just blank screens. It was like being inside an oil tank. And the Pit was full of bunk beds, twenty of them, stacked up. Plastic carapaces lay over the beds. Ahmed was working his way around the room, opening the carapaces one by one, and reclosing most of them.

Snowy was stark naked, but he wasn’t cold. He picked up his ration pack. It was a clear vacuum-packed bag containing dried banana, chocolate, and other goodies. He ripped into it with the only tool available to him, his teeth. The bag popped and air hissed. He dumped out the goodies on his bed and crammed some banana into his mouth. He felt like he’d been running a marathon. He’d been through cold sleep twice before, for training and evaluation purposes, just a week at a time. It was a peculiarity of the process that at no time did you feel cold, but you always woke up ravenous: something to do with your body slowly absorbing its stores to keep itself alive, according to the medicos.

But something was wrong with his bunk. He could see where he had been lying, his body had left a very clear imprint, like the gruesome dead-mother’s-bed scene in Psycho. He probed at the mattress. It was lumpy and hard. And the sheets on which he had been lying crumbled as he poked at them, like a mummy’s wrappings.

He felt a gathering sense of dread.

Ahmed was helping a girl from one of the upper bunks. Her name was June, so, naturally, she was known as Moon. She was a cutie, in or out of her clothes; but now, naked, she looked fragile, even ill, and Snowy felt nothing but an impulse to help her as she clambered awkwardly down from her bunk, flinching as her bare flesh brushed against the metal.

With Moon awake, Snowy started to feel self-conscious. He reached under his bunk, looking for his clothes.

But the floor seemed to be on a tilt. He straightened up, expecting his head to clear. But still the bare floor seemed askew, the vertical lines of the bunk frames leaning like drunks. Not good, Snowy thought. He could think of nothing reassuring that would tip up this hundred-ton emplacement.

He reached under his bed again. The cardboard box that had contained his clothes was gone. His clothes were still there, in a heap. But when he grabbed them the cloth just crumbled, like the sheets on his bed.

“Forget it,” Ahmed called, watching him. “Get your flight suit. They seem to have lasted.”

“Lasted?”

“It’s the plastic, I think.”

Snowy complied. He found his boots were still intact too, made of some imperishable artificial material. But he had no surviving socks, none at all; that might be a problem.

Snowy helped get some food inside Moon, while Ahmed continued his patroling.

The woken gathered in a circle, sitting on the lowest tier of the bunks. But there were only five of them, five out of the twenty who had been stored here. The five were Snowy, Ahmed, Sidewise, the girl Moon, and a young pilot called Bonner.

For a time they were silent, as they tucked into banana and chocolate and drank vials of water. Snowy knew that was a good idea. If you were dropped into some new situation it always paid to give yourself time to just sit and listen and think, and adjust to the new situation.

Snowy had pressed Ahmed about the CO. Ahmed showed him. Barking Madd’s body was shriveled and shrunken, literally mummified, just hardened flesh over the bone. The rest, the other fourteen, were the same.

Sidewise, predictably, couldn’t keep his mouth closed. Sidewise was an air warfare officer. He was a thin, intense man, and he had earned his handle for his habit of making sideways crablike moves whenever he got on a dance floor. Now he glanced around at the little group. “Fucking hell,” he said to Snowy. “So much for the safety margins.”

“Shut it,” Ahmed snapped.

Bonner asked Ahmed, “So what was the tally?”

Tally, for tally-ho, was the slang for a wake-up call. “There wasn’t one,” Ahmed said bluntly.

“So if not a tally, what woke us up?”

Ahmed shrugged. “Maybe the Pit has an automatic timer. Or maybe something just failed and it pitched us out.”

Bonner was a good-looking kid, though one of the gen-enged plagues had left him hairless from head to toe. Now he ran his hand over his bare scalp. His accent was faintly Welsh. “Maybe we just pushed it too hard. The Pit was supposed to be a cryostore for seeds and animal embryos and stuff. Insurance against the mass extinction. Not for humans—”

“Especially not humans like you, Bonner,” Snowy said. “Maybe your farts blew the gaskets.”

The bit of low humor seemed to relax the group, as Snowy had hoped.

Ahmed said, “This Pit might have been originally built for elephant embryos or whatever, but it was man rated. We all saw the lectures on the safety parameters, the reliability of the systems.”

“Sure,” Sidewise said. “But any system will fail, no matter how well it’s designed and built, if you give it enough time.” That silenced them. And Sidewise said, “Anybody noticed the clock?”

Most of the Pit’s instruments were dead. But there had been a backup mechanical clock that had drawn on a trickle of thermal energy from deep roots planted in the earth below. Before they submitted to the cold sleep they had all been shown the clock’s working — the cogs made of diamond that would never wear out, the dials that spanned the unthinkable time of fifty years, and so on. It had been a not-so-subtle psychological ploy to reassure them that no matter how long they were in the ground, no matter what became of the outside world, no matter what else failed in the Pit, they would know the date.

But now Snowy saw that the clock’s hands had jammed against the end of their dials.

Snowy thought of his wife, Clara. She had been pregnant when he had gone into cold sleep. Fifty years? The kid would be born, grown, with kids of its own. Maybe even grandkids. No. He rejected the thought. It made no sense; you couldn’t have a human life with a gap of fifty years in the middle of it.

But Sidewise was still talking. “At least fifty years,” he said relentlessly. “How long do you think it would take for Barking’s body to mummify like that, for all our clothes to rot away?” That was the trouble with Sidewise, Snowy thought. He was never shy of saying what everybody else didn’t even want to think about.

“Enough,” Ahmed snapped. He was short, stocky, squat. “Barking is dead. I’m senior here. I’m in charge.” He glared around at them. “Everyone happy with that?”

Moon and Bonner seemed to have withdrawn into themselves. Sidewise was smiling oddly, as if he knew a secret he wasn’t sharing.

Snowy shrugged. He knew Ahmed had served as a watch chief — the navy equivalent of a sergeant major. Snowy thought of him as competent, oddly thoughtful, but inexperienced. And, incidentally, not popular enough for a nickname. But there wasn’t anybody better qualified here, regardless of rank. “I suggest you get on with it, sir.”

Ahmed gave him a look of gratitude. “All right. Here’s the deal. We’ve had no tally. In fact, no contact from the outside. I can’t even tell how long since we last got a contact of any sort. Too many of the systems are down.”

Moon said, “So we don’t know what’s happening out there?”

Snowy said briskly, “Tell us what we do.”

“We get out of here. We don’t need protective gear. Enough of the external sensors are working to tell us that.”

That was a relief, Snowy thought. He wouldn’t have welcomed relying for protection on his NBC suit — nuclear-biological-chemical — if it had been subject to the same ferocious aging as his other clothes.

Ahmed hauled a steel trunk out from under one of the bunks. Inside were pistols, Walther PPKs, each packed in a plastic bag filled with oil. “I checked one already. We can test fire them outside.” He handed them around.

Snowy cracked the bag, wiped clean his pistol on crumbling bits of sheet, and tucked its reassuring mass into his belt. He rummaged through more of his surviving kit: helmets, life jackets, survival vests — a pilot’s equipment. The plastic components seemed more or less intact, but the cloth and rubber had failed. He took what he thought he would need. He regretted leaving behind his helmet, his venerable bone-dome, even if it was painted United Nations blue. Still, he somehow doubted he would be doing much flying today one way or another.

They clustered before the exit. The door to the facility was heavy, round-edged, and airtight, and operated by a wheel; it was like a submarine’s hatch. Ahmed began to break its seals.

They were all shitting themselves, Snowy realized, even if none of them wanted to show it to the rest.

“So what do you think we’ll find?” whispered Sidewise. “Russians? Chinese? Bomb craters, two-headed kids? Everybody wearing monkey masks, like Planet of the Apes?”

“Fuck off, Side, you twat.”

With an uncompromising motion Ahmed turned the wheel. The last seal broke with a crack. The door swung back.

Green light flooded in.

Cryobiology was actually a venerable industry.

The key to its utility was that far below the freezing point of water, molecules slowed the frenetic pace that permitted chemical reactions to proceed. So red blood cells could be stored for a decade or more. You could freeze, thaw, and reuse corneas, organ tissue, neural tissue. You could even freeze embryos. The cold was as much an enemy as an ally, of course; expanding crystals of ice had an unpleasant habit of destroying cell walls. So the medicos infused tissues with cryoprotective agents like glycerol and dimethyl sulfoxide.

Still, freezing and reviving a complex mature organism — such as a hundred kilograms of blasphemous Royal Navy pilot — presented more of a challenge. In Snowy’s body there were many different types of cells, each requiring a different freeze-thaw profile. In the end, a little subtle genetic engineering had done the trick. Snowy’s cells had been given the ability to manufacture natural antifreeze — in fact, glycoproteins, a trick borrowed from some species of polar fish — and the freezing was regulated at the level of the cells themselves.

Obviously it had worked. Snowy had come out of the process alive and functioning. After half an hour he barely felt a thing.

Of course he had been intended to come out fighting.

Officially this unit was under the command of UNPROFOR, the UN Protection Force. But everybody knew that was only a cover. The strategy had become known as sowing dragon’s teeth. As the intensity of global conflict had rapidly increased, post-Rabaul, new forms of deterrence had been devised. The idea was that it would be futile for any power to attempt an invasion if it knew that the ground was salted with groups of highly trained military personnel, fresh and fully equipped, ready to resume the battle. From these scattered teeth the dragon would regrow. That was the theory.

There were drawbacks, of course. The cold sleep process itself brought a risk of injury or fatality (but low, not 75 percent). And you never knew where you would be stationed; the freezing had been done at huge central depots, the subjects transported and deposited, all unconscious, at selected sites around the country, even abroad. But Snowy had known that his unit of Navy flyers would be kept together, which was more than reassuring.

And there were worse assignments. The tour of duty was limited to two years. For sure it was safer than being posted on a carrier to one of the world’s oceanic hellholes, the Adriatic or the Baltic or the South China Sea. In all, it was odd but it was just another posting.

Snowy had been happy to go along with it, even though it meant being locked away from his wife. He had expected to come out of the hole healthy and happy, a lot richer with the back pay he hadn’t been able to spend. Or, failing that, the grimmer possibility would be that he would have to come out fighting. But that was what he was trained for. Even then, he had expected to emerge into the middle of an ongoing high-tech war, to find a chain of command, everything basically functioning, to find something to fly. That was why they had salted away pilots in the first place. He hadn’t expected that they would be cracking the door cut off from any chain of command, completely ignorant of conditions outside — ignorant even of where he was. But that was what he faced.

Snowy took the lead. He stepped through the hatch.

Beyond the hatch, a stairwell was cut into concrete. The well led up to a rectangle of bright green light: leaves, traces of blue-white sky beyond. A forest?

The stairwell’s concrete, where it was exposed, was stained brown where metal fittings had rusted away. And when Snowy put his weight too close to the edge of a step, the concrete just crumbled. The stairs themselves were barely visible under a tangle of moss, leaves, debris of all kinds. Snowy wasted a little energy trying to clear this stuff off, but found that much of it was actually growing here, out of a layer of mulch over the concrete.

Ignoring the mess, he stepped up and just pushed his way out of the well.

At last he found himself standing on leaf-covered ground. He was panting hard. Evidently the cold sleep had taken more out of him than he had expected. The others followed him, one by one, brushing dead leaves and moss and mulch off their clothes.

The forest was built of tall trees, with low branches, heavy, spreading leaves. Oak, perhaps. Wind rustled, bringing warm air to Snowy’s face. It felt like late spring or early summer. The air smelled fresh, of nothing but forest, green and mulchy.

The Pit was set in the ground, half-concealed by a great concrete lid. But the lid was tilted askew and cracked, and plants were growing out of its surface.

Ahmed had a small black backpack. This contained a clockwork radio transceiver — which, like the pistols, had been stored in oil. Now he turned this on, wound it up, extended its aerial and began to walk around the little clearing.

Both Moon and Bonner looked very young and scared, lost in the green shade.

Sidewise stood by Snowy. Moodily he kicked the concrete carapace. “It’s amazing the power supply kept going as long as it did.”

Snowy said, “It’s like we just clambered out of Chernobyl.”

“I don’t think Chernobyl is a problem anymore.”

“What?”

“Snow, just how long do you think we’ve been stuck down that hole?”

Snowy braced himself. “More than fifty years?”

Sidewise grunted. “Look around you, pal. Those trees are oak. And look at this.” He led Snowy to a fallen tree. The trunk had snapped off maybe a meter above the ground. Much of the fallen trunk was coated with greenery, and fat, platelike fungi adhered to the upright stub of trunk, like disks stuck into the wood. “Snow,” said Sidewise, “you are surrounded by a mature forest. These are old trees. This one got so old it died without being felled. Come on, Snow. You remember those eco classes in training. What happens if you let a forest clearing recover?”

The grasses and herbs would be first to colonize the empty space. Within a year or so there would be Scots pine seedlings, birches, other deciduous trees sprouting from seeds left in the ground, or from stumps. Once there was some protection from the frost, Norway spruce and chestnut might take hold. Then, as conditions changed, different species would compete for light and space. After maybe fifty years, as the recovering forest darkened, the grasses on the floor would make way for shade-tolerant vegetation like bilberry and mosses. And after that, the oaks would return.

Snowy hadn’t paid a great deal of attention to this kind of stuff, at school, during his training, or later. Eco was always too depressing, nothing but lists of dead creatures. But — how long?

Sidewise poked at the grounded trunk. “Look at these bryophytes — the mosses and liverworts — and the lichens, fungi, insects burrowing away. You know, in our day a sight like this dead trunk was as rare as a wolf.”

“In our day?”

Ahmed had given up his stroll around the clearing. “Nothing,” he said. “Not a peep on any frequency. Not even GPS.”

“Maybe the radio’s out,” Moon said.

Ahmed pressed a green button on the set. “The self-test is okay.”

“Then,” said Bonner, “what do we do?”

Ahmed straightened up. “We keep ourselves alive. We get out of this damn forest. And we find somebody to report to.”

Snowy nodded. “Which way?”

“The maps,” said Bonner immediately.

Their training asserted itself, and they hurried back to the Pit.

The Pit had been equipped with external stores of paper maps, in case of the eventuality that a troop found itself revived like this without external direction or orientation. The maps were supposed to be contained in weatherproof boxes on the exterior of the Pit. The maps would also come with spins — specific instructions. Snowy knew they would all be reassured to find something to tell them what to do, maybe a clue as to what was going on.

But, try as they might, they couldn’t even find a trace of the map boxes. There was nothing but a surface of corroded, crumbling concrete, heavily colonized by mosses and grass.

Sidewise helped with the search, but Snowy could tell that his heart wasn’t in it. He had known the maps wouldn’t be here. Snowy began to feel vaguely scared of Sidewise, because he was so far ahead of the game; he really didn’t want to know what Sidewise had already figured out.

They gave up on the maps. Still Ahmed tried to take a lead, to be decisive, and Snowy admired him for that. Ahmed sniffed the air, looked around, and pointed. “The land is rising that way. So that’s the way we’ll go. If we’re lucky, we’ll break out of these woods. Agreed?”

He was rewarded by shrugs and nods.

II

There wasn’t much to take from the Pit — nothing but what they could plunder from the dead: all the weapons and ammo they could find, spare clothing, ration packs. They made backpacks from spare flight suits, and loaded up their gear.

They set off in the direction Ahmed had chosen. The sun looked to be setting, and that meant, Snowy thought, that they had to be traveling roughly north. Unless even that had gotten itself screwed up in the years they had lost in the Pit.

The forest was dominated by the great oaks, though they were interspersed with other species like sycamore, Norway maple, and conifers. There were plenty of birds — mostly starlings, it seemed to Snowy — but he was startled to see a rattle of green and yellow wings pass across the sun. Occasionally they saw animals — rabbits, squirrels, small, timid-looking deer, even what looked like a wolf, which had them all fingering their pistols.

After maybe an hour they came to a neat round hole in the ground. It was full of debris, but was obviously man-made. The bit of human design drew them insistently. They gathered around, sipping water from the small vials they carried.

Snowy said to Sidewise, “Did you see those green birds? They looked like—”

“Budgerigars. The descendants of escaped pets. Why not? There are probably parakeets and parrots too. Some of those deer looked like muntjac to me. Out of zoo stock, maybe. Even some of the trees look like imports — like that turkey oak back there. Like they taught us: Once you disturb the balance of nature, once you start importing species, it never goes back the way it used to be.”

Snowy said, “There was a wolf.”

“You sure it was a wolf?” Sidewise said sharply. “Didn’t it look too low, too fast?”

Come to think of it, Sidewise was right. It had looked a little furtive, low-slung. Rodentlike.

Bonner said, “All right, two-brains, what about this hole in the ground? Somebody’s removed a tree stump here, and done it deliberately.”

“Maybe,” said Sidewise coldly. “But holes in the ground last a long time. You can still find holes dug by hunter-gatherers tens of thousands of years ago. All this tells us is there hasn’t been another Ice Age yet.”

Ahmed glared at him. “You aren’t doing much for morale, Sidewise.”

Sidewise shot back, “And what about my morale? It does me no damn good to ignore what’s blindingly obvious all around us.”

There was a moment of strained silence. For a minute Snowy glimpsed Sidewise’s past, a past he never talked about: the too-brainy kid at school, impatient of the rest, constantly bullied back into line by his fellows.

“Let’s move on,” Bonner said gruffly. Ahmed nodded and led the way.

Soon they came to what looked like a track. It was nothing but a winding ribbon of earth, almost invisible, crooked and devious. But the vegetation here grew a little sparser, and Snowy could feel how the ground did not give under his feet as it did elsewhere. A track, then — and surely a human-made track, not animal, if the ground had been compacted as much as it felt.

They didn’t say anything. Nobody wanted this little bit of hope to be punctured by another lecture from Sidewise. But they all followed the track, walking single file, moving that much more briskly up the shallow slope.

Snowy already felt exhausted, strung out.

He found he wasn’t thinking of his wife, his buddies back home, the life that seemed to have vanished forever. Everything was too strange for that. But he longed, absurdly, for the snug safety of his cold-sleep bed, with its enclosing carapace and humming machines. Out here he felt very exposed — his PPK didn’t add up to much protection — and he was very aware that when darkness fell in this strange, transformed place they were going to be very vulnerable.

We have to find some answers before then, he thought.

After maybe another hour the trees thinned out, and with relief Snowy found himself walking out in the open. But he still couldn’t see much. He was on the breast of a broad shallow rise, its summit hidden over the nearby horizon. The ground was chalky, he saw, and the soil thin and heavily eroded. Nothing much grew here but heather, and shoulders of bare rock stuck out of the ground.

The sky was clear, save for a scattering of thin, high cloud. The setting sun cast long shadows on the ground. It was so low that Snowy would have expected the sunset to have started already, a tall Rabaul-ash light show. But there was no redness in the western sky; the sun shone bright and white. Was the ash gone?

Moon yelped. “Tracks! Vehicle tracks!” She was pointing a little way down the slope to their right, jumping up and down with excitement.

They all ran that way, their improvised packs bouncing on their backs.

She was right. The tracks were quite unmistakable. They had been made by some off-road vehicle, and they ran at an angle down the slope.

The mood was suddenly exultant. Bonner was grinning. “So there’s somebody around. Thank Christ for that.”

“All right,” Ahmed said. “We have a choice. We can keep on heading for the high ground, looking for a viewpoint. Or we can follow these tracks back downhill and find a road.”

The high ground would probably have been the smarter move, Snowy thought. But in the circumstances none of them wanted to let go of these traces of human activity. So they started downhill, following the twin scars in the hillside.

Sidewise walked beside Snowy. “This is dickheaded,” he muttered.

“Side—”

“Look at it. These are vehicle tracks, all right. But they have turned into gullies. Look over there. They’ve eroded right down to the bedrock. Snow, in an area like this, above the treeline, it can take centuries for a covering of soil and vegetation to re-establish itself once it’s removed. Centuries.”

Snowy stared at him. His thin face was gray in the fading light. “These tracks look like they were made yesterday, as if somebody just drove by.”

“I’m telling you they could be any age. I don’t fucking know.” He looked as if he was dying for a cigarette.

The tracks wound down the hillside, eventually leading them into a broad valley that cupped the silvery streak of a river. The tracks veered off the rough ground onto what was unmistakably a road following the valley wall, a neat flat shelf carved almost parallel to the valley’s contours.

The group clambered into the road surface with relief. They started to hike down the road, along the valley toward the lower ground, their mood staying high despite their fatigue.

But the road was in bad shape, Snowy saw. It was overgrown. There was still some asphalt — he could see it as black fragments in the green — but it had aged, becoming cracked and brittle. Plants and fungi had long since broken through the surface, and in fact as he walked he sometimes had to push through thickets of birch and aspen seedlings. It was less like walking along a road than over a sparsely vegetated ridge.

Sidewise was walking alongside him again. “So what do you think? Where are we?”

They had all been trained up in the basic geographical features of Europe and North America. “The valley isn’t glaciated,” Snowy said reluctantly. “So if we’re in Europe, we aren’t too far north. Southern England. France maybe.”

“But it’s been a long time since anybody maintained this road. And look down there.” Sidewise pointed to a line etched in the side of the far valley wall, just bare rock.

“So what?”

“See how level it is? I think this valley was flooded once. Dammed. At the water’s surface you get a lot of erosion — you get horizontal cuts like that — because when the flow is managed, the water levels fluctuate fast.”

“So where’s the dam?”

“We’ll come to it,” said Sidewise grimly.

After another hour of walking, they did.

They turned around a breast of the valley, and there it was. A branch of this roadway actually led down to the dam, and must have run over it to the valley’s far side.

But the dam was gone. Snowy could make out the piers that still clung to the shore, heavily eroded and overgrown with greenery. Of the central section, the great curving wall and gates and machinery that had once tamed the river, there was nothing left but a hummocky arced line on the valley floor, a kind of weir that barely perturbed the river as it ran over it.

Moon said, “Maybe somebody blew it up.”

Sidewise shook his head. “Nothing is impervious. There are always cracks and weaknesses, places the water can get into. And if you don’t do anything about it, the leaks get worse, until…” He fell silent. “All you need is time,” he finished lamely.

“Fucking hell,” growled Bonner. “Fucking buggering hell.”

It seemed to Snowy that the unavoidable truth was starting to sink into them all. Even Sidewise didn’t need to say any more to make it so.

Ahmed strode ahead a few paces, and peered further down the valley. He was a pilot; like them all, he had good eyes. He pointed. “I think there’s a town down there.”

Maybe, thought Snowy. It was just a splash of greenish gray. He could see no movement, no car windshields or windows glinting, no smoke rising, no lights. But they had nowhere else to go.

Before they left the higher ground Ahmed fired off a couple of the search-and-rescue flares he had retrieved from the shelter. There was no reply.

They followed Ahmed as he made bold, defiant strides along the grassed-over roadway, down the valley toward the town. The light began to fade. Not a single light came on in the town they approached; it was a well of darkness and silence.

In some places the river’s banks had reverted to marsh, with low, green-clad hummocks marking what might once have been buildings. Elsewhere the banks were lined with elder and graceful willows — old-looking willows, Snowy thought reluctantly — and the floodplain beyond was covered by a forest of poplar and ash. Beyond, he could see arms of the oak forest spreading over the low hills.

Long before they reached the center of the town they had to abandon the grown-over road, as it slipped under the surface of the broadening river. Further out into the river, Snowy could make out shapes, lines, under the shallow water.

“If you build around a river,” said Moon slowly, “you reclaim the land to either side. Right? But when you abandon the town, the water table is going to rise because you’re no longer pumping out groundwater for industrial use, and you’ll get flooded out.”

