The Science of Discworld I

24. DESPITE WHICH ...

THAT BLUE IN THE ROUNDWORLD SEA isn't a chemical, well, not in the usual Simple chemical' sense of the word. It's a mass of bacteria, called cyanobac-teria. Another name for them is 'blue-green algae', which is wonderfully confusing. Modern so-called blue-green algae are usually red or brown, but the ancient ones probably were blue-green. And blue-green algae are really bac­teria, whereas most other algae have cells with a nucleus and so are not bacteria. The blue-green colour comes from chlorophyll, but of a different kind from that in plants, together with yellow-orange chemicals called carotenoids.

Bacteria appeared on Earth at least 3.5 billion years ago, only a few hundred million years after the Earth cooled to the point at which living creatures could survive on it. We know this because of strange layered structures found in sedimentary rocks. The layers can be flat and bumpy, they can form huge branched pillars, or they can be highly convoluted like the leaves in a cabbage. Some deposits are half a mile thick and spread for hundreds of miles. Most date from 2 billion years ago, but those from Warrawoona in Australia are 3.5 billion years old.

To begin with, nobody knew what these deposits were, In the 1950s and 1960s they were revealed as traces of communities of bacteria, especially cyanobacteria.

Cyanobacteria collect together in shallow water to form huge, floating mats, like felt. They secrete a sticky gel as protection against ultraviolet light, and this causes sediment to stick to the mats. When the layer of sediment gets so thick that it blocks out the light, the bacteria form a new layer, and so on. When the layers fossilize they turn into stromatolites, which look rather like big cushions. The wizards haven't been expecting life. Roundworld runs on rules, but life doesn't, or so they think. The wizards see a sharp discontinuity between life and non-life. This is the problem of expecting becomings to have boundaries, of imagining that it ought to be easy to class all objects into either the category 'alive' or the category 'dead'. But that's not possible, even ignoring the flow of time, in which 'alive' can become 'dead', and vice versa. A 'dead' leaf is no longer part of a living tree, but it may well have a few revivable cells.

Mitochondria, now the part of a cell that generates its chemical energy, once used to be independent organisms. Is a virus alive? Without a bacterial host it can't reproduce, but neither can DNA copy itself without a cell's chemical machinery.

We used to build 'simple' chemical models of living processes, in the hope that a sufficiently complex network of chemistry could 'take off', become self-referential, self-copying, by itself There was the concept of the 'primal soup', lots of simple chemicals dis­solved in the oceans, bumping into each other at random, and just occasionally forming something more complicated. It turns out that this isn't quite the way to do it. You don't have to work hard to make real-world chemistry complex: that's the default. It's easy to make complicated chemicals. The world is full of them. The problem is to keep that complexity organized.

What counts as life? Every biologist used to have to learn a list of properties: ability to reproduce, sensitivity to its environment, utilization of energy, and the like. We have moved on. 'Autopoeisis', the ability to make chemicals and structures related to one's own reproduction, is not a bad definition, except that modern life has evolved away from those early necessities. Today's biologists prefer to sidestep the issue and define life as a property of the DNA mol­ecule, but this begs the deeper question of life as a general type of process. It may be that we're now defining life in the same way that 'science fiction' is defined, it's what we're pointing at when we use the term[34].

The idea that life could somehow be self-starting is still contro­versial to many people. Nevertheless, it turns out that finding plausible routes to life is easy. There must be at least thirty of them.

It's hard to decide which, if any, was the actual route taken, because later lifeforms have destroyed nearly all the evidence. This may not matter much: if life hadn't taken the route that it did, it could eas­ily have taken one of the others, or one of the hundred we haven't thought of yet.

One possible route from the inorganic world to life, suggested by Graham Cairns-Smith, is clay. Clay can form complicated micro­scopic structures, and it often 'copies' an existing structure by adding an extra layer to it, which then falls off and becomes the starting point of a new structure. Carbon compounds can stick on to clay surfaces, where they can act as catalysts for the formation of complex molecules of the kind we see in living creatures, proteins, even DNA itself. So today's organisms may have hitched an evolu­tionary ride on clay.

