The Science of Discworld

by Terry Pratchett

  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 Günther Wächtershäuser'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.* 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 li
fe 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.

  Everyone knows what science fiction is — until you start asking questions like 'Is a book set five years in the future automatically SF? Is it SF just because it’s set on another world, or is it simply fantasy with nuts and bolts on the outside? It is SF if the author thinks it isn’t? Does it have to be set in the future? Does the presence of Doug McClure mean that a movie is SF, or merely that the men-in-rubber-monster-suits quotient is going to be high? One of the best SF books ever written was the late Roy Lewis’s The Evolution Man; there is no technology in it more sophisticated than a bow, it’s set in the far past, the characters are barely more than ape-men … but it is science fiction, nonetheless.

  They were fortunate, given the names of some places in Australia, that they ended up merely sounding like a minor Star Trek species.

  TWENTY-FIVE

  UNNATURAL SELECTION

  THE LIBRARIAN KNUCKLED SWIFTLY through the outer regions of the University's library, although terms like 'outer' were hardly relevant in a library so deeply immersed in L-space.

  It is known that knowledge is power, and power is energy, and energy is matter, and matter is mass, and therefore large accumulations of knowledge distort time and space. This is why all bookshops look alike, and why all second-hand bookshops seem so much bigger on the inside — and why all libraries, everywhere, are connected. Only the innermost circle of librarians know this, and take care to guard the secret. Civilization would not survive for long if it was generally known that a wrong turn in the stacks would lead into the Library of Alexandria just as the invaders were looking for the matches, or that a tiny patch of floor in the reference section is shared with the library in Braseneck College where Dr Whitbury proved that gods cannot possibly exist, just before that rather unfortunate thunderstorm.

  The Librarian was saying 'ook ook' to himself under his breath, in the same way that a slightly distracted person searches aimlessly around the room saying 'scissors, scissors' in the hope that this will cause them to re-materialize. In fact he was saying 'evolution, evolution'. He'd been sent to find a good book on it.

  He had a very complicated reference card in his mouth.

  The wizards of UU knew all about evolution. It was a self-evident fact. You took some wolves, and by careful unnatural selection over the generations you got dogs of all shapes and sizes. You took some sour crab-apple trees and, by means of a stepladder, a fine paintbrush and a lot of patience, you got huge juicy apples. You took some rather scruffy desert horses and, with effort and a good stock book, you got a winner. Evolution was a demonstration of narrativium in action. Things improved. Even the human race was evolving, by means of education and other benefits of civilization; it had began with rather bad-mannered people in caves, and it had now produced the Faculty of Unseen University, beyond which it was probably impossible to evolve further.

  Of course, there were people who occasionally advanced more radical ideas, but they were like the people who thought the world really was round or that aliens were interested in the contents of their underwear.

  Unnatural selection was a fact, but the wizards knew, they knew, that you couldn't start off with bananas and get fish.

  The Librarian glanced at the card, and took a few surprising turnings. There was the occasional burst of noise on the other side of the shelves, rapidly changing as though someone was playing with handfuls of sound, and a flickering in the air. Someone talking was replaced with the absorbent silence of empty rooms was replaced with the crackling of flame and displaced by laughter ...

  Eventually, after much walking and climbing, the Librarian was faced with a blank wall of books. He stepped up to them with librarianic confidence and they melted away in front of him.

  He was in some sort of study. It was book-lined, although with rather fewer than the Librarian would have expected to find in such an important node of L-space. Perhaps there was just the one book ... and there it was, giving out L-radiation at a strength the Librarian had seldom encountered outside the seriously magical books in the locked cellars of Unseen University. It was a book and father of books, the progenitor of a whole race that would flutter down the centuries ...

  It was also, unfortunately, still being written.

  The author, pen still in hand, was staring at the Librarian as if he'd seen a ghost.

  With the exception of his bald head and a beard that even a wizard would envy, he looked very, very much like the Librarian.

  'My goodness ...'

  'Ook?' The Librarian had not expected to be seen. The writer must have something very pertinent on his mind.

  'What manner of shade are you ... ?'

  'Ook.'*

  A hand reached out, tremulously. Feeling that something was expected of him, the Librarian reached out as well, and the tips of the fingers touched.

  The author blinked.

  'Tell me, then,' he said, 'is Man an ape, or is he an angel?'

  The Librarian knew this one.

  'Ook,' he said, which meant: ape is best, because you don't have to fly and you're allowed sex, unless you work at Unseen University, worst luck.

  Then he backed away hurriedly, ooking apologetic noises about the minor error in the spacetime coordinates, and knuckled off through the interstices of L-space and grabbed the first book he found that had the word 'Evolution' in the title.

  The bearded man went on to write an even more a
mazing book. If only he had thought to use the word 'Ascent' there might not have been all that unpleasantness.

  But, there again, perhaps not.

  HEX let itself absorb more of the future ... call it ... knowledge. Words were so difficult. Everything was context. There was too much to learn. It was like trying to understand a giant machine when you didn't understand a screwdriver.

  Sometimes HEX thought it was picking up fragmentary instructions. And, further away, much further away, there were little disjointed phrases in the soup of concepts which made sense but did not seem to be sensible. Some of them arrived unbidden.

  Even as HEX pondered this, another one arrived and offered an opportunity to make $$$$ While You Sit On Your Butt!!!!! He considered this unlikely.

  The title brought back by the Librarian was The Young Person's Guide to Evolution.

  The Archchancellor turned the pages carefully. They were well illustrated. The Librarian knew his wizards.

  'And this is a good book on evolution?' said the Archchancellor.

  'Ook.'

  'Well, it makes no sense to me,' said the Archchancellor. 'I mean t'say, what the hell is this picture all about?'

  It showed, on the left, a rather hunched-up, ape-like figure. As it crossed the page, it gradually arose and grew considerably less hairy until it was striding confidently towards the edge of the page, perhaps pleased that it had essayed this perilous journey without at any time showing its genitals.