Nobody commented. They walked on, skirting the river and its marshy fringe.

At last they came to the town itself. There was a layout of streets here, you could see that, a roughly rectangular grid laid out over shallow slopes. But the roads were as ruined as the one they had followed here. The buildings themselves were just patterns of mounds and hummocks draped with green, most of them no more than waist height. The whole place looked like an overgrown graveyard. Snowy thought they could have passed by any of these heaps of green-clad rubble in the forest and thought it just another extrusion of rock, the product of nature’s mindless churning. Even the vegetation was much the same as in the open land beyond the town. It was only the patterns that told you that hands had built this place, that minds had planned it.

Here and there, though, more enduring fragments poked out of the drowning green. There was one looming, circular hill, as green clad as the rest. Snowy wondered if this might be a keep, the base of one of the Normans’ great castles, erected to enforce their occupation of England in the eleventh century. If so, it had lasted where much else had failed. They came across a row of columns, worn to stubs, that looked as if they had been clad in marble. They might have been the grandiose frontage of a bank or town hall.

And here was a statue, fallen on its back. Its face, pocked by lichen and eroded beyond recognition, peered up at the sky from an ocean of green. But the statue bore traces of charring, Snowy saw. He searched for a date, but couldn’t find one.

When he dug into the greenery that blanketed other anonymous mounds, he found more traces of fire, of soot and scorching. This place had burned, then, before it had been broken up. He was walking on tragedy, on overgrown horror. He wondered how deep he would have to dig before he found bones.

They came to a comparatively open space. This must have been a central square, maybe a marketplace. Ahmed called a halt. They dropped their packs, drank their water, and peered around. In the lengthening shadows of evening the ruined town was an eerie place, Snowy thought, neither quite natural nor human, neither one thing nor the other.

A little ratty creature scuttled from under Snowy’s feet, crisply pattering over the broken asphalt surface and disappearing into the richer green away from the square. It looked like a vole. And, following its tracks, Snowy made out the upright, wary form of a hare. With bewildering speed it turned and scuttled away.

“Voles and hares,” he muttered to Sidewise. “I thought we’d see cats and dogs.”

Sidewise shrugged, sweat and grime coating his face. “People have gone, right? Civilization has fallen, blah, blah, blah. Cats and dogs were pampered, domesticated, all the genetic variation bred out of them. They wouldn’t have lasted long without us.”

“I’d have thought cats would survive. Even little kittens used to go hunting.”

“Wild cats were perfect killing machines. But the domestic variety had smaller teeth, jaws, brains than their wild ancestors, because old ladies liked them better that way.” Sidewise winked. “I always thought cats were faking it. They weren’t so tough. Just a pain in the arse.”

“Where are the cars?” Moon asked. “I mean, I see the buildings, what’s left of them. What about the cars?”

“If you dig in the greenery you might find a few patches of rust, or bits of plastic.” Sidewise glared at Ahmed. “What, are you going to chew me out for lowering morale again? I’m only pointing out the bleeding obvious.”

“But we don’t have to deal with that right now,” Ahmed said, with an evenness Snowy admired. “What we need to do is obvious too.”

Snowy nodded. “We have to find shelter.”

Bonner clambered up onto a low mound that might once have been a wall. Now he pointed, roughly west. “That way. I can see walls. I mean, standing walls. Something that isn’t all covered in shit.”

With an unreasonable spark of hope, Snowy got to his feet. It was a church, he saw. A medieval church. He could make out the tall, narrow windows, the high doorway. But the doors and roof had long gone, leaving the building open to the sky. He felt disappointment — and yet a stab of admiration.

Sidewise seemed to share his thought. “If you’re going to build, build out of stone.”

“Where do you think we are? England, France?”

Sidewise shrugged. “What do I know about churches?”

Ahmed picked up his pack. “All right. There’s no roof, so we’ll have to make lean-tos. Bonner, Snowy, come with me and we’ll fetch some branches. And we’ll need a fire. Moon, Sidewise, you attend to that.” He looked around at their faces, which were shining like coins in the gathering dark. This would be the first time they had been out of each other’s sight since they had woken up, and even Snowy felt a pang of uncertainty. “Don’t go too far,” Ahmed said gently. “We’re alone here. There doesn’t seem to be anybody to help us. But we’ll be fine so long as we’re careful. If anything goes wrong — anything — shout or use your pistol, and the rest of us will come running. All right?”

They nodded and murmured. Then they moved off into the gathering dark, purposefully pursuing their allotted tasks.

The interior of the church was just another patch of greenery. There was a mound at one end that might once have been an altar, but there was no sign of pews or crucifixes, prayer books or candles. The roof was gaping open to the sky, not a trace left of the wooden construction that must once have spanned these slender, sturdy walls.

Under their lean-tos, on pallets of brush, with leaves for blankets, it wasn’t going to be such an uncomfortable night. They had all had plenty of survival training; this wasn’t so bad compared to that.

They stuck to their survival packs, munching on dried bananas and beef jerky. They didn’t eat any of the fruit from the forest. It was a little superstitious, Snowy thought, as if they wanted to cling to what was left of the past as long as possible, before committing themselves to this peculiar new present. But it was OK to take it slow. Ahmed was showing a good grasp of psychology in allowing that. It certainly wouldn’t make any difference in the long run.

They were all pretty exhausted after a walk of many klicks on their first day out of the Pit. Snowy wondered how they would have got on if they’d really had to fight; maybe this strategy wouldn’t have worked as well as the planners had imagined. And they all had trouble with their feet, with blisters and aches. It was the lack of socks that was the problem. Snowy worried about using up their limited supplies of ointments too fast. They would have to do something about that tomorrow.

But it was comforting to shelter in this relic of human construction, as if they were still cradled by the civilization they had come from. Still, they would keep their fire burning all night.

Snowy was relieved to find he was too tired to think too hard. Still, he woke.

He rolled on his back, restless. The air was hot — too damn hot for an English spring; maybe the climate had changed, global warming gone crazy or somesuch. The sky framed by the open roof was littered with stars, obscured here and there by cloud. There was a crescent moon, too narrow to banish the stars, as far as he could see unchanged from the patient face that had watched over his boyhood. He had learned a little astronomy, during training exercises in the desert, for navigation purposes. He picked out constellations. There was Cassiopeia — but the familiar W shape was extended by a sixth star. A hot young star, maybe, born since he had gone into the Pit. What a strange thought.

“I can’t see Mars,” Sidewise whispered, from out of the dark.

That startled Snowy; he hadn’t known Sidewise was awake. “What?”

Sidewise pointed to the sky, his arm a silhouette. “Venus. Jupiter. Saturn, I think. Where’s Mars?”

“Maybe it set already.”

“Maybe. Or maybe something happened to it.”

“This is bad shit, isn’t it, Side?”

Sidewise didn’t reply.

“Once I saw some Roman ruins,” Snowy whispered. “Hadrian’s Wall. It was like this. All grown over, even the mortar rotted away.”

“This was a different scale,” Sidewise murmured. “Even from Rome. We had a global civilization, a crowded world. Everything was linked up.”

“What do you think happened?”

“I don’t know. That fucking volcano, maybe. Famine. Disease. Refugees everywhere. War in the end, I guess. I’m glad I didn’t live through it.”

“Shut up, you two,” Ahmed murmured.

Snowy sat up. He peered out through a glassless window frame in the wall of the church. He could see nothing. The land was just a blanket of dark, no glimmer of lights, no glow of streetlights on the horizon. Maybe everywhere was dark like this. Maybe their fire was the only light in England — on the whole damn planet. It was a stupendous, unbelievable, unacceptable thought. Maybe Sidewise could grasp it properly, but Snowy sure couldn’t.

Some kind of animal howled, out in the night.

He threw a little more wood on the fire, and buried himself deeper in his mound of greenery.

Sidewise had been right. Mars was missing.

The replicators, Ian Maughan’s robot probes, had survived. The program had been designed as a precursor to human colonization of the planet. The replicating robots would have been instructed to build homes for human astronauts, to make them cars and computers, to assemble air and water, even grow food for them.

But the humans never came. Even their commands ceased to be received.

That wasn’t troubling, for the replicating robots. Why should it be? Until they were told otherwise, their only purpose was to replicate. Nothing else mattered, not even the strange silence from the blue world in the sky.

And replicate they did.

Many modifications were tried, incorporated, abandoned. It did not take long for a radically better design to converge.

The replicators began to incorporate the factory components within their bodies. The new kind looked like tractors, pilotless, trundling over the impassive red dust. Each weighed about a ton. It took each one a year to make a copy of itself — a much shorter reproduction time than before, because they could go where the resources were.

After a year, one of the new replicator types would become two. Which after another year had made two more copies, a total of four. And in another year there were eight. And so on.

The growth was exponential. The outcome was predictable.

Within a century the factory-robots were everywhere on Mars, from pole to equator, from the peak of Mons Olympus to the depths of the Hellas crater. Some of them came into conflict over resources: There were slow, logical, mechanical wars. Others began to dig, to exploit the deeper materials of Mars. If you mined, there was still plenty of resources to go around — for a while, anyhow.

The mines got deeper and deeper. In places the crust collapsed. But still they kept digging. Mars was a cold, hard world, rocky for much of its interior. That helped the mining. But as they dug deeper and encountered new conditions, the replicators had to learn quickly, adapt. They were capable of that, of course.

Still, the penetration of the mantle presented certain technical challenges. The dismantling of the core was tricky too.

Mars weighed one hundred billion billion times as much as any one of the tractor-replicators. But that number was small in the face of the doubling-every-generation rule. Because of the continuing conflicts, the pace of growth was a little slower than optimal. Even so, in just a few hundred generations, Mars had gone, all but a trace of its substance converted to the glistening bulks of replicators.

With the whole planet transformed to copies of themselves — using solar sails, fusion drives, even crude antimatter engines — the swarm of replicators had moved out through the solar system, seeking raw material.

The next day, roaming into the country around the town, Snowy saw birds, squirrels, mice, rabbits, rats. Once he thought he saw a goat; it fled at his approach.

Not much else. There didn’t even seem to be many birds around. The place was silent, as if all the living things had been collected up and removed.

Some of the rats were huge, though. And then there were the rat-wolves he thought he had glimpsed. Whatever they were, they fled at his approach.

Rodents had always been in competition with primates, Sidewise said. Even at the peak of their technical civilization, people had had to be content with keeping rodents out of sight, and out of the food. Now, with people out of the picture, the rats were evidently flourishing.

It was easy to hunt, though. Snowy set a few snares, in a spirit of experimentation. The snares worked. The hares and voles seemed peculiarly tame. Another bad sign if you thought about it, because it meant they hadn’t seen humans for a while.

At the end of the second day, Ahmed had them sit in the ruins of the church, in a rough circle on corroded stone blocks.

Snowy was aware of subtle changes in the group. Moon was looking down, avoiding everybody’s eyes. Bonner, Ahmed, and Sidewise were watching each other, and Snowy, with calculation.

Ahmed held up an empty ration packet. “We can’t stay here. We have to plan.”

Bonner shook his head. “The most important thing is finding other people.”

“We’re going to have to face it,” Sidewise said. “There are no other people — nobody who can help us, anyhow. We haven’t seen anybody. We’ve seen no sign that anybody has been in this area recently.”

“No contrails,” Ahmed said, pointing to the sky. “Nothing on the radio, on any frequency. No satellites. Something went wrong—”

Moon laughed hollowly. “You can say that again.”

“We can’t know how events unfolded. Before the end it must have become — chaotic. We were never recalled. Eventually, I suppose, we were forgotten. Until we were revived by chance.”

Snowy forced himself to ask the question. “How long, Side?”

Sidewise rubbed his nose. “Hard to say. If we had an astronomy almanac I guess we could figure it out from the changed positions of the stars. Failing that, your best guess is based on the maturity of the oak forest.”

Bonner snapped, “You’re so full of shit, you scrawny bastard. How fucking long? Fifty years, sixty—”

“Not less than a thousand years,” Sidewise said, his voice tight. “Maybe more. Probably more, actually.”

In silence, they let that sink in. And Snowy closed his eyes, imagining he was plunging off the deck of an aircraft carrier into the dark.

A thousand years. And yet it meant no more than the fifty-year gulf that he thought had separated him from his wife. Less, maybe, because it was just unimaginable.

“Some future this is,” Bonner said edgily. “No jet cars. No starships, no cities on the Moon. Just shit.”

Ahmed said, “We have to assume we are not going to find anybody else. That we’re alone. We have to plan on that basis.”

Sidewise snorted. “Civilization has collapsed, everybody is dead, and we’re stuck a thousand years in the future. How are we supposed to plan for that?”

“That river is probably clean,” Snowy said. “All the factories must have shut down centuries ago.”

Ahmed nodded gratefully at him. “Good. At last, something we can actually build on. We can fish, we can hunt; we can start that tomorrow. Sidewise, why don’t you use that brain of yours for something useful and think about the fishing? Figure out how we can improvise lines, nets, whatever the hell. Snowy, you do the same for the hunting. Further down the line, we’re going to have to find somewhere to live. Maybe we can find a farm. Start thinking about clearing the ground, planting wheat.” He glanced at the sky. “What do you think the season is? Early summer? We’re too late for a harvest this year. But next spring—”

Sidewise snapped, “Where do you think you’re going to find wheat? Do you know what happens if you leave corn or wheat unharvested? The ears fall to the ground and rot. Cultivated wheat needed us to survive. And if you leave cows unmilked for a few days, they just die of udder bursts.”

“Take it easy,” Snowy said.

“All I’m telling you is that if you want to farm, you’ll have to start from scratch. The whole damn thing, agriculture and husbandry, all over again from wild stock, plants and animals.”

Ahmed nodded stiffly. “We, Side. Not you. We. We all share the problems here. All right. So that’s what we’ll do. And in the meantime we gather, we hunt. We live off the land. It’s been done before.”

Moon fingered her clothing. “This stuff won’t last forever. We’ll have to find out how to make cloth. And our weapons will be pretty useless once the ammo is gone.”

Bonner said, “Maybe we can make more ammo.”

Sidewise just laughed. “Think about stone axes, pal.”

Bonner growled, “I don’t know how to make a fucking stone ax.”

“Neither do I, come to think of it,” Sidewise said thoughtfully. “And you know what? I bet there aren’t even any books to tell us how. All that wisdom, painfully acquired since we were buck naked Homo erectus running around in Africa. All gone.”

“Then we’ll just have to start that again too,” Ahmed said firmly.

Bonner eyed him. “Why?”

Ahmed looked up at the sky. “We owe it to our children.”

Sidewise said simply, “Four Adams and one Eve.”

There was a long, intense silence. Moon was like a statue, her eyes hard. Snowy noticed how close her hand was to her PPK.

Ahmed got to his feet. “Don’t think about the future. Think about filling your belly.” He clapped his hands. “Let’s move it.”

They dispersed. The crescent moon was already rising, a bonelike sliver in the blue sky.

“So,” Sidewise said to Snowy as they moved off, “how are you finding life in the future?”

“Like doing time, mate,” Snowy said bitterly. “Like doing fucking time.”

III

Maybe five kilometers from the base camp, Snowy was trying to build a fire.

He was in what must once have been a field. There were still traces of a dry stone wall that marked out a broad rectangle. But after a thousand years it was pretty much like any patch of land hereabouts, choked by perennial herbs and grasses, shrubs and deciduous seedlings.

He had made a fire board about the length of his forearm, with a dish cut into its flat side. He had a spindle, a stick with a pointed end; a socket, a bit of rock that fit neatly into his hand; and a bow, more sapling with a bit of plastic shoelace tied tight across it. A bit of bark under the notch served as a tray to catch the embers he would make. Nearby he had made a little nest of dry bark, leaves, and dead grass, ready to feed the flames. He knelt on his right knee, and put the ball of his left foot on the fire board. He looped the bow string and slid the spindle through it. He lubricated the notch with a bit of earwax, and put the rounded end of the spindle into the dish of his fire board, and held the pointed end in the hand socket. Then, pressing lightly on the socket, he drew the bow back and forth, rotating the spindle with increasing pressure and speed, waiting for smoke and embers.

Snowy knew he looked older. He wore his hair long now, tied back in a ponytail by a bit of wire. His beard was growing too, though he hacked it back with a knife every couple of days. His skin was like tough leather, wrinkled around the eyes, the mouth. Well, I am older, he thought. A thousand years older. I should look the part.

It was hard to believe that it was only a bit more than a month since they had come out of the Pit.

They didn’t need to do this kind of thing yet, this fire building from scratch. They still had plenty of boxes of waterproof matches, and a supply of trioxane packs — a light chemical heat source much used by the military. But Snowy was looking ahead to the day when they wouldn’t be able to rely on what had come out of the Pit. In some ways he was “cheating,” of course. He had used his thousand-year-old finely manufactured Swiss Army knife to make the bow and the fire board; later he would have to try out stone knives. But one step at a time.

This ancient field was close to an arm of the vast oak forest which, as far as they had scouted, dominated the landscape of this posthuman England. It was on a slight rise. To the west, further down the hill, a lake had gathered. Snowy could see traces of stone walls disappearing under the placid water. But the lake was choked with reeds and lilies and weeds, and on its surface he could see the sickly gray-green sheen of an algal bloom. Eutrophication, said Sidewise: Even now, artificial nutrients — notably phosphorus — were leaching out of the land into the lake and overstimulating the miniature ecology. It seemed incredible to Snowy that the shit long-dead farmers had pumped into their land could still be poisoning the environment around him, but it seemed to be true.

It was a strangely empty landscape. Silence surrounded him. There wasn’t even birdsong.

Some creatures had probably bounced back quickly once human hunting, pest control, and land use had ceased — hares, rabbits, grouse. Larger mammals reproduced so slowly that recovery must have taken longer. But there seemed to be various species of deer, and Snowy had glimpsed pigs in the forests. They’d seen no large predators. Even foxes seemed rare. There were no birds of prey either — apart from a few aggressive-looking starlings. Sidewise said that as their food chains had collapsed, the specialized top predators would have died out. In Africa there were probably no lions or cheetahs either, he said, even if they had escaped being eaten by the last starving human refugees.

Maybe, Snowy thought. He wondered about the rats, though.

Balance would return in the long run, of course. Variation, adaptation, and natural selection would see to that; the old roles would be filled one way or another. But it might not be anything like the community that had gone before. And, said Sidewise, since the average mammalian species lasted only a few million years, it would correspondingly be millions of years — ten, twenty maybe, twenty million years — before there would again be assembled a world of the richness it had enjoyed. So even if humans recovered and lasted, say, five million years, they wouldn’t see anything like the world Snowy had known as a kid.

Snowy was not a tree hugger, definitely. But there was something deeply disturbing about these thoughts. How strange it was to have lived to see it come about.

Still no smoke, still the damn embers hadn’t caught. He continued to work the bow.

The main problem with fire making was that it gave him too much time to think. He missed his friends, the camaraderie of navy life. He missed his work, even the routine bits — maybe the routine most of all, since it had given his life a definition it lacked now.

He missed the noise, he found, though that was harder to pin down: TV and the web and music, movies and ads, the logos and jingles and news. The one thing about the new world that would drive him crazy in the end, he suspected, was the silence, the huge, inhuman, vegetable silence. It gave him the shivers to imagine how it must have been in the last days, when all the machines had died, the winking logos and neon tubes and screens flickering and dying, one by one.

And he missed Clara. Of course he did. He had never known his kid, never even seen him, or her.

At the beginning he had been plagued by spasms of guilt: guilt that he was still alive where so many had gone into the dark, guilt that there was nothing he could do for Clara, guilt that he was eating and breathing and pissing and taking shits and covertly studying Moon’s butt while everybody he had ever known was dead. But that, mercifully, was fading. He had always been blessed, as Sidewise had once told him, by a lack of imagination.

Or maybe it was more than that.

In the clear light of this new time it seemed like it was his old life, in the crowded, murky England of the twenty-first century, which was the dream. As if he were dissolving into the green.

There was a rustle in the waist-high foliage, a dozen paces away. He turned that way, still and silent. A single grass stem, laden with seeds, nodded gracefully. He had set a snare over there. Was there something in the foliage — a curve of shoulder, a bright, staring eye?

He put down the bow and spindle. He stood, stretched, and casually walked toward the place he had seen the rustle. He slid his bow from his back, scooped an arrow from his rabbit-skin quiver, notched it carefully.

There was no movement in the foliage — not until he was almost on it — and then there was a sudden blur, a lunge away from his approach. He glimpsed pale skin flecked with brown, long limbs. A fox? But it was big, bigger than anything he’d seen here so far.

Without hesitating further he ran up to the thing, lodged his boot in the small of its back, and raised his arrow toward its head. The creature squirmed onto its back. It yowled like a cat, put its hands over its face.

He lowered the bow. Hands. It had hands, like a human, or an ape.

His heart thumping, he dropped the bow. He knelt over the creature, trapping its torso, and got hold of its wrists. It was spindly, lithe, but very strong; it took all of his power to force those hands away from the face. Still the creature spat and hissed at him.

But its face — no, her face — was no chimp’s, no ape’s. It was unmistakably human.

For long seconds Snowy sat there, astounded, astride the girl.

She was naked, and though her pale skin showed through, she was covered by a loose fur of straggling orange-brown hairs. The hair on her head was darker, a tangle of filthy curls that looked as if they had never been cut. She was not tall, but she had breasts, sagging little sacks with hard nipples protruding from the hair, and beneath the triangle of darker fur at her crotch there was a smear of what might be menstrual blood. And she had stretch marks.

Not only that, she stank like a monkey cage.

But that face was no ape’s. Her nose was small but protruding. Her mouth was small, her chin V-shaped with a distinct notch. Over blue eyes, her brow was smooth. Was it a little lower than his?

She looked human, despite her hairy belly. But her eyes were — cloudy. Frightened. Bewildered.

His throat tight, he spoke to her. “Do you speak English?”

She screeched and thrashed.

And suddenly Snowy had an erection like an iron rod. Holy shit, he thought. Quickly he rolled off the girl, reaching for his bow and his knife.

The girl couldn’t get up. Her right foot was trapped by his snare. She scrabbled over the moist ground until she was hunched over her foot. She rocked back and forth, crooning, obviously scared out of whatever wits she had.

Snowy’s spasm of lust faded. Now she looked like a chimp in her gestures, in her mindless misery, even though her body had felt like a woman’s under his. (Clara, forgive me, it’s been a long time…) The scrapings of shit on her legs, the puddles of droppings where she had been lying, put him off even more.

He rummaged in a pocket of his flight suit, and pulled out the remains of a ration pack. It still contained a handful of nuts, a bit of beef jerky, some dried banana. He pulled out the banana and held it out, a handful of curling flakes, toward the girl.

She shied back, pulling as far as she could on the wire.

He tried miming, putting a flake or two into his own mouth and exaggeratedly devouring it with every expression of enjoyment. “Yum, yum. Delicious.”

But still she wouldn’t take the food from his hand. Then again, neither would a deer or a rabbit, he thought. So he put the flakes on the ground between them and backed away.

She grabbed a couple of the flakes and crammed them into her mouth. She chewed and chewed at the bits of banana, as if extracting every bit of flavor from them, before finally swallowing them. She must never have tasted anything so sweet, he thought.

Or maybe it was just that she was starving. He had set the trap a couple of days before; she might have been here for forty-eight hours already. All the shit and piss, the way the fur on her legs was matted and stained, indicated that too.

As she ate he got a good look at the foot that had been caught in the snare. It was a simple loop snare, meant for the heads of rabbits and hares. In her efforts to get free she had pulled the snare tighter — it had worked just as it had been designed — and it had cut so deeply into her leg that it had made a grisly, bloody mess of her flesh, and he thought he could see the white of bone in the wound.