An alternative is Gunther Wachterhauser's suggestion that pyrite, a compound of iron and sulphur, could have provided an energy source suitable for bacteria. Even today we find bacteria miles underground, and near volcanic vents at the bottom of the oceans, which power themselves by iron/sulphur reactions. These are the source of the 'upflow of poisonous minerals' noticed by Rincewind. It's entirely conceivable that life started in similar envi­ronments.

A potential problem with volcanic vents, though, is that every so often they get blocked, and another one breaks out somewhere else. How could the organisms get themselves safely across the interven­ing cold water? In 1988 Kevin Speer realized that the Earth's rotation causes the rising plumes of hot water from vents to spin, forming a kind of underwater hot tornado that moves through the deep ocean. Organisms could hitch a ride on these. Some might make it to another vent. Many would not, but that doesn't matter -all that would be required would be enough survivors.

It is interesting to note that back in the Cretaceous, when the seas were a lot warmer than now, these hot plumes could even have risen to the ocean's surface, where they may have caused 'hyper-canes', like hurricanes but with a windspeed close to that of sound. These would have caused major climatic upheavals on a planet which, as we shall see, it not the moderately peaceful place we tend to believe it is.

Bacteria belong to the grade of organisms known as prokaryotes. They are often said to be 'single-celled', but many single-celled creatures are far more complex and very different from bacteria. Bacteria are not true cells, but something simpler; they have no cell wall and no nucleus. True cells, and creatures both single-celled and many-celled, came later, and are called eukaryotes. They probably arose when several different prokaryotes joined forces to their mutual benefit, a trick known as symbiosis. The first fossil eukary­otes are singe-celled, like amoebas, and appear about 2 billion years ago. The first fossils of many-celled creatures are algae from 1 bil­lion years ago ... maybe even as old as 1.8 billion years.

This was the story as scientists understood it up until 1998: ani­mals like arthropods and other complex beasts came into being a mere 600 million years ago, and that until about 540 million years ago the only creatures were very strange indeed, quite unlike most of what's around today.

These creatures are known as Ediacarans, after a place in Australia where the first fossils were found[35]. They could grow to half a metre or more, but as far as can be told from the fossil record, seem not to have had any internal organs or external orifices like a mouth or an anus (they may have survived by digesting symbiotic bacteria in their selves, or by some other process we can only guess at). Some were flattened, and clustered together in quilts. We have no idea whether the Ediacarans were our distant ancestors, or whether they were a dead end, a lifestyle doomed to failure. No matter: they were around then, and as far as anyone knew, not much else was. There are hints of fossil wormcasts, though, and some very recent fossils look like ... but we're getting ahead of the story. The point is that nearly all Ediacaran life was apparently unrelated to what came later.

About 540 million years ago the Pre-Cambrian Ediacarans were succeeded by the creatures of the Cambrian era. For the first ten million years, these beasties were also pretty weird, leaving behind fragments of spines and spikes which presumably are the remains of prototype skeletons that hadn't yet joined up. At that point, nature suddenly learned how to do joined-up skeletons, and much else: this was the time known as the Cambrian Explosion. Twenty mil­lion years later virtually every body-plan found in modern animals was already in existence: everything afterwards was mere tinkering. The real innovation of the Cambrian Explosion, though, was less obvious than joined-up skeletons or tusks or shells or limbs. It was a new kind of body plan. Diploblasts were overtaken by triploblasts ...

Sorry, Archchancellor. We mean that creatures began to put another layer between themselves and the universe. Ediacarans and modern jellyfish are diploblasts, two-layered creatures. They have an inside and an outside, like a thick paper bag. Three-layered crea­tures like us and practically everything else around are called triploblasts. We have an inner, an outer, and a within.

The within was the big leap forward, or at least the big slither. Within you can put the things you need to protect, like internal organs. In one sense, you are not part of the environment any more, there is a you as well. And, like someone who now has a piece of property of their very own, you can begin to make improvements. This is a lie-to-children, but as lies go it is a good one. Triploblasts played a crucial role in evolution, precisely because they did have internal organs, and in particular they could ingest food and excrete it. Their excreta became a major resource for other creatures; to get an interestingly complicated world, it is vitally important that shit happens.

