Stone spring

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The comet swam out of the dark. Its light bathed the planet that lay ahead, reflecting from a hemisphere that gleamed a lifeless bone-white.

Vast ice caps covered much of North America and central Asia. In Europe a single monstrous dome stretched from Scotland to Scandinavia, in places piled kilometres thick. To the south was a polar desert, scoured by winds, giving way to tundra. At the glaciation’s greatest extent Britain and northern Europe had been abandoned entirely; no human had lived north of the Alps.

At last, prompted by subtle, cyclic changes in Earth’s orbit, the climate had shifted – and with dramatic suddenness. Over a few decades millennia-old ice receded north. The revealed landscape, scoured to the bedrock, was tentatively colonised by the grey-green of life. Migrant herds and the humans who depended on them slowly followed, taking back landscapes on which there was rarely a trace of forgotten ancestors.

With so much water still locked up in the ice, the seas were low, and all around the world swathes of continental shelf were exposed. In northern Europe Britain was united with the continent by a bridge of land that, as it happened, had been spared the scouring of the ice. As the thaw proceeded, this north land, a country the size of Britain itself, became rich terrain for humans, who explored the water courses and probed the thickening forests for game.

But now, in the chill nights, eyes animal and human were drawn to the shifting light in the sky. The comet punched into the atmosphere. It disintegrated over North America and exploded in multiple airbursts and impacts, random acts of cosmic violence. Whole animal herds were exterminated, and human survivors, fleeing south, thought the Sky Wolf was murdering the land they had named for him. One comet fragment skimmed across the atmosphere to detonate over Scandinavia.

In time the skies cleared – but the remnant American ice caps had been destabilised. One tremendous sheet had been draining south down the Mississippi river system. Now huge volumes of cold water flowed through the inland sea that covered the Gulf of St Lawrence, chilling the north Atlantic. Around the world the ice spread from the north once more, and life retreated to its southern refuges. This new winter lasted a thousand years.

But even as the ice receded again, even as life took back the land once more, the world was not at rest. Meltwater fuelled rising seas, and the very bedrock rebounded, relieved of the weight of ice – or it sank, in areas that had been at the edge of the masses of ice and uplifted by its huge weight. In a process governed by geological chance, coastlines advanced and receded. The basic shape of the world changed around the people, constantly.

And to north and south of the rich hunting grounds of Europe’s north land, generation on generation, the chill oceans bit at the coasts, seeking a way to sever the land bridge.