What now? He could slug her and take her back to the base camp. But this wasn’t a prey animal, a rabbit or a hare; it wasn’t some interesting specimen, like the huge half-way-flightless parakeet Sidewise had caught stalking the fringe of a stagnant pond. This was a person, no matter what she looked like. And, he reminded himself, those stretch marks told him she had at least one kid out there waiting for her.

“Did I come all this way, across a thousand fucking years, to make the same mess of your life as I’ve made of mine? I don’t bloody think so,” he muttered. “Pardon me.” And without hesitating he leapt on her.

It was another wrestling match. He got her pinned to the ground, face down, her arms under her, his buttocks in the small of her back. He used his Swiss Army knife to cut the snare wire, and prized the loop out of the bloody gouge it had dug. Then he used up more of his precious supplies to clean away the dirt and dried blood and pus with antiseptic fluid — he had to pick strands of brown hair out of the scabs — and to apply sealant and cream to the wound. Maybe she would leave the stuff on long enough for the wound to get itself disinfected.

The moment he released her she was gone. He glimpsed a figure, upright and lithe, shimmering through the long grass toward the trees, limping but moving fast even so.

It was already late afternoon. They weren’t supposed to be alone in the dark, away from base: Ahmed’s standing orders. He longed to follow the girl into the green mysteries of the denser forest. But he knew he must not. Regretfully he gathered up his gear and set off back to the base camp.

Snowy was the last to join the group that evening.

They had decided to settle close to a lake a few kilometers from the ruined town. The site was in the lee of a compact, cone-shaped hill — apparently artificial, maybe an Iron Age barrow, or maybe just a spoil heap of some kind.

Ahmed made them gather round the stump of a fallen tree, where he sat, a bit grandly. Snowy wanted to tell the others of his encounter, of what he had found. But the mood wasn’t right. So he just sat down.

Moon had grown increasingly withdrawn as the weeks had worn away; now she just sat cross-legged before Ahmed, her eyes averted. But she was the center of everything, as always, all the wordless maneuvering. Sidewise had his usual detached dreaminess, but he was sitting facing Moon, and Snowy saw how his gaze strayed over the curve of her hip, the centimeters of calf she showed above her boot. Ahmed himself sat beside the girl, raised up on his tree stump, as if he owned her.

Bonner was the one whose lust for Moon showed most nakedly. He sat awkwardly, muscles tensed, with a great stripe of mud splashed across his face, a hunter’s camouflage marking. He looked like an animal himself, Snowy thought, as if the last bits of his training were barely holding him together.

They were breaking up, Snowy saw, drifting apart, with great fault lines running through their intense little set of relationships. There was hardly anything left of the timid group of Navy fliers who had huddled in the ruined church that first night, chomping on their rations. They might kill each other over Moon, if Moon didn’t kill them first.

And Ahmed, their leader, was aware of none of this. Ahmed, in fact, was smiling. “I’ve been thinking about the future,” he said.

Sidewise gave a muffled groan.

“I mean, the further future,” Ahmed said. “Beyond the next few months, even the next few years. However we get through the next winter, times are going to be hard for our children.”

At the talk of children, Snowy cast a glance at Moon. She was glaring at her hands, her nested fingers.

Ahmed said that during the industrialized period — and especially during the last few insane decades — mankind had used up all its accessible supplies of fossil fuels: coal, gas, oil. “The fossil fuels are probably forming again even now. We know that. But incredibly slowly. The stuff we burned up in a few centuries took around four hundred and fifty million years to form. But there will always be fuel for our descendants,” he said. “Peat. Peat is what you get when bog mosses, sedges, and other vegetation decompose in oxygen-starved wetlands. Right? And in some parts of the world peat-cutting for fuel continued right until the middle of the twentieth century.”

“In Ireland,” Sidewise said. “In Scandinavia. Not here.”

“Then we go to Ireland, or Scandinavia. Or maybe we’ll find it here. Conditions have changed a lot since we went into the cold sleep. Anyhow, if we don’t find peat we’ll find something else. We’ve inherited a burned-out world.” He tapped his temple. “But we still have our minds, our ingenuity.”

“Oh, for God’s sake,” Sidewise said explosively. “Ahmed, don’t you get it? We’re just a bunch of castaways — that’s it — castaways in time. For Christ’s sake, man, we only have one womb between us.”

“My womb,” said Moon now, without looking up. “My womb. You insufferable prick.”

“Bog iron,” Ahmed said smoothly.

They all stared.

Ahmed said, “You get iron oxide forming in bogs and marshes. When iron-rich groundwater comes into contact with the air, well, it rusts. Right, Sidewise? The Vikings used to exploit that stuff. Why don’t we?”

As the bickering went on, Snowy’s gaze was drawn to the nearby woods, the shadowed green. Sidewise is right, he thought. We are here by accident, just a kind of echo. We are just going to fall apart, and get pulled down by the green like all the ruined buildings, and just disappear, adding our bones to the billions already heaped in the ground. And it won’t matter a damn. If he hadn’t known it before, known it in his gut, he was convinced of it now, having encountered the ape-girl. She is the future, he thought; she, with her bright lion’s gaze, her naked little body, her nimbleness and strength — her wordless silence.

As they dispersed, Snowy took Sidewise to one side, and told him about the feral woman.

Sidewise asked immediately, “Did you fuck her?”

Snowy frowned his disgust. “No. I felt like it — I got a hell of a rod — but when I saw what she was really like, I couldn’t have.”

Sidewise clapped him on the shoulder. “No reflection on your manhood, pal. Weena is probably the wrong species, that’s all.”

“Weena?”

“An old literary reference. Never mind. Listen. No matter what El Presidente over there says, we ought to find out more about these critters. That’s a hell of a lot more important than digging peat. We need to figure out how they are surviving here. Because that’s the way we are going to have to live too. Go find your girlfriend, Snowy. And ask her if she’d like a double date.”

A couple of days after that, before Ahmed could implement his plans for rebuilding civilization, he fell ill. He had to retreat to his lean-to, dependent on the food and water the others brought him.

Sidewise thought it was mercury poisoning, from the spoil heap by the camp. Mercury had been used for centuries in the making of everything from hats to mirrors to bug-control potions to treatments for syphilis. The ground was probably saturated with it, relatively speaking, and even now, a thousand years later, it was still leaching by various slow-dispersal routes into the lake, where it worked its way up the food chain to maximum concentration in the bodies of fish, and the mouths of the people who ate them.

Sidewise seemed to think all of this was funny: that Ahmed, the great planner — the one who, among them all, had clung the longest and hardest to the expansionist dreams of the long-gone twenty-first century — had succumbed to a dose of poison, a lingering legacy of that destructive age.

Snowy didn’t much care. There were far more interesting things in the world than anything Ahmed said or did.

Like Weena, and her hairy folk of the forest.

Snowy and Sidewise built a kind of blind, a lean-to liberally sprinkled with grass and green leaves, not far from where Snowy had first encountered the ape-girl Sidewise had christened Weena.

Snowy glanced at Sidewise, stretched out in the blind’s shade. In the dense heat of this un-English summer, both of them had taken to going naked save for shorts, an equipment belt, and boots. Sidewise’s skin, brown and smeared liberally with dirt, was as good a camouflage as anything invented by the hand of man. Only five or six weeks out of the Pit he was unrecognizable.

“There,” hissed Sidewise.

Slim gray-brown figures — two, three, four of them — coalesced out of the shadows at the edge of the forest. They took a few cautious steps out onto the open ground. They were naked, but they were slim and upright, and they carried something in their hands, probably their usual crude stone hammers and knives. Standing in a loose circle, their backs to each other, they peered around with sharp jerks of their heads.

Sidewise being Sidewise, he had developed a story about where these diminutive hairy folk had come from. “Sewer kids,” he had said. “When the cities fell, who was going to last the longest? The scrubby little kids who were already in the drains and the sewers and living off the garbage. It might have been years before some of them even noticed anything had changed—”

Now the hairies ran across the grassy meadow toward a slumped, fallen form. It was a deer, a big buck, that Snowy and Sidewise had brought down with a slingshot and dumped here in the hope of attracting the hairies out of their forest cover. The hairies converged on the carcass. They began to hack away at the joints where the hind legs were attached to the lower body. And as they worked in their intent silence, one of them was always on her feet, peering around, keeping guard.

“That’s their way of working,” murmured Snowy. “Taking the legs — see?”

“Quick and easy,” said Sidewise. “About the easiest bit of butchery you can do. Hack off a leg, then beat it back to the forest cover before something with bigger teeth than you comes along to make a contest of it. They are coordinated, even if they don’t speak. See the way they are taking turns to be look-out? They are pack hunters. Or scavengers anyhow.”

Snowy wondered how come they were so cautious if Sidewise was right about there being no big predators around.

“They look human but they don’t act it,” Snowy whispered. “You see what I mean? They aren’t like a patrol. They’re looking around like cats, or birds.”

Sidewise grunted. “Those sewer kids must have had no culture, no learning. All they would have known was the sewers. Maybe that was why they stopped talking. In the sewers, maybe the cover of silence was more important than language.”

“They lost language?”

“Why not? Birds lose their flight all the time. To be smart costs. Even a brain the size of yours, Snow, is expensive; it eats a lot of energy from your body’s supply. Maybe this isn’t a world where being smart pays off as much as, say, being able to run fast or see sharply. It probably didn’t take much rewiring for language, even consciousness, to be shut down. And now the brains are free to shrink. Give them a hundred thousand years and they’ll look like australopithecines.”

Snowy shook his head. “I always thought men from the future would have big bubble heads and no dicks.”

Sidewise looked at him in the dark of the blind. “Being smart didn’t exactly do us a lot of good, did it?” he said sourly. He peered out at the hairies, rubbing his face. “Makes you think, looking at them, how brief it all was. There was a moment when there were minds there to understand: to change things, to build. Now it’s gone, evaporated, and we’re back to this: living as animals, just another beast in the ecology. Just raw, unmediated existence.”

They watched a little longer, as the hairy, naked folk tore the limbs off the fallen deer and, cooperating and squabbling in turns, hauled the haunches back to the shelter of the forest.

Then they returned to their base camp.

Where they found that Bonner was ripping up the place because Moon had disappeared.

“Where the fuck is she?”

Moon had set up her own little lean-to, more solidly built and private than the others. Snowy had always thought that if she could have put on a door with a padlock she would have. Now everything was gone — the backpack Moon had made from a spare flight suit, her tools and clothes, her homemade wooden comb, her precious store of washable tampons.

Bonner was rampaging through what was left, smashing apart the walls of the lean-to. Naked save for now-disintegrating shorts, with his bulked-up muscles and mud smeared over his face and chest and in his spiky hair, Snowy thought there was very little left of the timid young pilot he remembered looking after when they had first met, on assignment to a carrier in the Adriatic.

Ahmed came out of his own lean-to, wrapped in a silvered survival blanket. “What’s going on?”

“She’s gone. She’s fucking gone!” Bonner raged.

Sidewise stepped forward. “We can all see she’s gone, you moron—”

Bonner hit at him with a slashing blow. Sidewise managed to duck out of the path of the young pilot’s fist, but he was caught on the temple and knocked flat.

Snowy ran forward and grabbed Bonner’s arms from behind. “For Christ’s sake, Bon, take it easy.”

“That two-brained bastard has been fucking her. All the time he was fucking her.”

Ahmed seemed utterly dismayed — as well he might, thought Snowy, for if Moon was gone, taking their only hope of procreation with her, all his grandiose plans were ruined before they had started. “But why would she go?” he moaned. “Why be alone? What would be the point?”

Snowy said, “What’s the point of any of it? We’re all going to die here. It was never going to work, splot. All the bog iron in the world wouldn’t have made any difference to that.”

Sidewise managed a grin. “I don’t think Bonner is worried about the destiny of mankind right now. Are you, Bon? All he cares about is that the only pussy in the world has vanished, without him getting any of it—”

Bonner roared and swung again, but this time Snowy managed to hold him back.

Ahmed sloped back to his shelter, coughing.

When relative calm was restored, Snowy went to the rack where they had hung a row of skinned rabbits, and started preparing a meal.

Before the first rabbit kebab was cooked over the fire, Bonner had made up a pack. He stood there, in the gathering twilight, facing Sidewise and Snowy. “I’m pissing off,” he said.

Sidewise nodded. “You going after Moon?”

“What do you think, shithead?”

“I think she has good land craft. She’ll be hard to track.”

“I’ll manage,” Bonner snarled.

“Wait until morning,” Snowy said reasonably. “Have some food. You’re asking for trouble, going off in the dark.”

But the reasoning part of Bonner’s head seemed to have switched off for good. He glared at them out of his mask of mud, every muscle tense. Then, his clumsy pack bumping on his back, he stalked away.

Sidewise put another bit of rabbit on the fire. “That’s the last we’ll see of him.”

“You think he’ll find Moon?”

“Not if she sees him coming.” Sidewise looked reflective. “And if he tries to force her, she’ll kill him. She’s tough that way.”

The rabbit was nearly done. Snowy pulled it off the fire, and began to push bits of it off the spit and onto their crude wooden plates. Every night he had divided up their food into five portions. Now, with Bonner and Moon gone, he divided it into three.

He and Sidewise just looked at the three portions for a while. Ahmed was back in his shelter. Out of sight, out of mind. Snowy picked up the third plate and, with the blade of his knife, scraped off the meat onto the other two plates. “If Ahmed gets better, he can look after himself. If not, there’s nothing we can do for him.”

For a time they chewed on their rabbit.

“I’ll leave tomorrow,” Snowy said eventually.

Sidewise didn’t reply to that.

“What about you? Where will you go?”

“I think I’d like to explore,” Sidewise said. “Go see the cities. London. Paris, if I can get across the Channel. Find out more about what’s happened. A lot of it must have gone already. But some of it must be like the ruins of the Roman Empire.”

“Nobody else will ever see such sights,” Snowy said.

“That’s true.”

Hesitantly, Snowy said, “What about after that? I mean, when we get older. Less strong.”

“I don’t think that is going to be a problem,” Sidewise said laconically. “The challenge will be to pick how you want to go. To make sure you control at least that.”

“When you’ve seen all you want to see.”

“Whatever.” He smiled. “Maybe in Paris there will be a few windows left to smash. Thousand-year-old brandy to drink. I’d enjoy that.”

“But,” Snowy said carefully, “there will be nobody to tell about it.”

“We’ve always known that,” Sidewise said sharply. “From the moment we clambered out of the Pit into that ancient oak forest. It was obvious even then.”

“Maybe to you,” Snowy said.

Sidewise tapped his temple, where a healthy bruise was developing from Bonner’s punch. “That’s my big brain working. Churning out one useless conclusion after another. And all of it making no damn difference, none at all. Listen. Let’s make a pact. We’ll pick a meeting place. We’ll aim to rendezvous, every year. We may not make it every time, but you can always leave a message, something.”

They picked a site — Stonehenge, on the high ground of Salisbury Plain, surely still unmistakable — and a time, the summer solstice, easy to track with the timekeeping discipline Ahmed had instilled in them. It was a good idea. Somehow it was comforting to Snowy, even now, to think that his future would have a little structure.

When they had done eating, the dark was closing in. It wasn’t cold, but Snowy fetched himself a blanket of crudely woven bark and wrapped it around his shoulders. “Hey, Side. Was he right?”

“Who?”

“Bonner. Did you pork Moon?”

“Too right I porked her.”

“You fucking dark horse. I never knew. Why you?”

“Atavistic urges, mate. I think she was responding to my smarter than the average brain.”

Snowy mulled over that. “So our big brains are good for one thing, then.”

“Oh, yes. They were always good for that. Probably what they were for in the first place. All the rest was bullshit.”

“You fucking dark horse.”

IV

Snowy followed the ape people.

He didn’t live as they did. He used his snares to trap game up to the size of pigs and small deer, and used knives and fires and lean-tos for protection and butchery. But he walked where they walked.

They wandered impressively widely, through the great forests that blanketed southern England, forests that concealed the ruins of cities and cathedrals, palaces and parks. He became concerned if he lost sight of Weena, reassured when he found her again. He grew to know all the individuals in the little group — he gave them names, like Grandpa and Shorty and Doc — and he followed their lives, their triumphs and tragedies, as if he were watching a small soap opera.

They were frightened of the rats — the big ones, the rat-wolves that seemed to hunt in packs. He found that out quickly.

He wondered how he must seem to them. They were clearly aware of him, but he didn’t interfere with them or the food they gathered. So they let him be, unremarked. He was like a ghost, he thought, a ghost from a vanished past, haunting these new people.

After a few months, with the long, long summer of these late times at last drawing to a close, they came to a beach. Snowy thought he was somewhere on the Sussex shore, on Britain’s south coast.

The hairies did a little foraging at the fringe of the forest, ignoring Snowy as usual.

Snowy wandered along the beach. The forest washed right down to the shore, as if this were a Robinson Crusoe tropical island, not England at all. He found a place to sit, facing the crashing waves.

He picked up a handful of sand. It was fine and golden, and ran easily through his fingers. But there were black grains in there, he saw, and some bits of orange and green and blue. The multicolored stuff must be plastic. And the black stuff looked like soot — soot from Rabaul, the killer volcano, or from the fires that had swept the world as everything went to shit.

It’s all gone, he thought wonderingly. It really has. The sand was a kind of proof. Moon rock and cathedrals and football stadiums, libraries and museums and paintings, highways and cities and shanties, Shakespeare and Mozart and Einstein, Buddha and Mohammed and Jesus, lions and elephants and horses and gorillas and the rest of the menagerie of extinction — all worn away and scattered and ground down, mixed into this sooty sand he trickled through his fingers.

The hairies were leaving. He could see their slim forms sliding silently into the deeper forest.

He stood up, brushed the sand off his palms, shifted the pack on his back, and followed them.

I

The asteroid had once been called Eros.

Eros had its own miniature geography. Its ground was covered by impact craters, scattered rubble and debris, and strange pools of very fine, bluish dust, electrically charged by the relentless sunlight. Some three times as long as it was wide, it was like Manhattan Island hurled into space.

Eros was as old as the Devil’s Tail. Like the Chicxulub comet it was a relic of the formation of the solar system itself. But unlike the comet the asteroid had coalesced well within the clockwork of the inner system — inside the orbit of Jupiter, in fact. In the early days there had been mass destruction as the young asteroids, following their careening orbits, had smashed helplessly into each other. Most were shattered into clouds of dust, or thrown into the great maw of Jupiter, or into the crowded and dangerous inner system. The survivors, in their depleted swarms, followed orderly orbits around the brightening sun.

But even now gravity’s ghostly tug caused the asteroids’ orbits to resonate like plucked strings.

She surfaced reluctantly into the daylight.

She had had another bad dream. Her head felt muzzy, her limbs stiff. Through the crude roof of her treetop nest she saw the rustling green of the higher canopy, and slivers of bright blue tropical sky. Like the pallet under her body, the roof was just a pulled-together mass of twigs and leaves and slim branches, hastily constructed in the last hour before darkness, soon to be abandoned.

She lay on her back, her right arm pillowed under her head, her legs tucked up against her belly. Her naked body was covered with fine golden hair. At fifteen years of age she was in the prime of her life. Stretch marks on her belly and her small dugs showed that she had already given birth. Her eyes, crusted with sleep, were large, black, watchful: the mark of a slow readaptation to nocturnal living. Behind them a shallow brow led to a small, neat brainpan, its modest outline obscured by a thatch of curly dark hair.

A part of her never slept soundly, no matter how well she constructed her nests. Her dreams were always troubled by the huge spaces beneath her, into which she might fall. Since the treetops were the only safe place for her people to live, this didn’t make sense, but there it was. It was going to take more time yet for people to get used to their return to the trees.

It didn’t help, of course, that her only child so far had been taken by those spaces beneath her, his grip loosened from her fur by rain, his little body tumbling into the green depths.

She had never discussed this with anyone. In fact nobody discussed anything anymore. The days of endless talking were long gone, the larynxes and cognitive capacities of a loquacious folk put aside, irrelevant to life in the trees.

She didn’t even have a name. But perhaps something in her retained a deep memory of vanished, different days. Call her Remembrance.

She heard a rustling in the layers of vegetation beneath her, a trickle of discarded fruit husks falling through the leaves, the first tentative hooting pants of the males.

She rolled on to her belly and pressed her face into her bed of twigs. She could just make out the colony itself, a dark, pendulous mass in the deeper layers of the canopy, like a wooden submarine somehow lodged high in the green. All around the colony slim figures were moving, working, bickering. The business of the day was starting. And it didn’t pay to be a late arrival.

Remembrance stood upright and broke open her nest, like a bird bursting from its egg. With her small head raised to her full meter-tall height above the branch, she peered around at her world.

Everywhere the forest lapped in great green layers of life. The highest canopy was a roof far above her own elevation. To north, west, and east, beyond the trees, Remembrance could make out a blue, sparkling glimmer. The light off the ocean had always intrigued her. And though she could not make out the southern shore, she had a correct intuition that the ocean continued even there, making a great belt around the land: she knew that she lived on a vast island. But the ocean was another irrelevance, too far away for her to be troubled with.

This particularly dense pocket of forest had sprouted from a gorge cut deep into the bedrock. Sheltered by walls of hard rock, fed by streams that ran along the base of the gorge, this was a crowded, vibrant place, full of life — though here and there were bare patches cleared by borametz trees and their servants, a new kind of life.

But the gorge itself wasn’t natural. Long ago blasted out of ancient bedrock, it was the result of human road building. Erosion had taken its toll: When the drainage ditches and culverts were no longer maintained, the cutting slopes had collapsed. But nevertheless a patient geologist could have detected a fine dark layer in the sandstone that had slowly gathered at the bottom of the gorge. The dark layer was metamorphosing bitumen, a stratum still sprinkled here and there with fragments of the vehicles that had once come this way.

Even now the passing of humans left its mark.

A shadow flickered over the leaves that rustled around her, fast-moving, silent, cast by the low sun. Hastily she ducked down, seeking the safety of the green’s cover. It had been a bird, of course. The predators of the upper canopy had already started their day, and it did not do to be too visible.

With a last glance at the remains of her nest — littered by bits of shit and discarded hair, stained by her urine, soon to be forgotten — she began to clamber down.

As the tropical day brightened, the people had already spread out through the trees, lithe and graceful, beginning the day’s relentless search for fruit, bark-burrowing insects, and leaf-cupped water.

Remembrance, still listless, hung back, watching.

There were males and females alike, some of the women laden with clinging infants. The males also did a great deal of displaying, hooting, aggressive leaping to and fro. Here was something that had not changed down the long years: the structure of primate society was still the same, a flashy male-hierarchy superstructure imposed on top of a network of patient female clans.

In these middle layers of the canopy the taller trees thrust upward past the crowns of their smaller brethren. In this intermediate place, neither low nor high, the people were relatively safe from threats from above and below. And it was here, surrounded by the tall, slim stripes of the great trees’ trunks, that they had built their colony.

It was a ball some ten meters across. Its thick wall was made of twigs and dead leaves, crudely crammed together. The leaves had been softened by chewing before being pressed into gaps in the structure. The whole thing was neatly lodged in the crooks of the robust branches of the tree, in which it had been constructed over generations. And it was lived in: A thin stream of shit and piss slid down the tree’s great trunk, sewage trickling out of the openings that pocked the colony’s base.

This ball of spittle and twigs was the most advanced construction any posthumans were now capable of. But it was the result of instinct, not mind, as empty of conscious planning as a bower bird’s nest or a termite mound.