But where did all those triploblasts come from? Were they an offshoot of the Ediacarans? Or did they come from something else that didn't leave fossils?

It's hard to see how they could have come from Ediacarans. Yes, an extra layer of tissue might have appeared, but as well as that extra layer you need a lot of organization to exploit it. That organization has to come from somewhere. Moreover, there were these occa­sional tantalizing traces of what might have been pre-Cambrian triploblasts, fossils not of worms, which would have clinched it, but of things that might have been trails made by worms in wet mud.

And then again, might not.

In February 1998, we found out.

The discovery depended upon where, and in this case how -you look for fossils. One way for fossils to form is by petrification. There is a poorly known type of petrification that can happen very fast, within a few days. The soft parts of a dead organism are replaced by calcium phosphate. Unfortunately for palaeontologists, this process works only for organisms that are about a tenth of an inch (2 mm) long. Still, some interesting things are that tiny. From about 1975 onwards scientists found wonderfully preserved speci­mens of tiny ancient arthropods, creatures like centipedes with many segments. In 1994 they found fossilized balls of cells from embryos, early stages in the development of an organism, and it is thought that these come from embryonic triploblasts. However, all of these creatures must have come after the Ediacarans. But in 1998 Shuhai Xiao, Yun Zhang, and Andrew Knoll discovered fos­silized embryos in Chinese rock that is 570 million years old -smack in the middle of the Ediacaran era. And those embryos were triploblasts.

Forty million years before the Cambrian explosion, there were triploblasts on Earth, living right alongside those enigmatic Ediacarans.

We are triploblasts. Somewhere in the pre-Cambrian, sur­rounded by mouthless, organless Ediacarans, we came into our inheritance.

It used to be thought that life was a delicate, highly unusual phe­nomenon: difficult to create, easy to destroy. But everywhere we look on Earth we find living creatures, often in environments that we would have expected to be impossibly hostile. It's beginning to look as if life is an extremely robust phenomenon, liable to turn up almost anywhere that's remotely suitable. What is it about life that makes it so persistent?

Earlier we talked about two ways to get off the Earth, a rocket and a space elevator. A rocket is a thing that gets used up, but a space elevator is a process that continues. A space elevator requires a huge initial investment, but once you've got it, going up and down is essentially free. A functioning space elevator seems to contradict all the usual rules of economics, which look at individual transac­tions and try to set a rational price, instead of asking whether the concept of a price might be eliminated altogether. It also seems to contradict the law of conservation of energy, the physicist's way of saying that you can't get something for nothing. But, as we've seen, you can, by exploiting the new resources that become available once you get your space elevator up and running.

There is an analogy between space elevators and life. Life seems to contradict the usual rules of chemistry and physics, especially the rule known as the second law of thermodynamics, which says that things can't spontaneously get more complicated. Life does this because, like the space elevator, it has lifted itself to a new level of operation, where it can gain access to things and processes that were previously out of the question. Reproduction, in particular, is a wonderful method of getting round the difficulties of manufac­turing a really complicated thing. Just build one that manufactures more of itself. The first one may be incredibly difficult, but all the rest come with no added effort.

What is the elevator for life? Let's try to be general here, and look at the common features of all the different proposals for 'the' origin of life. The main one seems to be the novel chemistry that can occur in small volumes adjacent to active surfaces. This is a long way from today's complex organisms, it's even a long way from today's bacteria, which are distinctly more complicated than their ancient predecessors. They have to be, to survive in a more compli­cated world. Those active surfaces could be in underwater volcanic vents. Or hot rocks deep underground. Or they could be seashores. Imagine layers of complicated (because that's easy) but disorgan­ized (ditto) molecular gunge on rocks which are wetted by the tides and irradiated by the sun. Anything in there that happens to pro­duce a tiny 'space elevator' establishes a new baseline for further change. For example, photosynthesis is a space elevator in this sense. Once some bit of gunge has got it, that gunge can make use of the sun's energy instead of its own, churning out sugars in a steady stream. So perhaps 'the' origin of life was a whole series of tiny 'space elevators' that led, step by step, to organized but ever more complex chemistry.