Remembrance could see small faces peering out timidly through gaps in the colony’s crude wall. She remembered her time with her own child inside those dank, ill-smelling walls. The colony’s basic purpose was to shelter the most vulnerable from the forest’s predators: at night the prepubescent young, the old and sickly would cram within its walls. But only the smallest infants and their mothers were allowed to stay in its shelter during the day while the rest risked the open spaces to forage.

And, as stray canopy-filtered rays of sunlight caught the colony, the walls sparkled. Embedded in the packed-together twigs and leaves were bright stones gathered from the forest floor. There were even bits of glass. Across millions of years glass was unstable, becoming opaque as tiny crystals formed within it — but nevertheless these fragments had retained their shapes, bits of windshields or taillights or bottles, now retrieved and gathered to adorn the walls of this shapeless building.

It looked like decoration, but it was not. The glass and bright stones were meant for defense. Even now predators on these postpeople could be deterred by the remnants of buildings, by glittering stones and shining glass, haunted by deep-buried instincts developed in the time of the most dangerous killers who had ever walked the Earth. So Remembrance’s folk aped the structures of their ancestors, not even capable of imagining what they were imitating.

Once, of course, the trees had been the domain of primates, where they had been able to roam with little fear of predation. Monkeys and chimpanzees had not needed fortresses of leaf and twig. Times had changed.

As Remembrance lingered, a young male hissed at her. He had a bizarre white patch of fur on his backside, almost like a rabbit’s. She knew what he was thinking: He suspected she might be after the patch of bark he was working with his mother and siblings. People’s minds were not what their ancestors’ had been, but Remembrance was still capable of working out the beliefs and intentions of others.

But White-patch’s troop had been weakened today. Since the last time Remembrance had seen them, their elder son had gone. He might have left to seek some other colony, suspended somewhere in the forest’s green depths. Or, of course, he might be dead. The family members themselves showed they were still aware of the lost one’s absence in the way they looked over their shoulders at nobody, or left a space for a big male who never came. But soon their memories would heal over, and the brother would vanish into the unremembering mists of the past, as lost as had been all the children of man since the construction of the last tombstone.

Remembrance herself would never learn what had become of the other son. This was not an age of information. Nobody told anybody else anything anymore. All she knew for sure was what she saw for herself.

For Remembrance, though, this was an opportunity. She could probably fight this weakened group for a place on their tree. But her poor sleep had left her feeling brittle, restless. It was a mood that had plagued her since the loss of her child. The child’s death had been more than a year ago, but so sharp was the pain, so vivid was it still in her kaleidoscopic, unstructured mind, that it might have been yesterday. Like all her kind, Remembrance was a creature not of purposeful planning but of impulse. And today her impulse was not to fight these squabbling folk for the privilege of a place on their crowded branch, to peel back a bit of bark in search of grubs.

She turned away and began to make her way through the tangled levels of trees.

As she swung, clambered, and leapt her way from branch to branch, she began to feel better. The stiffness quickly worked out of her muscles, and it was as if she were coming fully awake. She even forgot, briefly, the loss of her child. She was still young; her kind often lived beyond twenty-five or even thirty years. And, long after a remote ancestor had crawled, baffled, out of a sewer into the greening daylight, her body was well adapted to her way of life, if not yet the deepest chapels of her mind.

So, as she worked in a blur of speed through the trees, she felt a kind of joy. Why not? Much had been lost, but that made no difference to Remembrance. Her brief moment in the light was here, now, and was to be cherished. As she soared through the dense twilight of the forest layers, her lips drew back from her teeth, and she laughed out loud. It was a reflex the children of man had never lost — even though, across Earth’s healing face, thirty million summers had flickered and gone.

Remembrance’s tropical forest was part of a great belt wrapped around the waist of the planet, a belt broken only by oceans and mountains. The forests were luxuriant — although they had taken thousands of years after the cessation of man’s ferocious logging to attain something like their former richness.

The reassembled world, engulfed by forest, had left little room for the descendants of mankind. And so Remembrance’s ancestors had left the ground and taken once more to the green womb of the canopies. There had already been primates here: monkeys whose ancestors had evaded the starving humans in the final days, survivors of the great extinction event. At first the posthumans were clumsier than the monkeys. But they were still smart, relatively — and they were desperate. Soon they completed the extinction that their forefathers had begun.

After that they had begun to proliferate. But the pressures that had driven them off the ground continued to pursue them.

Remembrance knew nothing of this. And yet she carried within her a molecular memory, a continuing unbroken line of genetic inheritance that stretched back to the vanished folk who had carved the mighty roadway out of the rock — and back, back far beyond them, to still more distant times when creatures not unlike Remembrance had clambered in trees not unlike this one.

She stopped at a branch laden with fat red fruit. She sat squat on the branch and began to feed briskly, shelling the fruit and sucking down the soft contents, letting the drained husks fall into the darkness below. But as she ate she kept her back to the trunk, her gaze darted fearfully around the shadows, and her motions were fast, furtive.

Despite her watchfulness, she was startled when the first chunk of rind hit her on the back of the head.

Cowering against the trunk, she looked up. Now she saw that the branches above her were heavy with what looked like fruit: fat, dark, pendulous. But those “fruit” were sprouting arms and legs and heads and glittering eyes, and clever hands that hurled rinds and bits of bark and twig down at her. They had probably lain in wait as she approached, and then just as silently converged on her position. They even threw lumps of warm shit.

And now the chattering began. It was a screaming, meaningless jabber that filled her head, disorienting her — as was its purpose. She huddled in the crook of the branch, her hands clapped over her ears.

The Chattering Folk were cousins of Remembrance’s kind. They used to be humans too. But the Chatterers lived differently. They were cooperative hunters. All of them, from barely weaned young upward, would work with a cold, instinctive discipline to bring down any prey, or battle any predator. The strategy worked: Remembrance had seen more than one of her kind fall before this treetop army.

Despite their different ways of living, up to a couple of million years ago the two kinds of people could still have crossbred, though their offspring would have been infertile. By now that was impossible. It had been a speciation, one of many. To the Chattering Folk, Remembrance was not kin, nothing but a potential threat — or perhaps, a meal.

She was cut off. There seemed to be a Chatterer on every branch. She could never get past them and reach the sanctuary of another tree. She had only one way off this bare trunk: across the ground itself.

She didn’t hesitate. Skittering down the trunk — letting herself fall for long distances, trusting on her reflexes to grab at branches and slow her descent — she escaped toward the deeper gloom of the forest floor.

At first the Chatterers followed her, and their bits of fruit and shit hailed around her, splattering against the bark. She heard them spread out through the tree from which they had ousted her, chattering and screaming their useless triumph.

At last she slid off the trunk. She intended to make for another great tree trunk a couple of hundred meters away, which might be far enough away from the Chatterers to give her a safe passage back to the canopy.

She stepped forward, her eyes wide and alert, walking upright.

Remembrance had narrow hips and long legs, relics of the bipedal days of ground-dwelling savannah apes. She was more upright than any chimp had ever been, more upright than Capo’s folk. But even upright, her legs remained slightly bent, her neck sloped forward. Her shoulders were narrow, her arms long and strong, and her feet were long and equipped with opposable toes — all good equipment for climbing, clinging, leaping. Arboreal life had reshaped her kind: Selection had reached back to ancient designs, much modified, their templates never abandoned.

She wasn’t comfortable here on the ground. When she looked up she saw layers of foliage, trees competing for the energy of the sun, cutting out all but the most diffuse light. It was like looking up at another world, a three-dimensional city.

By contrast the forest floor was a dark, humid place. Shrubs, herbs, and fungi grew sparsely in the endless twilight. Though leaves and other debris fell in a continual slow rain from the green galleries above, the ground cover was shallow: the ants and termites, whose mounds stood around the floor like eroded monuments, saw to that.

She came to a huge mushroom. She stopped and began to cram its tasty white meat into her mouth. She had eaten little so far that day, and she had used up a lot of energy in fleeing the Chatterers.

Beyond a stand of spindly saplings something moved through the shadows: huge shapes, grunting, snuffling at the dirt. Remembrance ducked behind the mushroom.

The creatures emerged from the shadows, dimly outlined in the gray-green twilight. They had bulky, hairy bodies, stocky heads, and short trunks that scraped at the ground and plucked foliage and fruit from the trees’ lower branches. A couple of meters tall at the shoulder, they looked like forest elephants, though they were tuskless.

These browsers’ small pointed ears and oddly curling tails gave away their ancestry. They were pigs, descended from one of the few species domesticated by mankind to survive the great destruction, and now shaped into this efficient form. The last true elephants, in fact, had gone with humans into extinction.

More large, hairy creatures shouldered their way into Remembrance’s view. They were elephantine forms too, the same size and shape as the pigs. But where the pigs had trunks but lacked tusks, these animals had no trunks, but carried great sweeping horns that curled before them and served as elephants’ tusks once had, clearing the ground and upturning roots and tubers. More skittish and aggressive than the pigs, these animals were descended from another generalist survivor of human farmyards, the goats.

The two kinds of browser, pig- and goat-elephants, worked the shallow ground, different enough to be able to share this space, loftily ignoring each other’s presence. Remembrance cowered, waiting for a chance to get away from these much-evolved descendants of farm animals.

And then she smelled a breath on her neck: the faintest trace of warmth, the putrid stink of meat.

Immediately she hurled herself forward. Ignoring the elephantine pigs and goats, she ran until she reached a tree trunk and swarmed up, clinging to crevices in the bark. She didn’t hesitate for a moment, not even to look back to see what it was that had so nearly crept up on her.

She caught glimpses, though. It was a creature the size of a leopard, with red eyes, long limbs, grasping paws, and powerful incisors.

She knew what it was. It was a rat. When you smelled rat, you ran.

But the rat followed.

To pursue its climbing prey, the rat-leopard’s kind had learned to climb too. The rat-leopard had claws, opposable fingers to grasp branches, forelimbs that could swing wide to allow it to hurl itself from branch to branch, even a prehensile tail. It wasn’t as good a climber as the best of the primates, like Remembrance. Not yet. But it didn’t need to be as good as the best. It only needed to be better than the worst, the weak and the ill — and the unlucky.

And so Remembrance climbed, on and on, ascending into the pale green light of the upper canopy, faster and faster, ignoring the bursting pain in her lungs and the ache in her arms. Soon she was dazzled by the light. She was reaching the upper reaches of the canopy. But still she climbed, for she had no choice.

Until she burst into open daylight.

She almost stumbled, so suddenly had she erupted out of the green. She clung to a narrow branch that swayed alarmingly under her, bright with leaves that, green and lush, drank in the sunlight.

She was perched right on top of the giant tree’s uppermost branch. The canopy was a blanket of green that stretched away to the ocean. But she could make out the rocky shoulders of the gorge within which her dense pocket of forest grew, the ancient roadway of her ancestors. She had nowhere to go. Panting, exhausted, her depleted muscles trembling, she could only cling to this spindly branch. The sun beat down, too hot. Unlike her remote ancestors she was not built for the open: Her kind had given up the ability to sweat.

But the rat did not follow her. She thought she glimpsed its red-rimmed eyes, glittering, before it descended back into the gloom of the forest.

For a heartbeat she exulted. She threw back her head and whooped her joy.

Perhaps it was that that gave her away.

She felt a breeze first. Then came an almost metallic rustle of feathers, a swooping shadow over her.

Claws dug deep into the flesh of her shoulders. The pain was immediately agonizing — and grew worse as she was lifted by those claws, her whole weight suspended from scraps of her own flesh. She was flying. She glimpsed the land wheeling beneath her — scraps of forest, swaths of green grassland and brown borametz groves, all laid over a broken, eroded volcanic landscape, and that belt of glimmering sea beyond.

In Remembrance’s world there were ferocious predators both above and below, like red mouths all around you, waiting to punish the slightest mistake. In escaping from one peril, she had run straight into the grasp of another.

The bird was like a cross between an owl and an eagle, with a fierce yellow beak and round forward-facing eyes, adapted for its forays into the gloom of the forest canopy. But it was neither owl nor eagle. This ferocious killer was actually descended from finches, another widespread generalist survivor of the human catastrophe.

The finch was hauling her toward a high complex of volcanic plugs, the eroded core of ancient volcanoes. The debris-littered ground nearby was green with grass, here and there browned by groves of borametz trees. And, tucked into the high ledges, Remembrance glimpsed nests: nests full of pink, straining mouths.

She knew what would happen if the finch succeeded in getting her to its nest.

She began to scream and struggle, pounding her fists against the legs and underbelly of the bird. As she fought, the hooked flesh in her shoulder ripped, sending blood streaming down her fur, but she ignored the pulses of agonizing pain.

The finch cawed angrily and flapped its wings, great tents of oily feathers that hammered at her head and back. She could smell the iron staleness of its blood-caked beak. But she was a big piece of meat, even for this giant bird. As she fought they spun toward the ground, hominid and bird tied up in their clumsy midair battle. At last she got her teeth into the softer flesh above the bird’s scaly talons. The bird screamed and spasmed. Its claws opened.

And she was falling through sudden silence. The only noise was her own ragged breathing, the buffeting of the air, like a wind. She could still see the bird, a wheeling shadow above her, fast receding. She reached for branches or rocks, but there was nothing to grab.

Oddly, now that she was lost in her own deepest nightmare of falling, she was no longer afraid. She hung limp, waiting.

She smashed into a tree. Leaves and twigs clutched painfully at her skin as she crashed through them. But the foliage slowed her, and she plummeted at last to the grassy ground. Battered, torn, bruised, she was only winded. For a few heartbeats she could not move.

A human’s shock would have been deeper. Who was to blame for this sequence of calamities? The rat, the bird of prey, a spell-casting enemy, a malevolent god? Why had this happened? Why me? But Remembrance asked herself no such questions. For Remembrance, life was not something to be controlled. Life was episodic, random, purposeless.

That was how things were now, for people. You didn’t live long. You didn’t get to shape the world around you. You barely understood much of what happened to you. All you thought about was now: drawing another breath, finding another meal, evading the next random killer.

Seeing what happened next.

When she had got her breath back she rolled to all fours and scuttled into the shade of the tree that had broken her fall.

II

Remembrance’s time might have been called the Age of the Atlantic.

Since the fall of man the continents’ chthonic dance had continued. That great ocean, born as a crack in Pangaea over two hundred million years ago, was continuing to widen as new seabed erupted endlessly along the line of the midocean ridge. The Americas had drifted westward, and South America had broken away from North to resume its interrupted career as an island continent. Meanwhile the cluster of continents around Asia had drifted east, so that the Pacific was slowly closing up. Alaska had reached out to Asia, rebuilding the Bering Strait bridge that had been made and undone repeatedly by the Ice Age glaciations.

There had been tremendous, protracted collisions. Australia had migrated north until it rammed itself into southern Asia, and Africa had crashed into southern Europe. It was as if the continents were crowding into the northern hemisphere, leaving the south abandoned save for lonely, icebound Antarctica. But Africa itself had fragmented, as the mighty wound of the ancient Rift Valley had deepened.

Where continents met, new mountain ranges were stitched. Where the Mediterranean had been there was now a mighty mountain range that reached eastward toward the Himalayas. It was the final extinction of the ancient Tethys. No trace of Rome had survived: the bones of emperors and philosophers alike had been crushed, melted, and gone swimming into the Earth itself. But while mountains were built, others evaporated like dew. The Himalayas were eroded to stumps, opening up new migration routes between India and Asia.

Nothing mankind had done in its short and bloody history had made the slightest bit of difference to this patient geographical realignment.

Meanwhile the Earth, left to its own devices, had deployed a variety of healing mechanisms, physical, chemical, biological, and geological, to recover from the devastating interventions of its human inhabitants. Air pollutants had been broken up by sunlight and dispersed. Bog ore had absorbed much metallic waste. Vegetation had recolonized abandoned landscapes, roots breaking up concrete and asphalt, overgrowing ditches and canals. Erosion by wind and water had caused the final collapse of the last structures, washing it all into sand.

Meanwhile the relentless processes of variation and selection had worked to fill an emptied world.

The sun climbed higher. Despite all that had happened to Remembrance, it was not yet midday.

She was stranded on a grassy plain, with purple volcanic hills in the distance, a few sparse stands of trees and shrubs, and a brown patch of borametz, the new kind of tree. Here, in the rain shadow of those purple hills, the rainfall was intermittent and erratic. The soil was habitually dry, and in such conditions trees were unable to establish themselves, and the grasses continued their ancient dominion — almost. Even vegetable communities evolved. And now the grasses had new competitors, in the borametz groves.

The tree that had saved her from the fall was barren of fruit, parched, clinging to life in the dry soil of this grassland. There was nothing to eat here — nothing but the scorpions and beetles that squirmed from beneath the rocks, bugs she popped into her mouth.

She made out a belt of forest, huddled against those remote purple hills, shimmering in the heat haze. Vaguely she realized that if she could get there she would be safer, she might find food, even people of her own kind.

But the forest was far away. Remembrance’s distant grandmothers would have easily walked across this stretch of open savannah. But not Remembrance. She was too clumsy a walker. And like Capo, a chimplike ape of a different time, her kind had regrown their hair and forgotten how to sweat.

So she sat there, her mind empty of plans, waiting for something to turn up.

Suddenly a slim head swooped down from the washed-out sky. Remembrance chattered and flinched back against the tree trunk. She saw black round eyes, wide with surprise, set in a slender, fur-covered face, and two long ears that swept back against an elegant neck. It was a rabbit’s head — but it was large, as large as a gazelle’s.

The rabbit-gazelle evidently decided that the cowering hominid was no particular threat to her. She proceeded to crop at the grass that grew thinly in the shade of the tree.

Cautiously Remembrance crept forward.

Her visitor was one of a herd, she saw now, scattered over the plain and grazing patiently on the grass. They were tall, some twice as tall as she was. Slim, graceful, they looked like gazelles — but they were indeed descended from rabbits, as their long ears and small white tails clearly demonstrated.

The legs of these animals were like gazelles’, too. Their forelegs were straight, and could be locked into position to support the animal with little effort. But halfway down their hind legs these rabbits had backward-bending joints that were in fact ankles. The lower leg was like an extended foot — balanced on two hooflike toes — and the knee was up near the torso, hidden in fur. Their back legs held in a permanent sprinter’s crouch, the rabbit-gazelles were constantly ready for flight, the most critical task in their lives. As they grazed, the youngest scuttling at the feet of their elders, the herd remained compact, and there was never a time when at least one of the adults was not scanning the grass.

The reason for all of this soon became apparent. One of the bigger bucks startled, went rigid, fled. The rest of the herd followed immediately, in a blur of speed and dust.

From the cover of a bluff of rocks a slim black form darted forward. It was another rat, this one shaped to run with the low-slung power of a cheetah. The rat-cheetah disappeared into the dust, pursuing the rabbit herd.

Stillness resumed. For a time, nothing moved over the grass-covered plain, nothing but the shimmer of the air. The sun slid away from its height. But the heat did not lessen, and thirst clawed at Remembrance’s throat.

She crept out of her hiding place. Her very human face, with straight nose, small mouth and chin, wrinkled in the bright afternoon light. She raised herself to her full height and sniffed. She heard a lowing, a clattering of tusks that sounded as if it were coming from the east, away from the sun. And she smelled the tang of water.

She began to run that way. She moved in scurries, hurrying from one patch of covering shade to the next, with frequent drops to an all-fours lope. This daughter of mankind ran like a chimp.

At last she crested a shallow bluff of eroded sandstone. She found herself facing a broad lake. It was fed by streams that snaked from more distant hills, but she could see that it was choked with reeds and fringed by a broad mud pan. She found an acacia to shelter under, and peered out, trying to find a way to get to the water.

Here, just as they always had, the herbivores had gathered to drink.

She saw more rabbits. There were skittish gazellelike creatures of the kind she had seen before. But there were also heavier-built, bisonlike powerhouses — and, running around their feet, smaller creatures that hopped and jumped. The rabbits, widespread and fast breeding, had, after the fall of man, radiated and adapted quickly. But not all of the new species had abandoned the ancient ways. There were still smaller browsers, especially in the forests where small beasts kicked and leapt and hopped as their ancestors always had.

Meanwhile warthogs snuffled and snorted in the muddy fringe of the lake, left all but unchanged by time. If there was no need to adapt, nature was conservative. And Remembrance made out huge, slow-moving creatures, marching serenely through the shallow water. They were related to the goats she had encountered in the forest, but these were giants, with tree trunk legs and horns that curled like mammoth tusks. They lacked trunks — none of these ruminants had evolved that particular anatomical trick — but, giraffelike, they had long necks that let them reach the succulent leaves growing on low-hanging tree branches, or the water of the lake.

A herd of different goat descendants stood knee-deep in the water. They had webbed feet that kept them from sinking in soft mud and sand. Each had a broad bill-like mask before its face. Sculpted from horns, these bills were used for browsing on the soft weeds found at the edge of the lakes. Sucking peacefully at the lakeside vegetation, these goats were like nothing so much as the hadrosaurs, the long-vanished duck-billed dinosaurs.

And, just as the hadrosaurs had been the most diverse group of dinosaurs before the comet fell, so this rediscovery of an ancient strategy was enabling a new radiation. Already many species of the duck-billed goats, subtly distinguished by differences in horn design, size, and diet preferences, were to be found at many of the water courses of the world’s tropical regions and elsewhere.

Meanwhile, all around this scene of relatively peaceful herbivorous thirst-quenching — just as there had always been — intent predatory eyes watched the herbivores at work.

Watching this scene with half-closed eyes it would not have been impossible to imagine that the animals obliterated by human action had been restored. But on this new African savannah the familiar roles had been taken up by new actors, descended from creatures that had best survived the human extinction event. These were those that had resisted all of mankind’s attempts at extirpation: the vermin, especially the generalists — starlings, finches, rabbits, squirrels — and rodents like rats and mice. Thus there were rabbits morphed into gazelles, rats become cheetahs. Only subtleties were changed — a nervous twitchiness about the rabbits, a hard-running intensity about the rats that had replaced the cats’ languid grace.

There was a sudden flurry of activity, a great clash like a bone breaking. Two of the great goat-elephants, males, had begun a dispute. Their heads bobbed and swayed atop long giraffelike necks, and their horns, elaborately curling before their faces, clashed like baroque swords.

Remembrance cowered deep into the shade of her acacia. As the great herbivores began to mill around her, disturbed by the battle, she wasn’t so safe. This tree, trunk and all, could be smashed up and devoured in a few heartbeats.

And now the watchful predators took advantage of the confusion.

A pack of them erupted from cover. Lean and vulpine, with long, powerful shanks and thickly padded feet, they were more rats. Working closely together, they moved wedgelike to separate one older goat-elephant from the rest of the herd. His huge horn-tusks chipped and scarred by a lifetime of battles, this big male bellowed his rage and fear and began to run. The rats settled into the pursuit, running closely together.

These rat derivatives were like dogs, yet they were not dogs. Their characteristic rodents’ incisors had been subtly modified from teeth designed for processing seeds and insects into blades with stabbing points. Their rear molars were like shears, well equipped for shredding meat. And they moved more closely than any dog pack had ever run, with a liquid, slithering power. But, like a dog pack, their basic strategy was to chase the goat-elephant until he was exhausted.

Soon the prey and his pursuers had passed out of sight.

The goat-elephants settled down once more to their drinking and fighting — though some of them turned their great heads to the place where the old one had stood, remembering his absence.

Remembrance took the opportunity to creep forward.

The water was scum laden. But she scooped it up in her hands and let it trickle into her mouth, leaving her palms and fingers coated with fine green slime.

From the water, two yellow eyes watched her with abstract instinct. It was a crocodile, of course. These ancient survivors had ridden out the human apocalypse as they had survived so many before: by living off the gruesome brown food chain of the dying lands, by burrowing into the welcoming mud in drought. And even now no animal, no pig or rabbit or primate, no fish or bird, reptile or amphibian — not even the rodents — had managed to dislodge the crocodiles from their watery kingdom.

Remembrance shuddered, and backed off from the water’s edge.

A new predator stalked over the bluff toward the lake. Again Remembrance scurried for cover, screened by the huge, impassive bodies of a herd of duck-billed goats.

This predator was more rodent stock; in fact it came from a kind of mouse. But its behavior was not like any dog or cat’s. It came to the edge of the water, and lifted itself up on its massive hind legs. The herbivores at the water’s edge cowered away. But the mouse-raptor had no interest in the creatures milling before it. With lordly dismissal it dipped its ferocious muzzle to taste the water. Then it stalked back to dry land where it used its small, feeble-looking hands to pluck at the grass, as if testing it.

It looked like one of the great carnivorous dinosaurs of the Cretaceous days. Its forearms were small, its tail was thickened for balance, and its hind legs were awesomely powerful machines of muscle and bone. Its incisors had developed into ferocious slashing weapons, to be deployed by thrusts of the heavy head. The mouse-raptor was a land shark, like a tyrannosaur, a body design rediscovered and made devastatingly effective. And yet this arrogant creature retained the small ears and brown fur of the diminutive rodents from which it had derived.

The mouse-raptor seemed satisfied with the water and the grass. It squealed, spat, and drummed its tail on the ground. From the distance there was a series of answering calls, drums, and cries.

More mouse-raptors approached the lake. They fanned out over a swath of grassland, sniffing the air. A few kits ran around the legs of the adults, wrestling and nipping at each other with the ancient playful curiosity of predators.

When they had gathered, the adult mouse-raptors turned, opened their throats, and set up a kind of synchronized wailing. In response, a herd of another kind of animal came lumbering toward the water.

These were big creatures, as big as the goat-elephants. Nervously they huddled together, querulously jostling. But even as they stumbled toward the water, under the apparent guidance of the mouse-raptors, they cropped hastily at the grass under their feet.

Their bodies were coated with sparse fur. Their heads were crested, their skulls shaped to allow anchorage for the tremendous cheek muscles that worked their immense lower jaws: Their heads looked rather like those of robust pithecines, in fact. Their ears, plastered back over their massive skulls, were huge and veined, great radiator fins designed to extract waste heat from their huge bodies. Though their hind legs were massive, enabling them to support their weight, they had the peculiar wrong-way-bending look of the rabbit-gazelles: legs meant for fleeing.

These animals were ugly, elephantine. But they had not descended from goat or pig. They had forward-looking eyes under heavy browridges, huge dark eyes that peered at the world, baffled and fearful. They walked on all fours, but they supported themselves on the folded knuckles of their hands, a posture that had once been called knuckle-walking.

Like Remembrance, their ancestors had once been human.

Remembrance waited until the big dull animals had settled to their drinking, jostling querulously, their ears spreading in the cooling air of the afternoon. Then she crept away.

It had taken millions of years for the great rebound of life to be completed.

Today, to the north of Remembrance’s tropical forest, a great band of temperate woodland and grassland marched around the Earth, stretching from Europe-Africa across Asia to North America. Here more rabbit types browsed the cool foliage, while things like hedgehogs and pigs worked the undergrowth. In the trees there were birds and squirrels — and many, many bats. This diverse group of mammals had continued to proliferate and diverge, and now there were some nocturnal flyers who had lost their eyes altogether, others who had learned to compete with the birds for the richer pickings of the day.

Further north still, coniferous forests grew, evergreen trees whose spiky leaves were always ready to take advantage of the sun’s thin ration of light. Browsing animals lived on the young twigs and needles in the summer, and on bark, mosses, and lichen the rest of the year. Many of them were goats. Especially common were the hadrosaur-like duckbill forms. Their predators included the ubiquitous mice and rats — but there were also carnivorous squirrels and huge birds of prey that seemed to be trying to emulate the pterosaurs of the oxygen-rich Cretaceous skies.

On the northern fringes of the continents a belt of tundra had formed. Here the descendants of pigs and goats cropped the thin foliage of summer, and huddled together to endure the winter. Like the vanished mammoths, some of these creatures had grown huge, the better to retain their warmth, until they were great round boulders of flesh. On the tundra the predatory rats had grown their incisors into huge stabbing instruments, the better to penetrate those thick layers of fur and fat. They looked something like the saber-toothed cats of earlier times. There were even populations of migrant bats who had learned to subsist on the vast swarms of insects that formed during the brief tundra spring.

None of these new species, of course, would ever bear a human name.

There was one key difference in this latest recovery of life, compared to its last great trauma after Chicxulub. The rodents had not evolved until some ten million years after the comet impact. This time, though, when the days of recovery came, the rodents were everywhere.

Rodents were formidable competitors. They were born with incisors ready to gnaw. These great teeth were deep-rooted in powerful jaws: Once rats had been able to gnaw through concrete. Their teeth enabled them to eat food hard and tough enough to be inaccessible to other mammals. But the rodents’ ability to proliferate and adapt was more fundamental. Rodents lived fast and bred young. Even among the giant species like the rat-cheetahs, females had short gestation times and produced large litters. Many of those kits would die, but every one of those dead babies was raw material for the relentless processes of adaptation and selection.

Given empty spaces to fill, the rodents evolved quickly. In the grand recovery after the disappearance of man, the rodents had been the big winners. By now, on land at least, Earth could be described as a kingdom of rats.

All this had left little room for the descendants of humans.

Crowded out by increasingly ferocious and confident rodents, the posthumans had given up the strategy — superior intelligence — that had brought them such success, and disaster. They had retreated, seeking sheltering niches and passive strategies. Some had become small, timid, fast-breeding runners. They were like vermin. Some even burrowed into the ground. Remembrance’s folk had returned to the ancestral trees, but now the rats were invading even that ancient shelter.

The elephantine humans had tried another approach: becoming so huge they were protected simply by their immense size. But this had not been entirely successful. You could tell that from the design of their gazellelike back legs. Elephants could not have run very quickly, but they had not needed to; in their day no predator existed that could have taken on a full-grown proboscidean. Facing the power of the rodent predator families, the elephantine posthumans had had to retain the power to flee.

But even this had not been enough.

The mouse-raptors were social creatures. Their sociability was deep rooted, reaching back to the colony structures of the marmots and prairie dogs, which had lived in hierarchical “towns” of millions of animals. They scouted, seeking prey or water. They kept sentry watch for each other. They hunted cooperatively. They communicated: The adults called to each other continually with cries, squeals, and the drummings of those powerful tails that sent long-range shudderings through the ground.

For the posthumans, the sociability of these raptors made them simply too effective as predators. The numbers of the big herbivores had steadily dwindled.

But that was bad for the raptors too. And so, in time, the elephantines and the mouse-raptors had developed a kind of symbiosis. The mouse-raptors learned to protect the herds of slow-witted elephantines. Their presence would deter other predators. By their behavior and signals they could warn the elephantines of other dangers, such as fires. They could even guide them to water and good grazing.

All the raptors asked for in return was to take their share of meat.

The elephantines passively accepted all this. They had no choice. And over enough time, selection had shaped the elephantines to fit the new conditions. If the raptors chased away the other predators for you, why be fast? And if they did your thinking for you, why be smart?

As their bodies had bulked up, the people’s minds had shriveled, casting off the burden of thought. They were like domesticated chickens, whose brains had been sacrificed to make longer guts and a more effective digestive system. It wasn’t so bad when you got used to it. Under the mouse-raptors’ unthinking guidance their numbers had even increased. It wasn’t so bad, so long as you turned away when your mother or your sister or your child was taken.

Not such a bad life, to be farmed by rodents.

The light began to leak out of the sky. So Remembrance found another stand of acacias, and crawled gingerly into the branches of the tallest tree. It would have to do. At least she was off the ground.

As the light died, so the stars appeared — but it was a crowded sky.

The sun, in its endless swimming around the Galaxy, was now passing through a wisp of interstellar dust and gas, a wisp mighty enough to span light years. Human astronomers had seen this coming. It was the vanguard of a mighty bubble blown in the gas by an ancient supernova explosion, and at its heart was a region where stars were being built. And so the new sky was spectacular, full of bright, hot new stars.

But there was nobody on Earth who might understand any of this. Remembrance spent a sleepless night listening to the squeals, thrums, and roars of predators, while unnamed constellations drifted over the sky.

III

The first few hundred asteroids the astronomers discovered had orbited in their orderly belt between Mars and Jupiter, comfortably far from Earth. These space rocks had been a curiosity, nothing but a theoretical challenge to students of the origins of the solar system.

It had been quite a shock when Eros was discovered.

Eros was found to sail within Mars’ orbit — in fact at its nearest to Earth, it came to within less than a quarter of the closest approach of Mars and Earth. Later, more asteroids were found that actually crossed the orbit of Earth, making them candidates for eventual collision with the planet.

Eros, that first rogue, was never forgotten. As long as people cared about such things, the asteroid became a kind of mute hero among its kind, better known than any other.

At the beginning of the twenty-first century Eros was the target of the first space probe to orbit an asteroid. The probe was called NEAR, for Near Earth Asteroid Rendezvous. At the end of the mission the probe was made to land gently on the asteroid’s ancient ground. Those first astronomers had given their asteroid the romantic name of the Greek god of love. There was much talk of how the probe NEAR had “kissed” the target rock, and the press had been predictably excited that the contact had occurred only a little before Valentine’s Day.

But under the circumstances the asteroid’s name could not have been more inappropriate.

It had long been believed that Eros, with its eccentric orbit taking it endlessly across the orbit of Mars, was in no danger of collision with the Earth. In fact, it seemed much more likely to collide with Mars itself.

But Mars was gone.

And, over long enough periods, as it responded to the subtle tweaks of the planets’ gravitational pulls, the spinning of the sun, and its own intricate, intrinsic dynamic instabilities, the orbit of the asteroid evolved. One million years after the demise of mankind, Eros had sailed close to Earth — very close, close enough to be visible to a naked eye, had anyone been looking.

Some twenty-nine million years after that, it was coming closer still.

Stuck in her acacia tree, Remembrance itched. She scrabbled at her fur, hunting for the ticks and bugs that loved to feast on your blood, or lay their irritating eggs under your skin. But there were places she couldn’t reach, like the small of her back, and naturally the bugs congregated there.

It was a painful reminder of how alone she was. As language had declined, the habit of grooming had returned to serve its old function of social cement. (It had never really gone away anyhow.) But Remembrance had had no grooming since before her last sleep, when she had huddled with her mother in her nest.

Hot, itchy, hungry, thirsty, lonely, Remembrance waited in her acacia stand until the sun had once more climbed high in the sky.

Then, at last, she clambered down.

The elephant people and their rodent keepers had gone. Across the empty, dust-strewn grassland, little stirred. The silence was as heavy as the heat. Through dusty haze, she could see a dark smudge to the east that might have been a herd of elephantine pigs or goats, or even hominids. To the west there was a little pocket of motion, a glimpse of brown fur. Perhaps it was a predatory rat with her kits.

To the north, where the mountains loomed purple, she could see that splash of dull greenery. She still had no other impulse than to make straight for the forest’s alluring comfort.

Naked, her hands empty, she set off across the plain, slumping every now and again to let her knuckles carry some of her weight. She was a tiny figure crossing a huge, bare landscape, accompanied by nothing but the shadow under her feet.

She found no water, nothing to eat save handfuls of sparse grass. As she lumbered on, she was increasingly distracted by thirst. The silence settled still more heavily. Soon it was as if there were nothing in her life but this walk, as if her memories of a life of green and family were as meaningless as her dreams of falling.

She found herself walking down a shallow slope into a broad bowl of land kilometers across. Before this great depression she hesitated.

A valley was incised across the heart of the bowl — a valley once cut by a river — but even from here she could see that the valley was dry. The vegetation was different from that in the plain beyond. There were no trees here, few shrubs, and only occasional splashes of grass green. Instead, there was a broad mass of rustling violet leaves.

To distrust anything new was a good rule of thumb. But this great bowl lay right across her path, cutting her off from her forested slope, still far away. She could see there were no animals here, no herbivores, no prowling predators.

So she set off, wary, watchful.

The belt of violet purple turned out to be flowers growing in thick clumps, some tall enough to reach her waist, amid spindly, pale blades of grass. She walked on until she was surrounded by the clamoring purple. But there was still no water.

Once there had been a city here. Even now, so long after the city’s fall, the soil was so polluted that only metal-tolerant plants could survive here — such as the violet-petaled copper flowers that waved over the soil.

Eventually the purple flowers grew thinner. At the very heart of this strange place she came to the river’s shallow bank. The channel was dry, filled only with drifting dust: Ancient geological shifts had long since diverted away the water that had cut this channel. Remembrance clambered down the eroded banks and tried digging into the dusty substrate, but there was no moisture to be had here either.

As she climbed out of the shallow bowl it wasn’t long before Remembrance came to another obstacle.

There were trees here — twisted, stubborn-looking trees — and termite mounds, and broad, low ant colonies, scattered like statues over an otherwise dry and lifeless plain. It was not a forest — it wasn’t crowded enough for that — it was more like an orchard, with the individual trees well spaced, surrounded by their little gardens of termite mounds and ant nests. These were borametz trees, the new kind. The orchard stirred deep, instinctive feelings of unease in Remembrance. Something inside her knew that this was not the kind of landscape within which hominids had evolved.

But this stark landscape of trees and termites was another barrier across her path, stretching to left and right as far as she could see. And, as the sun began its swift descent to the horizon, she was growing ever thirstier and hungrier.

Tentatively she walked forward.

Something tickled her foot. She yelped and jumped back.

She had disturbed a double line of ants. They were walking to and from a nest — she could see the holes in the ground — along a trail that led to the broad roots of one of the trees. She crouched down and began to swipe at the ants with her cupped palms. She scooped up more dust than insects, but she managed to cram a few of the ants into her mouth and crunched the gritty goodies. More ants clambered around her feet, intent on their task, oblivious to the sudden fate of their fellows.

The tree that was the destination of these ants was unspectacular: It was low and squat, with a thick, gnarled trunk, branches coated with small round leaves, and broad roots that spread across the ground before plunging into it like digging fingers.

Remembrance walked up and inspected the borametz tree skeptically. No fruit clung to its low branches. There were what looked like hard-shelled nuts growing in clusters from the base of the trunk, close to the roots of the tree. But there were very few of the nuts, less than a dozen. When she tried to prize them off, she found they were bound too strongly for her fingers, and the shells were too tough for her teeth. She pulled off a few leaves and chewed them experimentally. They were bitter and dry.

She gave up, dropping the last of the leaves, and made her way to a more promising food source. The nearest termite mound was as tall as she was, a great rough cone of hardened mud. She went back to the tree to look for a twig. She’d done a little termite-fishing in the past, though she was not as good as Capo had been. She was not even as expert as chimps had been in the age of man. But she might be able to get enough of the squirming goodies to allay her hunger -

She glimpsed a lunging head, incisors like blades scything through the air. Rat. She leapt upward, reaching for the branches of the borametz. The branches were narrow, tangled, and hard to grasp. But she forced her way into them, for they were all the cover she had.

It was a mouse-raptor: one of the colony that had herded the posthuman elephantines to the lake. Squealing its high-pitched rage the raptor reared up on its huge hind legs, slashed at the lower foliage with its blood-stained incisors, and rammed the borametz’s trunk with its massive skull.

Young, restless, inquisitive, the raptor had never hunted this kind of animal. Tracking Remembrance this far had been a good game. But now the raptor had played enough, and had become curious about how she would taste.

The borametz’s gnarled bark scraped Remembrance’s skin painfully. The raptor couldn’t reach up into the branches. But under the battering of its huge head the whole tree shook, and Remembrance knew she would soon fall, like a piece of fruit. Growing frantic, she squirmed through the branches, trying to get further away from the raptor.

But the branches of the borametz were fragile and easily snapped. They had evolved that way, to discourage birds, bats, and climbing mammals from trying to make a living here.

The branch under her belly gave way suddenly. She fell through the air and hit the ground, but the dirt collapsed under her in a cloud of dust.

Shocked, she fell through a further body-length, landing hard. Winded, she lay on her back. She looked up at a patch of sky and the head of the raptor, framed by a ragged, broken roof of packed earth.

And then the surface beneath her gave way in turn. She fell again, followed by dust and chunks of earth. She landed hard, once again, deeper still. Rubble fell across her face, clogging her mouth and nose and eyes.

There was a smell like milk: milk laced with urine and feces. Something swarmed over Remembrance’s belly — something small, but heavy and hot and hairless. She grabbed blindly. She found herself clutching a torso, naked, slithery, moist. Arms and legs beat at her feebly. It was like holding a hairless baby.

But now one of those little hands reached her chest, and claws sliced into her skin. She yelled and hurled the creature away. She heard it land with a thump, and slither away into the dark.

But they were all around her — she heard them in the dark, sliding and rustling, saw them in the indistinct light.

Mole people. That was how they seemed. They had loose, fleshy skin that hung in folds around their necks and bodies. They were hairless: Their heads were bald, their pink scalps wrinkled, and they lacked eyelashes and eyebrows. Their ears were small, vestigial; their noses had pulled forward into snouts. They even had whiskers. And they had no eyes: There were only layers of skin covering the sockets where their eyes had been.

They had the arms and legs and torsos and heads of people. But they were all small, none of them larger than a child among her own kind, and yet many of them were adults. She saw breasts and functional penises on those small bodies.

Blind or not, they were flinching from the light. They swarmed away, disappearing into tunnels cut into the ground. The nails of their hands were shovel-like claws, equipped for digging. One touch of those claws had left deep furrows in Remembrance’s shoulder.

She was in a nest, a nest of people who squirmed and burrowed. She screamed, driven by a deep horror of these distorted posthumans, a horror she couldn’t understand, and she reached up toward the light.

And found herself staring straight into the eyes of the mouse-raptor. It hissed and braced to leap.

She hurled herself into an empty tunnel.

The walls were packed hard and worn smooth by the passage of many, many squirming bodies, and she was immersed in the characteristic stink of milk and piss. The tunnels had been built by the mole folk to take their own slim, scrabbling little bodies, and they were too small for Remembrance. She had to crawl on her belly, dragging herself along with arms and legs that soon ached painfully. It was a nightmare of enclosure.

But there was light. Narrow chimneys snaked to the surface. Thin, angled, they were intended to allow the passage of air while excluding any predator. But enough light diffused down to give her partial impressions of what she was passing through.

Tunnels, branching everywhere, a whole network of them. She could hear echoing spaces beneath and around her, chambers and tunnels and alcoves branching away forever. She caught occasional glimpses of the mole folk — a scrabbling limb or retreating rump, or smoothed over eye sockets gazing blindly.

Fear and dread filled her mind. But she had no choice but to go on.

Without warning she fell through a thin wall, and tumbled through into a crowded chamber. Babies instantly swarmed over her, biting and scratching.

This large chamber was crowded with children, miniature versions of the adults she had first glimpsed. The place stank overwhelmingly of blood and shit and milk and vomit. Struggling, she pushed the babies away. Almost all of them were female. Their soft, hot little bodies were somehow even more repulsive than the adults’. She turned and tried to clamber back up to the tunnel from which she had fallen.

But now adults came tumbling out of the tunnel. These newcomers did not retreat, as had those she had first encountered. These mole folk were soldiers, come to protect the birthing chamber from the intruder.

The first of the soldiers leapt at her, its digging claws extended. Remembrance raised her arm to protect her throat. Under the mole creature’s soft weight she fell back into the wriggling heap of infants.

The soldier was an adult, a female. But her breasts were as tiny as a child’s, her pudenda undeveloped. She was sterile. Nevertheless, squirming, biting, and scratching, she fought as ferociously as if her own children were at risk.

Remembrance might have succumbed to the soldier’s assault, but she got in a lucky kick. The heel of her foot caught the soldier just below her breastbone. The little creature went flying back, colliding with those who were trying to follow her, so they dissolved into a wriggling mass of limbs and claws.

Dimly making out a tunnel mouth on the far side of the chamber, Remembrance hurled herself that way. She went on all fours, wading through mewling infants.

But still the soldiers pursued her. She struggled on through the tunnels, selecting branches at random. She could not tell if she were climbing upward or deeper into the ground. But for now nothing mattered but to flee.

She broke through another wall, fell, landed on something hard, like a heap of rocks. No, not rocks — they were nuts, big heavy nuts, the nuts of the borametz tree. Stumbling further, she found an immense heap of seed and roots. This huge chamber was crammed full of food.

Still the soldiers came, swarming, snuffling.

She leapt to the far side of the chamber and dug herself in against the wall, behind a pile of the heavy seeds. She picked up nuts and hurled them as hard as she could. She could hardly miss, and she was rewarded by the crack of the heavy shells on those eyeless heads. There was whimpering and confusion as the front line of the soldiers pushed back into those who followed, trying to get away from this missile-throwing demon.

But not all the soldiers retreated. Several stayed at the mouth of the tunnel, hissing and spitting at her.

Remembrance, exhausted and battered, really didn’t care. She couldn’t get out of here, but the soldiers couldn’t get to her either. She stopped hurling the nuts.

She smelled dampness. She found a place in the earth wall behind her where a thin tree root pushed through. She had broken the root, and now it was dripping a thin, watery sap. She clamped the root to her mouth and began to suck down the sap. It was sweet, and it trickled over her parched throat. She found some tubers under the nut pile. In the near dark, she bit into sweet flesh, sating her hunger.

She lay down over what was left of her stolen roots, with heavy nuts grasped against her chest. Soon the hissing of the impotent soldiers seemed no more disturbing than the noise of a distant rainstorm. Her energy drained, shocked, bewildered, she actually dozed.

But there was movement in the chamber, scrabbling, slithering. Reluctantly she poked her head above the barrier of nuts. She saw mole folk moving around the chamber, but these were not soldiers. They seemed to have forgotten she was here. They were picking up nuts and passing them out of the chamber, into the tunnel entrance. She had no idea what they were doing. She didn’t have the intellectual capacity even to formulate the question. All that mattered was that they were no threat to her.

She slumped back into her improvised nest and, nibbling on a bit of root, fell asleep.

The mole folk’s underground way of life had started as a response to the aridity of this place — that and the usual ferocious predation. Even the rats couldn’t get you if you burrowed in the ground.

Of course there had been prices to be paid. People had shrunk, generation by generation, the better to fit into the growing complexes of burrows. And over time bodies had been shaped by the restrictions of tunnel life: useless eyes were lost, nails became digging claws, body hair evaporated save for vibrissae, whiskers, which sprouted from lengthening muzzles, the better to help them feel their way in the dark.

The aridity had also promoted cooperation.

The mole folk lived off roots and tubers, riches buried in the ground. But in the dryness the tubers grew large and widely spaced. It was better for the plants that way, because big tubers did not desiccate so easily. A solo mole person, however, burrowing away at random, was likely to starve long before stumbling across the scattered bounty. But if you were prepared to share what you found, then having many colony members digging in all directions brought a more likely chance of success for the group as a whole.

All posthumans were social, like their ancestors, but they specialized in the way they had developed that sociality. These mole folk had taken sociality about as far as you could go. They came to live like social insects, like ants or bees or termites. Or perhaps they were like naked mole rats, the peculiar hive-dwelling rodents that had once infested Somalia, Kenya, and Ethiopia, now long extinct.

This was a hive. There was no conscious mind at work here in the hive. But then consciousness wasn’t necessary. The hive’s global organization emerged from the sum of the interactions of its members.

Most of the inhabitants of the colony were female, but only a few of those females were fertile. These “queens” had produced the infants Remembrance had stumbled upon in the birthing chamber. The rest of the females were sterile — indeed they never entered puberty — and their lives were devoted to the care not of their own children, but of those of their sisters and cousins.

For the genes it made sense, of course. Otherwise it would not have happened. The colony was one vast family, bound together by inbreeding. By ensuring the preservation of the colony, you could ensure that your genetic legacy was transmitted to the future, even if not directly through your own offspring. In fact, if you were sterile, that was the only way you could pass on your genes.

More sacrifices. As the bodies of these colony people had shriveled, so had their brains. You didn’t need a brain. The hive would take care of you — rather as the mouse-raptors took care of the elephant folk they farmed. There were better things to be done with your body’s energy than fuel an unnecessary brain.

And, with time, the mole folk were even giving up that most precious of all mammalian inheritances: hot-bloodedness itself. As they rarely ventured out of their burrows, the mole folk did not need such expensive metabolic machinery — and a cold-blooded scout cost less food than a hot-blooded one. It was done without sentiment. With time, the colony folk would grow smaller yet, smaller than any hot-blooded mammal’s design could maintain. In another few million years these mole folk would swarm like tiny lizards, competing with the reptiles and amphibians who had always inhabited the microecology.

So the mole folk scuttled through their spit-walled corridors, their whiskers twitching, fearful and ignorant. But in their dreams their residual eyes, covered by flesh, would flicker and dart as they dreamed strange dreams of open plains, and running, running.

She lost track of time. Suspended in the suffocating heat of the chamber, she slept, ate roots and tubers, sucked water from the tree roots. The mole folk left her alone. She was in there for days, not thinking, with no impulse to act save to eat, piss, shit, sleep.

At last, though, something disturbed her. She woke, looked up drowsily.

In the dim, diffuse light, she saw that mole folk were clambering into the chamber, and out again through a narrow passageway in the roof. They moved in a jostling column, the flaccid skin on their pale bodies crumpling as they pressed against each other, their whiskers twitching, clawed hands scrabbling.

Though the mouse-raptor and other dangers lingered at the back of her mind, Remembrance found herself longing for openness — for a glimpse of day, for fresh air, for green.

She waited until the mole folk had passed. Then she clambered over the low heapings of roots and pushed her way into the narrow breach in the roof.

It was a kind of chimney that led up toward a crack of purple-black sky. The sight of the sky drove her on, and she wedged her body ever more tightly into the narrow, irregular chimney, scrabbling at the dirt with her hands and feet, knees and elbows, forcing her chest and hips through gaps that seemed far too small for them.

At last her head broke above ground level. She took in great gulps of fresh air and immediately felt invigorated. But the air was cold. The twisted forms of the borametz trees occluded a star-laden sky. It was night, the most natural time for the mole folk to venture to the surface. She forced her arms out of the hole, got her hands onto the surface, and with a tree-climber’s strength she pushed herself upward, prizing her body out of the chimney like a cork from a bottle.

The mole folk were everywhere, running on hind legs and knuckles, snuffling, shuffling, and squirming. But their movement was orderly. They moved in great columns that wound through the termite heaps and ant nests, to and from the borametz trees. They were picking off the nuts that grew in clusters at the roots of the trees, nuts that were sometimes as large as their heads. But they did not seem to be trying to break them open, to get at their flesh. They weren’t even taking them into their underground stores. In fact, she saw now, they were actually bringing nuts up from the underground chambers.

They were taking the nuts, one at a time, out to the fringe of the borametz grove. There workers dug into the dirt, scattering the thin grass to make little pits into which the nuts were dropped and buried.

Each borametz was the center of a symbiotic community of insects and animals.

Symbioses between plants and other organisms were very ancient: The flowering plants and the social insects had actually evolved in tandem, one serving the needs of the other. And it was the social insects, the ants and termites, who had been the first to be co-opted by the new tree species’ reproductive strategies.

Every symbiosis was a kind of bargain. Attendants, insect or mammal, would remove the borametz trees’ seeds from their root bases, but they would not devour them. They would store them. And when conditions were right they would transport them to a place suitable for planting, usually at the fringe of an existing grove, where there would be little competition with established trees or grasses. And so the grove would grow. In return for their labors the attendants were rewarded with water: water brought up even in the most arid areas from deep water tables by the borametz’s exceptionally deep-growing roots.

It had not been hard for the mole folk, with their cooperative society and still-agile primate hands and brains, to learn how to emulate the termites and the ants and begin to tend the borametz trees themselves. Indeed with their greater sizes, they were able to move larger weights than the insects, and the development of new borametz species with large seed cases had resulted.

For the borametz it was a question of efficiency. The borametz had to expend much less energy on each successful seedling than its competitors. And so it was a reproductive strategy that enabled the borametz to flourish where other tree species could not. Little by little, as their attendants carried their seeds from their orchards into the meadows, the borametz species were moving out into the grasslands. At last, more than fifty million years after the triumph of the grasses, the trees were finding a way to fight back.

The borametz trees embodied the first great vegetable revolution since the flowering plants that had arisen in the days before Chicxulub. And in the ages to come — like the initial emergence of plants on land that had enabled animals to leave the sea, like the evolution of the flowering plants, like the rise of the grasses — this new vegetable archetype would have a profound impact on all forms of life.

As she sat on the ground, still panting, watching the mole folks’ baffling behavior, Remembrance heard a familiar soft footstep, an awful hissing breath. She turned her head, slowly, trying to be invisible.

It was the mouse-raptor — the juvenile, the same one that had strayed from its herd of elephant folk to chase her here. It was standing over a line of mole folk who scurried back and forth from tree to planting ground, oblivious to the threat that loomed over them.

It was as if the raptor were taking a small revenge. Few rodents could get through the mighty shells of the borametz nuts. As the borametz spread, the seed-eating stock from which this raptor had sprung — along with birds and other species — would soon be threatened with dwindling food supplies, dwindling ranges — and, in some cases, extinction.

The raptor made its choice. It bent down, balancing with its long tail, and used its delicate front claws to scoop up a bewildered mole woman. The raptor turned her over and stroked her soft belly, almost tenderly.

The mole woman struggled feebly, cut off from the colony for the first time in her life, divorced from its subtle social pressures. It was as if she had suddenly surfaced from an ocean of blood and milk, and she was truly terrified, for the first and last time. Then the raptor’s head descended.

Her companions hurried on past the feet of her killer, their flow barely disturbed.

The mouse-raptor turned, its small ears twitching. And it stared straight at Remembrance.

Without hesitation she plunged straight back into her hole in the ground.

Remembrance stayed in the food chamber for several more days. But she was no longer able to settle back into the exhausted fog that had enveloped her.

In the end it was the madness of the mole folk that drove her out.

Even for this arid area, the season had been dry. The mole folk were having increasing difficulty in finding the roots and tubers on which they relied. The stock in the chamber dwindled steadily, and started to be replaced by other vegetation, like the violet leaves of copper flowers. But this unwelcome diet contained toxic elements. Gradually the poisons built up in the bloodstreams of the mole folk.

At last, everything fell apart.

Again Remembrance was startled awake by a rush of mole folk through the nearly empty food store. But this time they did not move in their orderly columns out through the vents. Instead they swarmed madly, surging up and out of the chamber, shattering its roof in their eagerness to be on the surface.

Remembrance, keeping out of the way of blindly scrabbling claws, followed gingerly. She emerged, this time, into full daylight.

All around her the mole folk swarmed. There were many, many of them, running over the ground, a carpet of squirming bare flesh. The air was full of their milky stink, the scraping of their skins against one another. There were far more than could have come from her own colony: Many hives had emptied as a burst of madness swept through the poisoned, half-intoxicated population.

Already the predators were showing interest. Remembrance saw the stealthy form of a rat-cheetah and a pack of doglike postmice, while overhead birds of prey began their descent. For those who sought flesh this was a miracle, as these little packets of meat just bubbled out of the ground.

It was all a response to the shortage of food. The mole folks’ overcrowded burrows had emptied as they swarmed everywhere in a mindless search for provision. But in their intoxicated state they were unable to keep themselves from danger. Many of this horde would die today, most in the mouths of predators. In the long run it did not matter to the hives. Each colony would retain enough breeding stock to survive. And it wasn’t necessarily a bad thing for their numbers to be reduced in these times of semidrought. Mole folk reproduced quickly, and as soon as the food supply picked up, the empty burrows and chambers would be full again.

The genes would go on: That was all that mattered. Even this periodic madness was part of the grander design. But many small minds would be extinguished today.

As the predators started to feed — as the air filled with the crunch of bone and gristle, the squeals of the dying, the stink of blood — Remembrance slipped away from this place of madness and death, and resumed her long-broken journey toward the distant purple hills.

IV

Remembrance came at last to a great bay, a place where the ocean pushed into the land.

She clambered down exposed sandstone bluffs. Once this area had been under the sea, and sediment had been laid down over millions of years. Now the land had been uplifted, and rivers and streams had cut great gouges in the exposed seabed, revealing deep, dense strata — in some of which, sandwiched between thick layers of sandstone, were embedded traces of shipwrecks and debris from vanished cities.

At last Remembrance reached the beach itself. She scampered along its upper fringe, sticking to the shade of the rocks and scrub grass. The sand was sharp under her feet and knuckles, and got into her fur. This was a young beach, and the sand was still full of jagged edges, too new to have been eroded smooth.

She came to a freshwater stream that trickled down from the rocks toward the beach. Where the water decanted onto the sand, a small stand of trees clung to life. She ducked down and pushed her mouth into the cool water, sucking up great mouthfuls. Then she clambered into the stream itself and scraped the water through her fur, trying to get rid of the sand and fleas and ticks.

That done, she crawled into the shade of the trees. There was no fruit here, but the leaf-strewn floor, cold and damp, harbored many toiling insects that she popped into her mouth.

Before her the sea lapped softly, the water bright in the high sunlight. The sea meant nothing to her, but its distant glimmer had always attracted her, and it was oddly pleasing for her to be here.

In fact the sea had been the savior of her kind.

Torn by great tectonic forces, Africa’s Rift Valley had eventually become a true rift in the fabric of the continent. The sea had invaded, and the whole of eastern Africa had sheared off the mainland and sailed away into what had been the Indian Ocean, there to begin its own destiny. So chthonically slow was this immense process that the mayfly creatures living on this new island had scarcely noticed it happening. And yet, for Remembrance’s kind, it had been crucial.

After the fall of mankind, there had been pockets of survivors left all over the planet. Almost everywhere the competition with the rodents had been too fierce. Only here, on this rifted fragment of Africa, had an accident of geology saved the posthumans, giving them time to find ways to survive the rodents’ ruthless competitive onslaught.

Once this place, East Africa, had been the cradle that had shaped mankind. Now it was the final refuge of man’s last children.

There was something in the water. Cautiously, Remembrance cowered back into the shade.

It was a great black shape, sleek and powerful, swimming purposefully. It seemed to roll, and a fin a little like a bird’s wing was raised into the air. Remembrance made out a bulbous head lifting above the water, with a broad sievelike beak. Water showered from two nostrils set in the top of the beak, sparkling in the air, expelled with a sharp whooshing noise. Then the great body flexed and dove back under the surface. She caught a last glimpse of a tail, and then the creature had vanished. Despite its immense bulk, it left scarcely a ripple in the water.

In this giant’s wake more slim, powerful bodies leapt from the water, three, four, five of them. They swept through graceful arcs and plunged back into the sea, and then rose to leap again and again. Their bodies were shaped like those of fish, but these dolphinlike creatures were evidently not fish. They were equipped with beaks like birds, stretched into long orange pincers.

Behind the “dolphins,” in turn, came more followers, likewise hopping and buzzing over the ocean surface. Much smaller, these were true fish. Their wet scales glistened, and fins like wings fluttered at the sides of their slim, golden bodies as they made their short, jerky flights over the water.

The “whale” was not a true whale, the “dolphins” not dolphins. Those great marine mammals had preceded humanity into extinction. These creatures were descended from birds: In fact, from the cormorants of the Galapagos Islands in the Pacific, which, blown there from mainland South America by contrary winds, had given up flight and taken to exploiting the sea. Their descendants’ wings had become fins, their feet flukes, their beaks a variety of specialized instruments — snappers, strainers — for extracting food from the ocean. Some of the species of “dolphin” had even regrown the teeth of their ancient reptilian ancestors: The genetic design for teeth had lain dormant in birds’ genomes for two hundred million years, waiting to be re-expressed when required.

Invisibly slow on any human timescale, adaptation and selection were nevertheless capable, given thirty million years, of turning a cormorant into a whale, a dolphin, or a seal.

And, strangely enough, all the swimming birds Remembrance saw were indirect legacies of Joan Useb.

As Remembrance watched, a dolphinlike creature erupted out of the water right in the middle of the cloud of flying fish. The fish scattered, their fin-wings buzzing, but the beak of the “dolphin” snapped closed on one, two, three of them before its sleek body fell back into the water.

The sun was starting its long descent toward the sea. Remembrance stood up, brushed herself free of sand, and resumed her cautious knuckle-walk along the fringe of the beach, but something overhead distracted her. She glanced up at the sky, fearing it was another bird of prey. It was a light like a star, but the sky was still too bright for stars. As she watched, it slid over the roof of the sky.

The light in the sky was Eros.

NEAR, the humble, long-dead probe, had spent thirty million years swimming with its asteroid host through the spaces beyond Mars. Its exposed parts were heavily eroded, metal walls reduced to paper thinness by endless microscopic impacts. At the touch of a gloved astronaut’s hand it would have crumbled like a sculpture of dust.

But NEAR had survived this far, among the last of all of mankind’s artifacts. If Eros had kept up its eccentric dance around the sun, perhaps NEAR could have survived longer yet. But it was not going to get that chance.

The asteroid’s passage through the atmosphere would be mercifully swift. The fragile probe, returning to the planet where it had been made, would flash to vapor only fractions of a second before the great body with which it had long ago rendezvoused was itself destroyed.

Earth’s evolutionary laboratories had been stirred many times by monstrous interventions from without. Now, here was another stirring. And over the bright scene on which Remembrance gazed, a curtain would soon be drawn.

Remembrance herself would survive, as would the children she would bear in the future. Once again the great work would begin: Once again the processes of variation and selection would sculpt the descendants of the survivors to fill shattered ecological systems.

But life was not infinitely adaptable.

On Remembrance’s Earth, among the new species there were many novelties. And yet they were all variations on ancient themes. All the new animals were built on the ancient tetrapod body plan, inherited from the first wheezing fish to have crawled out of the mud. And as creatures with backbones, they were all part of a single phylum — a great empire of life.

The first great triumph of multicellular life had been the so-called Cambrian explosion, some five hundred million years before the time of mankind. In a burst of genetic innovation, as many as a hundred phyla had been created: each phylum a significant group of species representing a major design of body plans. All backboned creatures were part of the phylum of chordates. The arthropods, the most populous of the phyla, included creatures like insects, centipedes, millipedes, spiders, and crabs. And so on. Thirty phyla had survived life’s first great shaking down.

Since then species had risen and fallen, and life had suffered major disasters and recoveries over and over again. But not one new phylum had emerged, not one, not even after the Pangaean extinction event, the greatest emptying of all. Even by the time of that ancient event, life’s capability for innovation was much constrained.

The stuff of life was plastic, the mindless processes of variation and selection inventive. But not infinitely so. And with time, less.

It was a question of the DNA. As time had worn away, the molecular software that controlled the development of creatures had itself evolved, becoming tighter, more robust, more controlled. It was as if each genome had been redrafted over and over, each time junk and defects were combed out, each time the coherence of the whole was improved — but each time the possibility for major change was reduced. Extraordinarily ancient, made conservative by the inward-looking complexity of the genomes themselves, life was no longer capable of a great innovation. Even DNA had grown old.

This epochal failure to innovate was an opportunity lost. And life could not take many more hammer blows.

The light in the sky was strange. But, Remembrance’s instinctive calculus quickly computed, it was no threat. In this she was wrong. Purga, who had watched the Devil’s Tail similarly slide silently overhead, might have told her that.

Before the sun had touched the horizon she at last reached her forest in the lee of the volcanic hills, her target for many days. Remembrance peered up at the tall trees before her, the canopy that strained up toward the sky. She thought she saw slim shapes climbing there, and perhaps those dull clots of darkness were nests.

They were not her people. But they were people, and perhaps they would be like her.

She pulled herself off the ground and clambered upward into the comforting green of the canopy.

Something fluttered past her head. It was a flying fish, coming from the sea. As she watched, it sailed into the forest canopy, flapping its fin-wings earnestly, and settled clumsily onto a nest, air wheezing into primitive lungs.

I

Ultimate dug listlessly in the dirt, hoping to find a scorpion or beetle. She was a mound of orange-colored fur on the rust-tinged ground.

This was a flat, dry plain of crimson red rock and sand. It was as if the land had been scraped bare by some vast blade, and the bedrock wind-burnished to a copper sheen. Once there had been mountains to the west, purple-gray cones bringing relief to eyes wearied by flatness. But long ago the wind had torn all the mountains down, leaving great fans of scattered rocks over the plains, rocks that had themselves eroded to dust, leaving no trace.

Half a billion years after the death of the last true human, a new supercontinent had assembled itself. Dominated by desert, as red as the ancient heart of Australia, it was like a vast shield fixed to the blue face of the Earth. On this New Pangaea, there were no barriers, no lakes or mountain ranges. Nowadays it didn’t matter where you went, from pole to equator, from east to west. Everywhere was the same. And there was dust everywhere. Even the air was full of red dust, suspended there by the habitual sandstorms, making the sky a butterscotch-colored dome. It was more like Mars than Earth.

But the sun was a ferocious disk, pumping out heat and light, much brighter than in the past. Any human observer would have cowered from that great fire in the sky.

Under that tremendous glare the heat lay heavy on the land, by day and by night. There was no sound save for the wind and the scratching of the few living things, no sense that things had ever been different on this red planet. The land felt empty, a huge place of resonant silence, a stage from which the actors had departed.

As it happened, far beneath the dust where Ultimate dug — buried under half a billion years of deposits, under the salt and the sandstone of New Pangaea — was the place that had been known as Montana. Ultimate was not far above Hell Creek, where the bones of Joan Useb’s mother had at last joined those of dinosaurs and archaic mammals in the strata she had searched so assiduously.

Ultimate had no way of knowing her peculiar place in history, still less of understanding. But she was among the last of her kind.

Ultimate went home. Home was a pit carved in the harder rock. It offered some shelter from the wind. This was where Ultimate and her kind eked out their lives.

The pit looked artificial. Its floor was smooth, its terraced walls steep. The pit was in fact a quarry, made half a billion years earlier by human beings, dug deep into the bedrock. Even after all this time, even as mountains had come and gone, the quarry had survived almost intact, a mute memorial to the workings of man.

Trees grew on the floor of the pit, standing stately and alone, like sentinels, with their satellite termite colonies towering all around. They were stubby, ugly trees with perennial needlelike leaves, defiant of time. Little else lived here save the people, and other symbiotes of the trees, and many, many tiny creatures that toiled in the dust.

As Ultimate clambered down the pit’s walls, the wind changed and began to blow from the west, from the direction of the inland ocean. Gradually the humidity rose. At last, over the ruined mountains to the west, heavy black clouds began to gather.

Ultimate peered into the western sky. It had never rained here, in Ultimate’s lifetime. Most clouds coming from the distant ocean dumped their rainfall long before they reached a place like this, deep in the supercontinent’s interior. It took a mighty storm indeed to breach those immense defenses of arid plain, a once-in-a-lifetime monster. But that was what was approaching now. You could feel it in the air, feel that something was wrong.

The people hurried back to their Tree, and clambered into its welcoming branches. Hurried, yes — but still they moved with a languid slowness, as if they were swimming through the air’s dense heat.

At ten years old, Ultimate looked something like a small monkey. She was long-limbed, with a narrow torso, narrow shoulders: Even now, in these distant descendants of mankind, the basic body plan of the primates persisted. Her slim body was coated in thick fur, bright red, red as the sand. She had a small head with a large brow and a mobile, expressive face — a very human face, in fact. Small flaps of skin, rather like eyelids, could cover her ears, nose, anus, vagina to trap precious moisture. Her brow was swollen, almost as if her kind had re-evolved the big forebrains of the human age, but behind that brow there was only spongy bone, a great system of sinuses that worked as a refrigeration system to keep her brain cool.

And, though she was fully grown, her body was childlike. Ultimate was functionally female — people still gave birth — but there were no males any more, and gender was meaningless. She had no breasts, not even vestigial nipples. Nowadays there was no need for mother’s milk, just as there was no need for the elaborate superstructure of a large brain. The Tree took care of all of that for you.

And she was not bipedal. That was obvious as she made her way back to the Tree: her arms and legs were made for swinging and climbing, her feet for grasping, not walking upright. That particular locomotive experiment had been thoroughly buried long ago. Compared to her ancestors, she was slow-moving, lethargic, like all her kind.

At the Tree, Ultimate looked for her daughter.

The infant’s leafy cocoon nestled in the crook of a low branch. Threads of orange hair littered over her swollen brow, the little girl was safely enfolded in soft white down. As the Tree’s sap passed along the pale thread of the belly-root that wormed into her stomach, the child stirred and murmured, her tiny thumb clamped firmly in her mouth, dreaming vegetable dreams.

Something was wrong. Ultimate was not capable of much in the way of analysis, but her instinct was unmistakable. She prodded at the tangled red fur on the child’s little belly, and smoothed out the fluffy cottonlike lining of the cocoon. The little girl mewled, turning blindly in her sleep. Nothing Ultimate did made that feeling of wrongness go away. Uncertain, she patted the walls of the cocoon back into place.

The wind rose, like a great breath.

Ultimate clambered higher into the Tree’s welcoming branches. Hastily she pulled her own cocoon into place around her body, sealing up the leaves. The leaves were thick and tough, like plates of leathery armor. The others were doing the same, people huddling on the branches, so that it looked as if the Tree were suddenly sprouting huge black fruits.

The clouds streamed overhead, blotting out the intense heat of a too hot sun. Ultimate stared. Curiosity wasn’t much use now, when there was so little difference in the world across great stretches of time and space. But today was different. She had never felt air as moist and heavy and oppressive as this, never seen black clouds that boiled and bubbled like that.

And in the last moment before the storm hit, she glimpsed something new.

Settled on the timeworn plain, it was a sphere. It was twice as tall as she was. It was not blue like the evening sky, nor rust red like the ground, nor the color of sand and dirt like most of the creatures in the world. Instead, it was a shimmering mixture of purple and black, the colors of the night.

On this day of strangeness, here was something extraordinary. She gaped, unable to comprehend. But she sensed that this new thing was not of her world. In that she was right.

But now lightning cracked, and she buried her face in the green, mewling. The leaves closed around her, sealing themselves up seamlessly. In the warm darkness the air grew moist and comforting. But when the belly-root came probing for the valvelike orifice on her stomach, just below her navel, she pushed it away. She was here for shelter; she had nothing to give the Tree today.

And then the storm hit.

Wind and dust came out of the west like a red wall. Dried plants were shattered. Even the scattered, stately Trees were shaken, branches ripped away. People and other symbiotes were wrenched from their cocoons, utterly terrified.

The first few raindrops, landing like bullets, heralded an immense downpour. The rain was so heavy it even began to erode the rock-hard surfaces of the ancient termite mounds. There was nothing to absorb the water, no grass to consolidate the loose soil. Within minutes water was running down every dried-out gully and streambed. A great muddy wave came cascading into the quarry. The water seethed around the roots of the trees, turbulent, tinged red by mud.

But the rain dissipated as quickly as it had begun. The clouds cleared, racing deeper into the heart of the supercontinent. The flood quickly subsided, sinking into the parched sand.

There hadn’t been such a storm since Ultimate’s mother had first opened her eyes. Nothing in Ultimate’s experience had prepared her for such a catastrophic downpour. But the Tree, in its slow vegetable way, understood.

Even as Ultimate cowered, shocked, in her cocoon, she felt the leathery skin pulse around her. She longed to stay here in the moist dark rather than face whatever lay beyond these enclosing walls. But she was made to feel uneasy, restless. The Tree wanted her to leave, to go to work.

She set her back against the cocoon wall and pushed. The leaves came free of one another with a moist, sucking noise. She tumbled out of the Tree, and landed in mud.

All around her people were falling out of the Tree. They took experimental steps and knuckle-walks. The mud felt strange: It was heavy, clinging, crimson stuff that stuck to their legs and feet and hands.

The ferocious sun was shining once more, and the mud was already drying, the water escaping into the air, the ground baking hard. But for these rare minutes the ground was a cacophonous swarm of noise and motion. With visible speed, tendrils, leaves, and even flowers were pushing out of the mud. They had come from seeds that had lain dormant for a century. Soon sacs began to pop. Like tiny artillery pieces, they shot new seeds through the air. Entire reproductive cycles were being completed in minutes.

Insects emerged from their own encysted hiding places to dance and mate over the transient pools. On the ground there were more insects — ants, scorpions, cockroaches, beetles, and their much morphed descendant species. Many of the ants were leaf eaters, and Ultimate could see great chains of them trooping back and forth from the burgeoning plants bearing bits of greenery for their nests.

And there were many, many small lizards. They were hard to see, so well did their reddish skin match the color of the ground. Everywhere they hunted. Some of them had no smarter strategy than to sit with their mouths open by the ant columns, waiting for clumsy insects to stumble in.

One small, sturdy cactuslike plant, a ball of leathery skin and defensive spikes, dragged its upper roots from the soil, abandoning a deep, extensive root system. On roots that quivered like clumsy legs, it tottered toward the still-running water. When it got there, the walking plant subsided into the mud, as if with a sigh. Immediately the inefficient vegetable muscles that had powered its short journey began to dissolve, and new roots began to work their way into the moist ground.

All over the pit people were feeding on the sudden plants, reptiles, amphibians, insects. They were mostly adults: Children were rare in these straitened times; the Tree saw to that.

Ultimate, a rainstorm virgin, stared at all this, gaping.

A froglike creature erupted from the ground. It hopped and stumbled to the nearest of the temporary ponds, where it leapt into the water and began to croak noisily, guiding the emerging females who followed to it. Soon the pond was a splashing frenzy of amphibian mating. Ultimate grabbed one of the frogs. It was like a slimy sac of water. She popped it into her mouth. Briefly she felt its coldness, its heart hammering against her tongue, as if in disappointment that its century-long wait in a cocoon of hardened mud was going to end in such ignominy. Then she bit down, and delicious water and salty blood gushed into her mouth.

But already the pools were drying, the water hissing into the parched earth. The frogs’ spawn had hatched, and tadpoles, fast metamorphosing, were feeding on algae, tiny shrimps, and each other. They swarmed out of the water after their parents — and were snapped up by a mass of tiny lizards in a quivering feeding frenzy. But already the young frogs were digging their way into the mud, constructing for themselves mucus-lined chambers in which they would wait out the decades until the next storm, their skins hardened, their shriveled metabolisms slowing into suspended animation.

People were stumbling away from the feeding grounds now. Some were carrying the heavy seeds of the Tree, huge pods as large as their own heads. Like the frogs, this strange day was the Tree’s once-a-century opportunity to have the seeds of its next generation buried for it by its armies of symbiotes.

Ultimate saw Cactus chasing a small, scuttling lizard with a plump tail full of stored fat.

Cactus had been born about the same time as Ultimate, and as they had grown up they had learned about the world together, sharing, competing, fighting. Cactus was small and round. This was unusual for her people, who were generally skinny and long limbed, the better to lose their body’s heat — and she was prickly tempered, indeed like a cactus. Cactus was a kind of companion, even a sister, but she wasn’t Ultimate’s friend. You had to be able to see somebody else’s point of view to call them a friend, and that ability had long been given up. People didn’t have friends these days — no friends apart from the Tree.

Ultimate wanted to follow Cactus, but she was distracted. Suddenly she longed for salt. That was the Tree’s message to her, imprinted in the organic chemistry it had fed her while in the cocoon. The Tree needed salt. And it was up to her to find it. She remembered where a salt bed was, a few hundred meters away. She was helplessly drawn that way.

But in that direction stood the sphere, that enigmatic ball of black and purple that lowered silently over the teeming landscape.

She hesitated, caught between conflicting impulses. She knew the sphere was wrong. The great tide of human intelligence had long withdrawn, but the people had retained a good understanding of the land, its geography, and resources: efficient foraging was an essential skill if you were to find food and water in this desperately arid landscape. So she understood very well that the sphere shouldn’t be here. But that was the way to the salt.

Despite her unease, she set off.

The salt lick was almost at the foot of the sphere. She saw how mud had lapped up against its oddly gleaming surface. She tried to ignore the sphere, and began to scrabble in the sticky dirt.

There was no shortage of salt. A hundred million years ago, as the continents had danced toward their spontaneous assembly of this New Pangaea, a great inland sea had formed over much of North America. It had become landlocked, leaving only scattered lakes of brine. But that vanishing sea had left behind a vast bed of salt deposits, a shining plain that had stretched for hundreds of kilometers. The salt bed had been covered by debris washed down from the ruins of the fast-eroding mountains, and now lay buried under meters of rust-red sand, but it was still there.

Before long she had made a hole as deep as her arm could reach, and she was bringing up handfuls of dirt laced with gray-white salt. She chewed on the dirt, letting the salt crystals melt in her mouth, and spitting out the sand. With the salt in her belly, stored for later transmission to the Tree, Ultimate was released from her compulsion.

And again she became aware of the sphere. It had moved from where she had first seen it. And it hovered above the ground; a finger’s-width of light could be seen beneath it.

She approached the sphere, walking on her hind feet and her knuckles, a dim curiosity alight in her eyes. Her fear wasn’t strong. There were few novelties in her desert world. But likewise there were few threats. In a landscape like a tabletop, predators had a difficult time sneaking up on even the slowest and dullest of victims.

With a tentative fingertip she stroked the sphere’s surface. It was neither warm nor cold. It was smooth, smoother than anything she had felt before. The hairs on her hand prickled, as if charged. And she could smell something, a smell like the quintessence of the desert itself, an electric smell of scorching, of burning, of dryness.

The burnt-metal smell was in fact the result of exposure to hard vacuum: a legacy of space.

Their foraging done, one by one the people returned to the Tree, climbed into its branches, and folded themselves securely inside its leaves.

Ultimate pulled leathery leaves around her body. The belly-root snaked out quickly, probing for the valve on her stomach, and nestled into her like a reattached umbilical. As her salt-laden fluids began to circulate into the Tree, so Ultimate was rewarded by a soothing sense of security, of peace, of lightness. This mood was induced by chemicals leaked into her body as she exchanged blood for Tree sap, but it was no less comforting for that. This was her immediate reward for feeding the Tree, just as her longer-term reward was life itself. The Tree did not take without giving. Posthuman and Tree were neither of them parasites on the other. This was a true symbiosis.

But there was something wrong. Ultimate felt uneasy, wordlessly disturbed.

Even though the warm sap filled her head with green sleepiness, she kept thinking of how the child had been lying in her cocoon, her thumb in her mouth, the belly-root curled before her. Something had been wrong. Every instinct told her so.

The sap pulsed harder into her gut, and soporific chemicals washed through her. This drastic injection meant the Tree wanted her to stay here, where she was, safe in her cocoon. But still that nagging sense of wrongness pulled at her.

She pulled the belly-root out of her stomach, and pushed hard with her shoulder and legs. The cocoon popped open, and she tumbled to the ground.

Briefly she was overwhelmed by light and warmth. Though the day was still bright the sun was low. Inside the cocoon, time swam at a different pace from the world outside — a pace chosen by the Tree. But the ground was hard and dust-strewn. Save for a few raindrop stipples, it was as if the storm had never been.

Nobody was around. All the cocoons were closed — all but one. Cactus was gazing down at her, her small head protruding from her own half-sealed cocoon. With a look of playfulness, Cactus pushed her way out of her enclosing leaves and tumbled easily to the ground beside Ultimate.

Ultimate’s sense of anxiety was still growing.

She hurried around the base of the Tree to find her baby’s cocoon in the crook of the low branch. But it was sealed tight, and would not yield when she tried to open it. As if this were a game, Cactus joined her. The two of them dug their fingers into the seams between the sealed-up leaves, straining and pushing and grunting.

Once it would have occurred to a person to use a tool to open this pod. Not anymore. Toolmaking was gone, all the artifacts of man had long since rotted away save for a few pithecine nodules buried in lost strata. And Ultimate and Cactus weren’t even very good at solving unusual problems, for in their flat world they encountered few novelties.

At last, however, the cocoon opened with a pop.

Here was Ultimate’s baby, still swaddled in the white cottonlike material of the cocoon’s interior. But, Ultimate saw immediately, the cottony stuff had grown thicker. It had closed around the baby’s face, and tendrils of it were pushing into her mouth, nose, eyes, and ears.

Cactus flinched, an expression of revulsion on her face.

Both of them knew what this meant. They had seen it before. The Tree was killing Ultimate’s baby.

A new Pangaea.

A hundred million years after Remembrance had gone to her unmarked grave, the Americas had begun to slide east once more. As the Atlantic closed, so Africa drifted north of the equator, in the process pushing Eurasia further north still. Meanwhile Antarctica sailed north to collide with Australia, and that new assemblage began to push into east Eurasia. So a new supercontinent had been born. Africa was the central plain of the new assemblage, with the Americas pressing to the west, Eurasia to the north, Australia and Antarctica to the east and south. In the interior, far from the mediating effect of oceans, severe conditions took hold — ferociously hot and arid summers, killingly cold winters.

All barriers to movement had been eliminated. There was a brutal free-for-all as plants and animals migrated in all directions. It was a chilling parallel to the great global mixing that humans had forced during their few thousand years of dominance of the planet — and, just as it had been before, a world united was a world reduced. There had been a rapid pulse of extinctions.

And as time wore away, things got worse.

The new supercontinent immediately began to age. The great tectonic collisions had thrown up new mountains, and as they eroded, their debris enriched the plains with chemical nutrients like phosphorus. But now there were no new mountain-building events, no new uplift. The last mountains wore away. Rainwater and groundwater, percolating through the soil, leached out the last nutrients — and when they were gone there was nothing to replace them.

New red sandstones were laid down, rust red, red as the lifeless Martian deserts had been — the signature of lifelessness, of erosion and wind, heat and cold. The supercontinent became a great crimson plain spanning thousands of kilometers and marked only by the worn stumps of the last mountains.

Meanwhile the reduction in sea levels exposed shallow continental shelves. As they dried out they quickly began to weather, drawing oxygen out of the air. On land many animals simply suffocated to death. And in the oceans, as the pole-to-equator temperature gradient flattened out, the circulation of the ocean slowed. The waters stagnated.

On land, in the sea, species fell away like leaves from an autumn tree.

In a desiccating world the familiar games of competition, of predator and prey, were not so effective anymore. The world didn’t have the energy to sustain great complex food webs and pyramids.

Instead, life had fallen back on much more ancient strategies.

Sharing was as old as life itself. Even the cells of Ultimate’s body were the result of mergers of more primitive forms. The most ancient bacteria had been simple creatures, living off the sulfur and heat of hellish early Earth. For them the emergence of cyanobacteria — the first photosynthesizers, which used sunlight to turn carbon dioxide into carbohydrates and oxygen — was a disaster, for reactive oxygen was a lethal poison.

The survivors won by cooperating. A sulfur eater merged with another primitive form, a free-living swimmer. Later an oxygen-breathing bacterium was incorporated into the mix. The three-part entity — swimmer, sulfur-lover, oxygen breather — became capable of reproduction by cell division and could engulf food particles. In a fourth absorption some of the growing complexes engulfed bright green photosynthetic bacteria. The result was swimming green algae, the ancestors of all plant cells. And so on.

Throughout the evolution of life there had been more sharing, even of genetic material. Human beings themselves — and their descendants, including Ultimate — were like colonies of cooperative beings, from the helpful bacteria in their guts which processed foods, to the mitochondria absorbed eons ago that powered their very cells.

So it was now. Joan Useb’s intuition, long ago, had been right: One way or another, the future for mankind had been cooperation, with one another and the creatures around them. But she could never have foreseen this, the final expression of that cooperation.

The Tree, a remote descendant of the borametz of Remembrance’s time, had taken the principle of cooperation and sharing to its extremes. Now the Tree could not survive without the termites and other insects that brought nutrients to its deep roots, and the furry, bright-eyed mammals who brought it water, food, and salt, and planted its seeds. Even its leaves, strictly speaking, belonged to another plant that lived on its surface and fed on its sap.

But likewise the symbiotes, including the posthumans, could not have survived without the succor of the Tree. Its tough leaves sheltered them from predators, from the harsh heat of the climate, even from the once-in-a-century rainstorms. Sap was delivered through the belly-roots, just as the Tree took back its nutrients by the same conduits: infants were not breast-fed but were swaddled by the Tree, nurtured by these vegetable umbilicals. The sap, drawing on the deepest groundwater, sustained them through the mightiest supercontinental droughts — and, laden with beneficent chemicals, the sap healed their injuries and illnesses.

The Tree was even involved in human reproduction.

There was still sex — but only homosexual sex, for there was only one gender now. Sex served only for social bonding, pleasure, comfort. People didn’t need sex for breeding anymore, not even for the mixing of genetic material. The Tree did it all. It took body fluid from one “parent” in its sap and, circulating it through its mighty bulk, mixed it and delivered it to another.

People still gave birth, though. Ultimate herself had given birth to the infant that now lay in its leafy cradle. That heritage, the bond between mother and child, had proved too central to give up. But you no longer fed your child, by breast or otherwise. All you had to give your child was attention, and love. You no longer raised it. The Tree did all that, with the organic mechanisms in its leafy cocoons.

Of course there was still selection, of a sort. Only those individuals who worked well with the Tree and with each other were enfolded and allowed to contribute to the circulating stream of germ material. The ill, the weak, the deformed, were expelled with vegetable pitilessness.

Such a close convergence of the biologies of plant and animal might have seemed unlikely. But given enough time, adaptation and selection could turn a wheezing, four-finned lungfish into a dinosaur, or a human or a horse or an elephant or a bat — and even back into a whale, a fishlike creature, once again. By comparison hooking up people and trees with an umbilical connection was a trivial piece of re-engineering.

In the myths of vanished humanity there had been a kind of foreshadowing of this new arrangement. The Middle Ages’ legends of the Lamb of Tartary had spoken of the Borametz, a tree whose fruit was supposed to contain tiny lambs. All of mankind’s legends were forgotten now, but the tale of the Borametz, with its twining of animal and plant, found strange echoes in these latter days.

But there were costs, as always. Their complex symbiosis with the Tree had imposed a kind of stasis on the postpeople. Over time the bodies of Ultimate and her kind had specialized for the heat and aridity, and had simplified and become more efficient. Once the crucial linking was made, Tree and people became so well adapted to each other that it was no longer possible for either of them to change quickly.

Since the snaking umbilicals had started to worm their way into posthuman bellies, since people had first huddled in the protective enclosure of borametz leaves, two hundred million years had worn away unmarked.

But even now, even after all this time, the symbiotic ties were weak compared to more ancient forces.

In its slow vegetable way, the Tree had concluded that for now the people could not afford another baby. Ultimate’s infant was being reabsorbed, her substance returning to the Tree.

It was an ancient calculation: in hard times it paid to sacrifice the vulnerable young, and to keep alive mature individuals who might breed again in an upturn.

But the infant was almost old enough to feed herself. Just a little longer and she would have survived to independence. And this was Ultimate’s baby: the first she had had, perhaps the only one she would ever be permitted to have. Ancient drives warred. It was a failure of adaptation, this battling of one instinct against another.

It was a primordial calculus, an ancient story told over and over again, in Purga’s time, in Juna’s, for uncounted grandmothers lost and unimaginable in the dark. But for Ultimate, here at the end of time, the dilemma hurt as much as if it had just been minted in the fires of hell.

It took heartbeats to resolve. But in the end the tie of mother to child defeated the bonds between symbiotes. She dug her hands into the cottony stuff and dragged her baby from the cocoon. She pulled the belly-root from the infant’s gut, and bits of white fiber from her mouth and nose. The child opened her mouth with a popping sound, and turned her head this way and that.

Cactus watched, astonished. Ultimate stood there panting, her mouth open.

Now what? Standing there holding the baby — in defiance of the Tree that had given her life — Ultimate was out on her own, beyond instinct or experience. But the Tree had tried to kill her baby. She had had no choice.

She took a step away from the Tree. Then another step. And another.

Until she was running, running past the place where she had dug the salt — the sphere was gone now, faded from her memory — and she kept running, her baby clutched in her arms, until she came to the walls of the quarry, up which she scrambled in a flash.

She looked back into the great pit, its floor studded with the lowering, silent forms of the borametz trees. And here came Cactus, running after her with a defiant grin.

II

The land was bare. There were a few stubby trees, and shrubs with bark like rock and leaves like needles, and cacti, small and hard as pebbles and equipped with long toxin-laden spines. Protecting their water, these plants were little balls of aggression, and Ultimate and Cactus knew better than to tackle such risky fare until it was essential to do so.

You had to watch where you put your feet and hands.

There were pits in the desert’s crimson floor. They were bright red, a little like flowers, barely visible against the red soil, but with knots of darkness at their centers. Foolish lizards and amphibians, and even the occasional mammal, would tumble unwarily into these waiting traps — and they would not emerge, for these pits were mouths.

These deadly maws belonged to creatures that lived in narrow burrows under the ground. Hairless, eyeless, their legs reduced to scrabbling finlike stubs with sand-digging claws, they were rodents, among the last remnants of the great lineages that had once ruled the planet.

This time of openness and lack of cover did not favor large predators, and the survivors had been forced to find new strategies. The frantic activity and sociability of their ancestors long abandoned, these burrowing rat-mouths spent their lives in holes in the ground, waiting for something to fall into their mouths. Shielded from the excesses of the climate, moving from their burrows only when driven to mate, the rat-mouths had slow metabolisms and very small brains. They made few demands of life, and in their way were content.

But for creatures as smart as Ultimate and Cactus, the rat-mouths weren’t hard to avoid. Side by side, the companions moved on.

The companions came to a little gully. It was nearly choked: the rainstorm had filled it with pebbles and stones. But there was still a trickle of silty runoff water. Ultimate and Cactus crouched down, Ultimate shielding her baby, and they pushed their faces into the water, sucking at it gratefully.

Ultimate found green here, in the damp. It was a kind of leaf, prostrate, dark, slightly crimped. Its form was very ancient, too primitive even to have the wherewithal to grow up toward the light. It was actually a descendant of a liverwort, all but unchanged by the passage of time, a barely modified copy of one of the first plants ever to colonize land — a land that had not looked so different from this harsh place. The times had come around, and the liverwort found room to live. Curious, Ultimate plucked the leaf from the rock it clung to, chewed it — it was waxy, sticky — and kissed her baby, letting bits of the leaf trickle into her mouth. The baby chomped with a sucking noise, her little eyes rolling.

Close to one of those pebblelike cactuses, Ultimate spotted a beetle, silvery-backed, toiling to push a dried-up pellet of dung along a miniature crevasse. Ultimate briefly considered making a grab for the beetle.

But as the beetle passed the shade of the cactus, a tiny crimson form shot out of the darkness. It was a lizard, smaller than Ultimate’s little finger, and its head was a lot smaller than the beetle itself. But nevertheless the lizard clamped its jaws on the toiling beetle’s rear end. Ultimate could hear the minuscule crunch: The beetle waved its legs and antennae, but it could not get away. The lizard, its burst of energy expended, spread great sail-like fans from its neck and legs. The cooling fans made the lizard look twice its resting size, though its red color gave it good camouflage against the Pangaean dust. Saved from overheating, it began the slow, luxurious process of sucking out the beetle’s salty vitals from within its carapace.

But it wasn’t to be given that luxury. From nowhere a small bird came running on to the scene. Black-feathered, its wings vestigial stubs hidden beneath its skin, it was flightless. Without hesitation, and with lethal precision, the bird lunged at the lizard with a yellow beak full of tiny teeth. The lizard released the beetle and tried to squirm away under the cactus, its sail-fans folding. But the bird had hold of one fin and it pulled the lizard back into the light, shaking its tiny body.

The mutilated beetle crawled away — only to be scooped up by Cactus’s little paw and delivered to her mouth.

There were plenty of birds around; that great, ancient lineage was much too adaptable not to have found a place even in this harsh, much-changed world. But few birds flew nowadays. Why fly when there was nothing to flee, nowhere to go that wasn’t exactly the same as here? So the birds had taken to the ground, and in the great shriveling, had adopted many forms.

Meanwhile, disturbed by the bird’s attack, more lizards erupted from under the cactus. There were many of them, all of them smaller than the sail-fan caught by the bird, smaller than Ultimate’s own fingernail. They were so tiny, Ultimate saw, they had to clamber over pebbles and irregularities in the dirt as if they were hills and valleys. They scurried in all directions, disturbed from their daily slumber, and made for cover in the rocks and pebbles.

Ultimate watched, fascinated.

As new Pangaea’s great drying had continued, the larger species had fallen away. In the barren emptiness of the supercontinent there was nowhere for a creature the size of Ultimate to hide, still less a gazelle or a lion. On the largest scales the ancient game of predator and prey had broken down.

But on smaller scales, a new ecology proliferated. Under Ultimate’s feet there were holes in the rocks, crevasses in the sand, holes in borametz tree trunks, tangles in root systems. Even in the flattest landscape there was topography where you could hide from predators, or wait to ambush prey, or simply hole up and avoid the rest of the world — if you were small enough.

But if the world of the smallest scales was still rich in opportunity, it was a world largely excluded from the hot-blooded kind.

All hot-bloods had to maintain their high body temperatures. But there was a limit to how much insulating hair and fat you could grow before you became an immobilized little puff-ball, how fast a pulse you could sustain. The last of the shriveling mole folk, their tiny hearts rattling heroically, had been as small as a centimeter. But a centimeter-scale was still huge. There was plenty of room below this, plenty of ways to live.

But all these niches were taken by insects and reptiles and amphibians. Small and skinny, the cold-bloods hid from the heat of the sun and the chill of the night, under rocks and in the shade of trees and cacti. In a handful of dirt it was possible now to find tiny, perfectly formed descendants of frogs and salamanders, snakes — and even the endlessly enduring crocodiles. There were even tiny lungfish, silvery little creatures hastily adapted for the land as the inland waters had dried. This largest of continents was dominated by the smallest of animals.

Without the Tree’s support, such large hot-blooded mammals as Ultimate’s postpeople could never have survived so long. They were like throwbacks to easier times, out of place in this marginal environment. As the Earth warmed relentlessly, as the great desiccation continued, even the Tree-based communities shrank back and died, one by one. And yet they were here: And yet here was Ultimate, the latest link in a great chain that now passed back through a hundred million grandmothers, morphing and changing, loving and dying, back to Purga herself, and into the formlessness of the still deeper past beyond.

Ultimate and Cactus watched the tiny scramblings in the dirt. Then, hooting, the posthumans fell on the scrambling lizards. Most of them were too small to catch — you would close your hand around them only to see them squirming out the other side — and even when Ultimate managed to cram one into her mouth it was too small a morsel to be satisfying.

But they didn’t need to eat the lizards. They were playing. Even now you could have fun. But in the silence of New Pangaea their whoops and hooting cries echoed from the bare rocks, and as far as could be seen they were the only large creatures moving, anywhere.

The sunset came quickly.

The air was scrubbed clean of dust by the rains. As soon as the sun touched the horizon, darkness striped across the flattened land, small ridges, dunes, and pebbles casting shadows tens of meters long. The light in the sky faded from blue to purple, quickly sinking to black at the zenith. It was like a sunset on the airless Moon.

Ultimate and Cactus huddled together, the baby between their bodies. Every night of her life Ultimate had spent in the Tree’s enfolding vegetable embrace. Now the shadows were like raptor fingers reaching out for them.

But as the temperature fell, so Ultimate’s adaptations to the desert came into operation.

Her skin was actually hot to the touch. During the day her body stored heat, in layers of fat and tissue. In the cooler air of night, her body was able to radiate a lot of the heat back to the environment. If she had not been able to perform this trick of refrigeration she would have had to lose the heat by sweating — and that would have used up water she couldn’t afford to waste. And Cactus and Ultimate were breathing, deeply and slowly. That way a maximum amount of oxygen was extracted from each lungful of air, and a minimum amount of water was lost. Meanwhile Ultimate’s body was manufacturing water from the carbohydrates in the food she had eaten. She would finish the night with more water in her body’s stores than when she had started.

But still, for all this remarkable physiological engineering, the two of them could do little but sit and endure the night, breathing slowly, lapsing into a kind of dull half dream as their bodies’ functioning slowed to a crawl.

While above them a bewildering sky unfolded.

Ultimate had a grandstand perspective view of the Galaxy. The huge spiral arms were corridors of brightness that spanned the sky, studded with pinpricks of sapphire-blue young stars and ruby-red nebulae. At the center of the disk was the Galactic core, a bulge of yellow-orange stars like the yolk of a fried egg: the light had taken twenty-five thousand years to travel here to Earth from that crowded core.

In human times the sun had been embedded in the body of the huge flat disk, so that the Galaxy had been seen edge-on, its glory diminished by the obstructing dust clouds that littered the disk. But now the sun, following its slow orbit around the core, had sailed out of the Galaxy’s plane. Compared to the random scattering of a few thousand lamps that had marked man’s sky, this was like glimpsing the lights of a hidden city.

Ultimate cowered.

A bony hook rose in the sky. It was the Moon, of course, an old Moon, tonight a narrow crescent. The same patient face that had peered down on Earth since long before the birth of man was all but unchanged across half a billion years. And yet this thin crescent Moon shone more brightly over the new supercontinent than it had over the more equable lands in the past. For the Moon shone by reflected sunlight — and the sun had grown brighter.

Had she known where to look Ultimate might have made out a dim smudge in the sky away from the Galaxy’s disk, easily visible on the clearest nights. That remote smudge was the great galaxy known as Andromeda, twice the size of its neighbor. It was still a million light-years distant from Earth’s Galaxy — but in human times it had been twice as far away as that, and even then it had been visible to the naked eye.

Andromeda and the Galaxy were heading for a collision, still another half-billion years distant. The two great star systems would pass through each other like mingling clouds, with direct collisions between stars rare. But there would be a vast gush of star formation, an explosion of energy that would flood the disks of both galaxies with hard radiation. It would be a remarkable, lethal light show.

But by then there would be little left alive on Earth itself to be troubled by the catastrophe. For the brightening of the sun was life’s final emergency.

Morning came with its usual stark suddenness. Scuttling lizards and insects disappeared into the nooks and crannies where they would ride out the day, waiting for the richer opportunities of evening.

The baby mewled. Her fur stuck up in clumps, and the pucker where the belly-root should sit looked inflamed. She kept up her complaints, her bulbous little head turning to and fro, until Ultimate had chewed some more liverwort and dribbled it into her mouth. Cactus, too, was grumbling, picking dirt and bits of dried shit from her fur.

This morning it didn’t seem such a good idea to be out here in the middle of nowhere, so far from home. But as she held her baby Ultimate knew she had to stay away from the Tree — stay away, or lose her baby. She clung to that one irreducible fact.

Ultimate and Cactus began to work their random way across the landscape, heading roughly away from the quarry. Just as they had yesterday, they ate where they could — though they found no water — and they avoided the rat-mouths and other hazards.

And, at some point past noon, when the sun had begun its climb down the sky, Ultimate suddenly found herself facing the sphere once again.

She had forgotten it existed. It did not occur to her to wonder how such an immense object might have gotten here from there, in the quarry.

Cactus showed no interest, once she had figured out that you couldn’t eat the sphere. She passed on, grumbling to herself, picking bits of crimson dust out of her fur.

Her baby asleep in her arms, Ultimate walked up to the sphere’s purple-black bulk. She sniffed it and, this time, tasted it. Again that unidentifiable electric tang subtly thrilled her. She lingered, somehow drawn. But the sphere offered her nothing.

But suddenly Cactus was howling, thrashing on the ground. Ultimate whirled, crouching. Cactus’s left leg was somehow pinned, and blood spurted from her foot — and Ultimate heard the crunch of bone, as if poor Cactus’s limb had been taken in some vast mouth.

But there was no mouth to be seen.

No teeth and claws held Cactus. But slashes appeared on her chest and torso, dripping with startlingly bright blood, as if out of nowhere. Still she fought. She swung her fists, kicked, tried to bite even as she screamed. She was landing blows — Ultimate could hear the meaty sound of flesh being struck, and there were peculiar bits of discoloration in the air over Cactus, purple and blue. And the blood itself was starting to outline her assailant in crimson splashes. Ultimate could make out a long cylindrical torso, stubby legs, a wide, snapping mouth.

But Cactus was losing her fight. Her legs and upper body became trapped under the shimmering mass. She turned to Ultimate, and reached out her hand.

Instincts warred in Ultimate. It might have been different if she could have imagined how Cactus was feeling, the mortal fear that flooded through her. But Ultimate could not; empathy had been lost in mankind’s great shedding, along with so much else.

She had hesitated too long.

That great blurred mass raised itself up and came crashing down on Cactus. A thicker, richer blood gushed from the helpless posthuman’s mouth.

Ultimate’s shock evaporated. With a squeal of terror she turned and ran, her squealing baby clutched to her chest, her feet and her free hand clattering over the dusty ground. She kept going until she came to an eroded ridge of crimson rock.

She flung herself to the ground, and looked back. Cactus was still. Ultimate could make out nothing of the vast transparent thing that had destroyed her. But new creatures had emerged, as if from nowhere. They looked like frogs, with sprawling bodies, leathery amphibian skin, splayed, clawed feet, and wide mouths equipped with needle-sharp teeth for rending and gouging. Already the first of them had opened up Cactus’s chest and was feasting on the still-warm organs within.

The invisible predator had done its job. It lay exhausted in a pool of Cactus’s blood. It was too weary even to feed itself, and it relied on scraps brought to it by its greedy siblings. The meat could be seen being shredded by its grinding teeth and then passing into its gullet and stomach, where digestive processes would begin to absorb and transform it.

As the world had emptied and been eroded flat, the lack of cover was the killer. In a landscape like a pool table, you just couldn’t hide a one-ton salamander, even if it was painted as red as the rocks. That was why most of the big animals had quickly disappeared, outcompeted by their smaller cousins.

But these creatures had adopted a novel strategy: the ultimate camouflage. The great redesign had taken many tens of millions of years.

Invisibility — or at least transparency — had been a strategy adopted by some fish in earlier times. There were transparent substitutes for most of the body’s biochemicals. A substitute had to be found for hemoglobin, for example, the bright red protein in blood cells that combined with oxygen to transport that vital substance through the body.

Of course no land-going creature could ever be truly invisible. Even in these arid times all animals were essentially bags of water. If you were actually immersed in water — where those long-extinct fish had once swum — something approaching true invisibility could be achieved. But light moved differently through air and water; in the air the final land-going “invisible” actually looked like a big bag of water sitting in the dirt.

Still, it worked pretty well. As long as you kept still you were hard to see — just a mistiness, a slight distortion here and there that might easily be mistaken for a bit of heat shimmer. You could huddle against a rocky outcrop, ensuring that you presented only your least visible angles to any prey. You even had fur, transparent-like fiber-optic cable, which transmitted bits of background color to baffle your prey further.

But even so, few species had adopted the stratagem, for invisibility was a blight.

Every invisible was blind, of course. No transparent retina could trap light. On top of that the creature’s biochemistry, limited by the use of transparent substances, was a lot less efficient. And there was no shielding, even for its innermost parts, from the ferocious light, heat, and ultraviolet radiation from the sun, or from the cosmic radiation that had always battered the planet despite its great shield of magnetism. Its organs were transparent, but not transparent enough to let through all the damaging radiation.

Already Cactus’s killer was in agony, and soon the cancers developing in its transparent gut would kill it. And it was neotenous. It would die without reaching puberty. None of the invisible kind had ever lived long enough to breed true, nor would their genetic material, damaged by radiation, ever have been able to produce a viable offspring.

Sickly, helpless from birth, these wretched creatures began dying before they emerged from their eggs.

But that didn’t matter, not from the point of view of the genes, for the family benefited.

This amphibian species had reached a compromise. Most of its young were born as they always had been. But perhaps one in ten was born invisible. Like the sterile workers in a hive, the invisible lived through its brief, painful life and died young, all for a single purpose: to retrieve food for its siblings. Through them — through their offspring, not its own — the invisible’s genetic legacy would live on.

It was an expensive strategy. But it was better to sacrifice one in ten of each generation to a brief life of agony than to succumb to extinction.

The presence of food in its stomach and waste in its lower gut made the invisible easy to spot, of course. So when they were hungry again its siblings would starve it, waiting for all the waste to pass out of its system, rendering it as transparent as possible. And then they would set it to work once more, under the lethal sun, hoping to have it snatch one more meal for them before it died.

The sphere had made its own observations of these events.

The sphere was a living thing, and yet it was not. It was an artifact — and yet it was not that either. The sphere had no name for itself, or for its kind. Yet it was conscious.

It was one of a great horde that now spanned the stars, in a great belt of colonization that swept around the Galaxy’s limb. And yet the sphere had come here, to this ruined world, seeking answers.

Memories stretched deep. Among the sphere’s kind, identity was a fluid thing, to be split and shared and passed on through components and blueprints. The sphere could think back, deep through thousands of generations, but it was a memory trail that ended in mist. The replicating hordes had forgotten where they came from.

In its way, the sphere longed to know. How had this great star-spanning swarm of robots first originated? Had there been some form of spontaneous mechanical emergence, cogs and circuits coming together on some metallic asteroid? Or had there been a Designer, some other, who had brought the progenitors of these swarming masses into being?

For a million years the sphere had studied the distribution of the replicators through the Galaxy. It wasn’t easy, for the great disk had rotated twice since the origin of its kind, and the stars had swum about, smearing the robot colonists across the sky. Great mathematical models had been built to reverse that great turning, to restore the stars as they once must have been, to map back the replicators’ half-forgotten expansion.

And at last the sphere had converged on this system, this world — amid a handful of others — as the putative origin. It had found a world of organic chemistry and creatures interesting in their way. But it was a dying world, overheated by its sun, the life-forms restricted to the fringes of a desert continent. There was no sign of organized intelligence.

And yet, here and there, the ancient rocks of the supercontinent had been marked deliberately, it seemed to the sphere, with cuts and gouges and great pits. Once there had been mind here, perhaps. But if so it was vanished from these wretched, crawling creatures.

The sphere represented a new order of life. And yet it was like a child, wistfully seeking its lost father. The last traces of the Martian robots’ original blueprint, assembled by long-dead NASA engineers in computer laboratories in California and New England and much modified since, had been lost. It was somehow appropriate that this greatest, and strangest, of all of mankind’s legacies should have been created entirely accidentally — and that those created should have been abandoned to their fate.

There was nothing more to be learned here. With an equivalent of a sigh, the sphere leapt to the stars. The small world dwindled behind it.

Ultimate huddled in the dirt until the scavenging siblings had finished feeding. Then she stumbled away, clutching her baby, not even noticing that the sphere had vanished.

III

Ultimate kept heading west, away from the borametz quarry.

At night she wedged herself with her infant into crevices between rocks, trying to emulate the comforting enclosure of the Tree’s cocoon. She ate whatever she could find — half-desiccated toads and frogs buried in the mud, lizards, scorpions, the flesh and roots of cacti. She fed the child a chewed-up pulp of meat and vegetable matter. But the child spat out the coarse stuff. She was still missing her belly-root, and she mewled and complained.

Ultimate walked, and walked and walked.

She had no strategy in mind save to keep moving, to keep her infant out of the chemical clutches of the Tree, and to wait to see what turned up. If her thinking had been sophisticated enough she might have hoped to find more people, somewhere she could stay, maybe even a community that lived independently of the Trees.

It would have been a futile hope, for there were no more such communities anywhere on Earth. She didn’t know it, but she had nowhere to go.

The land began to rise slowly. Ultimate found herself walking on coarse sand and gravel fans.

After half a day of this she came to a place of low, smooth-shouldered hills. She could see how these eroded stumps went marching off to the horizon, to north and south, for kilometer after kilometer, all the way to the dust-laden horizon and beyond. She was walking through the remnant of a once-great mountain chain, thrown up by the ancient stitching together of the continents. But the dust-laden winds of New Pangaea had long since worn down the mountains to these meaningless stumps.

When she looked back she could see her own footprints, accompanied by the scraping of knuckles, marked by the messy places where she had stopped to feed or make waste or sleep. They were the only trail through these silent hills.

It took her two days to cross the mountains.

After that the land began to descend again.

On the plain, a little more vegetation grew. There were spiky trees with gnarled branches and clumps of needlelike leaves, like bristlecone pines. Around their roots sheltered a few leaping mice — hardy rodent survivors, ferocious conservers of water — and many, many lizards and insects. She chased tiny things like geckos and iguanas, and munched their flesh. But on this looser ground Ultimate had to be cautious, watching for rat-mouths embedded in the ground, and for the quivering, invisible mass of an ambush hunter.

As the land descended further, the view to the west opened up. She saw a great plain. Beyond a kind of coastal fringe, the land was white, white as bone, a sheet that ran all the way to a knife-sharp, geometrically flat horizon. A thin wind moaned in her face. On its breath, she could already taste the salt. Nothing moved, as far as she could see.

She had come to a fragment of the dying inland ocean. There was still water out there — it took a long, long time to dry out a sea — but it was a narrowing strip of water so saline it was all but lifeless, and it was framed by this great white rim of exposed salt flats, a sheet that extended to her horizon.

Tucking her baby’s face into the fur on her breast, Ultimate continued her dogged descent.

She reached the place where the salt began. Great parallel bands showed where water had once lapped. She scooped up a bit of the salty dirt and licked it. She spat out the bitter stuff immediately. There was vegetation here, tolerant of the salty soil. There were small, spiky yellow shrubs that looked like the desert holly and honeysweet and spurge that had once clung to life in the Californian deserts of North America. Experimentally she broke off a bit of the holly’s foliage and tried to chew it, but it was too dry. Frustrated, she hurled the bit of twig away over the salt.

And then she saw the footprints.

Curious, she fit her own feet into the shallow indentations in the ground. Here had been toes, here a scuffing that might have been caused by a resting knuckle. The prints could not have been made recently. The mud was baked hard as rock, and her own weight left no mark.

The prints set out, straight as an arrow, across the salt pan, marching on toward the empty horizon. She followed for a step or two. But the salt was hard and harsh and very hot, and when it got into minor cuts and scrapes on her feet and hands it stung badly.

The footprints did not turn back. Whoever had made them had not returned. Perhaps the walker had been intent on reaching the ocean proper, on walking all the way across North America: After all, there were no barriers now.

She knew she could not follow, not into the belly of this dead sea.

And it would have made no difference even if she had. This was New Pangaea. Wherever she went she would have found the same crimson ground, the same searing heat.

She stayed on the desolate, silent beach for the rest of the day. As it descended the overheating sun grew huge, its circular shape quivering. Its harsh light turned the salt plain a washed-out pink.

This had been the last significant journey ever undertaken by any of her ancient, wandering lineage. But the journey was over. This parched, dead beach had been the farthest point of all. The children of humanity had done with exploring.

As the light failed she turned away and began to walk up the sloping ground. She did not look back.

In the years after Ultimate’s death Earth would spin on, ever more slowly, its waltz with its receding Moon gradually running down.

And the sun blazed ever brighter, following its own hydrogen logic.

The sun was a fusion furnace. But the sun’s core was becoming clogged with helium ash, and the surrounding layers were falling inwards: The sun was shrinking. Because of this collapse the sun was getting hotter. Not by much — only by around 1 percent every hundred million years — but it was relentless.

For most of Earth’s history, life had managed to shield itself from this steady heating-up. The living planet used its “bloodstream” — the rivers and oceans and atmosphere and the cycling rock, and the interactions of trillions of organisms — to remove waste and restore nutrients to where they were needed. Temperature was controlled by carbon dioxide — a vital greenhouse gas, and the raw material for plants’ photosynthesis. There was a feedback loop. The hotter it got, the more carbon dioxide was absorbed by the weathering rocks — so the less greenhouse effect there was — and so the temperature was adjusted back down. It was a thermostat that had kept the Earth’s temperature stable for eons.

But as the sun got hotter, so more carbon dioxide got trapped in the rocks, and the less there was available for the plants.

Eventually, fifty million years after Ultimate’s time, photosynthesis itself began to fail. The plants shriveled: grasses, flowers, trees, ferns, all gone. And the creatures that lived off them died too. Great kingdoms of life imploded. There was a last rodent, and then a last mammal, a last reptile. And after the higher plants had disappeared, so did the fungi and slime mold and ciliates and algae. It was as if evolution had, in these final times, reversed itself, and life’s hard-won complexity was shed.

At last, under a blazing sun, only heat-loving bacteria could survive. Many of them had descended with little modification from the earliest life-forms of all, the simple methane eaters who had lived before poisonous oxygen was spread into the atmosphere. For them it was like the good old days before photosynthesis: the arid plains of the last supercontinent were briefly streaked with gaudy, defiant colors, purples and crimsons draped like flags over the eroded rocks.

But the heat climbed, relentlessly. The water evaporated, until whole oceans were suspended in the atmosphere. At last some of the great clouds reached the stratosphere, the atmosphere’s upper layer. Here, assailed by the sun’s ultraviolet rays, the water molecules broke up into hydrogen and oxygen. The hydrogen was lost to space — and with it the water that might have reformed. It was as if a valve had been opened. Earth’s water leaked rapidly out into space.

When the water was gone, it got so hot that the carbon dioxide was baked out of the rocks. Under air as dense as an ocean, the dried seabeds grew hot enough to melt lead. Even the thermophiles wilted. It was the last extinction event of all.

But on rocky ground as hot as the floor of an oven, the bacteria had left behind desiccated spores. In these toughened shells, virtually indestructible, the bacteria, dormant, rode out the years.

There were still convulsions as asteroids and comets sporadically fell onto the parched land, more unremarked Chicxulubs. Now there was nothing left to kill, of course. But as the ground flexed and rebounded, huge quantities of rock were hurled into space.

Some of this material, taken from the edge of each impact zone, was not shocked, and therefore it reached space unsterilized. That was how the bacterial spores left Earth.

They drifted away from Earth and, propelled by the gentle, persistent pressure of sunlight, they created a vast diffuse cloud around the sun. Encysted in their spores, bacteria were all but immortal. And they were hardy interplanetary travelers. The bacteria had coated their DNA strands with small proteins that stiffened the helical shapes and fended off chemical attack. When a spore germinated it could mobilize specialized enzymes to repair any DNA damage. Even some radiation damage could be fixed.

The sun continued its endless circling of the Galaxy’s heart, planets and comets and spore cloud and all.

At last the sun drifted into a dense molecular cloud. It was a place where stars were born. The sky was crowded here; dazzling young stars jostled in a great swarm. The fiercely hot sun with its ruined planets was like a bitter old woman intruding in a nursery.

But, just occasionally, one of the sun’s spaceborne spores would encounter a grain of interstellar dust, rich with organic molecules and water ice.

Battered by the radiation of nearby supernovae, a fragment of the cloud collapsed. A new sun was born, a new system of planets, gas-stuffed giants and hard, rocky worlds. Comets fell to the surface of the new rocky planets, just as once Earth had been impact-nourished.

And in some of those comets were Earthborn bacteria. Only a few. But it only took a few.

Still the sun aged. It bloated to monstrous proportions, glowing red. Earth skimmed the diffuse edge of the swollen sun, like a fly circling an elephant. The dying giant star burned whatever it could burn. The final paroxysms lit up the great shell of gas and dust that lingered around the sun. The solar system became a planetary nebula, a sphere shimmering with fabulous colors, visible across light-years.

These glorious spasms marked the final death of Earth. But on a new planet of a new star, the nebula was just a light show in the sky. What mattered was the here and now, the oceans and the lands where new ecosystems assembled, where creatures’ changing forms tracked changes in their environment, where variation and selection blindly worked, shaping and complexifying.

Life always had been chancy. And now life had found ways of surviving the ultimate extinction event. In new oceans and on strange lands, evolution had begun again.

But it had nothing to do with mankind.

Exhausted, dust-laden, her body covered by a hundred minor scrapes, bruises and prickles, her baby cradled in her arms, Ultimate limped to the center of the ancient quarry.

The land seemed beaten flat, with the sun poised above, a great glowing fist. And at first glance there was no sign that anything still lived on this desert world, none at all.

She approached the Tree itself. She saw the big pendulous folded-over shapes of cocooned people, inert and black. The Tree stood there, silent and still, neither reproving nor forgiving her small betrayal.

She knew what she had to do. She found a folded-up ball of leaves. Carefully she prized the leaves open, shaping them into a makeshift cradle. Then she placed her baby carefully inside.

The baby gurgled and wriggled. She was comfortable, here in the leaves; she was happy to be back with the Tree. But already, Ultimate saw, the belly-rope had snaked into its orifice in the child’s stomach. And white tendrils were pushing out of pores in the cradling leaves, reaching out for the baby’s mouth and nose, ears and eyes.

There would be no pain. Ultimate had been granted that much knowledge, and comfort. Ultimate stroked the child’s furry cheek one last time. Then, without regret, she folded up the leaves and sealed them up.

She clambered off the ground, found her own favorite cocoon, and snuggled inside, neatly closing up the big leathery leaves around her. Here she would stay until a better time: a day miraculously cooler and moister than the rest, a time when it might be possible for the Tree to release Ultimate from this protective embrace, to send her out into the world once more — even to seed her belly with another generation of people.

But there would never be another impregnation, never another birth, never another doomed child.

One by one the cocoons would shrivel as their inhabitants, sealed in green, were absorbed back into the bulk of the borametz — and in the end the borametz itself, of course, would succumb, thousands of years old, tough and defiant to the last. The shining molecular chain that had stretched from Purga through generations of creatures that had climbed and leapt, and learned to walk, trod the dirt of another world, and grown small again, and mindless, and returned to the trees — at last that great chain was broken, as the last of Purga’s granddaughters faced an emergency she could not withstand.

Ultimate was the last mother of all. She couldn’t even save her own child. But she was at peace.

She stroked the belly-root and helped it worm its way into her gut. The Tree’s anesthetic and healing chemicals soothed her aching body, closed her small wounds. And as psychotropic vegetable medications washed away the sharp, jagged memory of her lost baby, she was filled with a green bliss that felt as if it would last forever.

It wasn’t such a bad way for the long story to end.