The Science of Discworld

by Terry Pratchett

  It seemed a lot fuller than he would have expected.

  'Not just blobs — there's all sorts of stuff! Some of it's wiggling ...'

  'Is that a plant or is it an animal?'

  'I'msure it's a plant.'

  'Isn't it... walking ... rather fast?'

  'I don't know. I've never seen a plant walking before.'

  The wizardery of UU was filtering back in the building as the news got around. The senior members of the faculty were clustered around the omniscope, explaining to one another, now that the impossible had happened, that of course it had been inevitable.

  'All those cracks under the sea,' said the Dean. 'And the volcanoes, of course. Heat's bound to build up over time.'

  'That doesn't explain all the different shapes, though,' said the Senior Wrangler. 'I mean, the whole sea looks like somebody had just turned over a very big stone.'

  'I suppose the blobs had time to consider their future when they were under the ice,' said the Dean. 'It suppose you could think of it as a very long winter evening.'

  'I vote for lavatories,' said the Lecturer in Recent Runes.

  'Well, I'm sure we all do,' said Ridcully. 'But why at this point?'

  'I mean that the blobs were ... you know ... excusing themselves for millions and millions of years, then you're get a lot of, er, manure ...'the Lecturer ventured.

  'A shitload,' said the Dean.

  'Dean! Really!'

  'Sorry, Archchancellor.'

  '... and we know dunghills absolutely teem with life ...' the Lecturer went on.

  'They used to think that rubbish heaps actually generated rats,' said Ridcully. 'Of course, that was just a superstition. It's really seagulls. But you saying life is, as it were, advancing by eating dead men's shoes? Or blobs, in this case. Not shoes, of course, because they didn't have any feet. And wouldn't have been bright enough to invent shoes even if they did. And even if they had been, they couldn't have done. Because there was, at that time, nothing from which shoes might be made. But apart from that, the metaphor stands.'

  'There still are blobs in there,' said the Dean. 'There's just lots of other things, too.'

  'Any of it lookin' intelligent?' said Ridcully.

  'I'mnot certain how we'd spot that at this stage ...'

  'Simple. Is anything killing something it doesn't intend to eat?'

  They stared into the teeming broth.

  'Bit hard to define intentions, really,' said the Dean, after a while.

  'Well, does anything look as if it is about to become intelligent?'

  They watched again.

  'That thing like two spiders joined together?' said the Senior Wrangler after a while. 'It looks very thoughtful.'

  'I think it looks very dead.'

  'Look, I can see how we can settle this whole evolution business once and for all,' said Ridcully, turning away. 'Mister Stibbons, can HEX use the omniscope to see if anything changes into anything else?'

  'Over a moderately sized area, I think he probably can, sir.'

  'Get it to pay attention to the land,' said the Dean. 'Is there anything happening on the land?'

  'There's a certain greenishness, sir. Seaweed with attitude, really.'

  'That's where the interesting stuff will happen, mark my words. I don't know what this universe is using for narrativium, but land's where we'll see any intelligent life.'

  'How do you define intelligence?' said Ridcully. 'In the long term, I mean.'

  'Universities are a good sign,' said the Dean, to general approval.

  'You don't think that perhaps fire and the wheel might be more universally indicative?' said Ponder carefully.

  'Not if you live in the water,' said the Senior Wrangler. 'The sea's the place here, I'll be bound. On this world practically nothing happens on the land.'

  'But in the water everything's eating each other!'

  'Then I'll look forward to seeing what happens to the last one served,' said the Senior Wrangler.

  'No, when it comes to universities, the land's the place,' said the Dean. 'Paper won't last five minutes under water. Wouldn't you say so, Librarian?'

  The Librarian was still staring into the omniscope.

  'Ook,' he said.

  'What's that he said?' said Ridcully.

  'He said "I think the Senior Wrangler might be right",' said Ponder, going over to the omniscope. 'Oh ... look at this ...'

  The creature had at least four eyes and ten tentacles. It was using some of the tentacles to manoeuvre a slab of rock against another slab.

  'It's building a bookcase?' said Ridcully.

  'Or possibly a crude rock shelter,' said Ponder Stibbons.

  'There we are, then,' said the Senior Wrangler. 'Personal property. Once something is yours, of course you want to improve it. The first step on the road to progress.'

  'I'm not sure it's got actual legs,' said Ponder.

  'The first slither, then,' said the Senior Wrangler, as the rock slipped from the creatures tentacles. 'We should help it,' he added firmly. 'After all, it wouldn't be here if it wasn't for us.'

  'Hold on, hold on,' said the Lecturer in Recent Runes. 'It's only making a shelter. I mean, the Bower Bird builds intricate nests, doesn't it? And the Clock Cuckoo even builds a clock for its mate, and no one says they're intelligent as such.'

  'Obviously not,' said the Dean. 'They never get the numerals right, the clocks fall apart after a few months, and they generally lose two hours a day. That doesn't sound like intelligence to me.'

  'What are you suggesting, Runes?' said Ridcully.

  'Why don't we send young Rincewind down again in that virtu-ally-there suit? With a trowel, perhaps, and an illustrated manual on basic construction?'

  'Would they be able to see him?'

  'Er ... gentlemen ...' said Ponder, who had been letting the eye of the omniscope drift further into the shallows.

  'I don't see why not,' said Ridcully.

  'Er ... there's a ... there's ...'

  'It's one thing to push planets around over millions of years, but at this level we couldn't even give our builder down there a heavy pat on the back,' said the Dean. 'Even if we knew which part of him was his back.'

  'Er ... something's paddling, sir! Something's going for a paddle, sir!'

  It was probably the strangest cry of warning since the famous 'Should the reactor have gone that colour?' The wizards clustered around the omniscope.

  Something had gone for a paddle. It had hundreds of little legs.

  THIRTY

  UNIVERSALS AND PAROCHIALS

  CHANCE MAY HAVE PLAYED A GREATER ROLE than we imagine in ensuring our presence on the Earth. Not only aren't we the pinnacle of evolution: it's conceivable that we very nearly didn't appear at all. On the other hand, if life had wandered off the particular evolutionary track that led to us, it might well have blundered into something similar instead. Intelligent crabs, for example. Or very brainy net-weaving jellyfish.

  We have no idea how many promising species got wiped out by a sudden drought, a collapse of some vital resource, a meteorite strike, or a collision with a comet. All we have is a record of those species that happen to have left fossils. When we look at the fossil record, we start to see a vague pattern, a tendency towards increasing complexity. And many of the most important evolutionary innovations seem to have been associated with major catastrophes…

  When we look at today's organisms, some of them seem very simple while others seem more complex. A cockroach looks a lot simpler than an elephant. So we are liable to think of a cockroach as being 'primitive' and an elephant as 'advanced', or we may talk of 'lower' and 'higher' organisms. We also remember that life has evolved, and that today's complex organisms must have had simpler ancestors, and unless we are very careful we think of today's 'primitive' organisms as being typical of the ancestors of today's complex organisms. We are told that humans evolved from something that looked more like an ape, and we conclude that chimpanzees are more primitive, in an evol
utionary sense, than we are.

  When we do this, we confuse two different things. One is a kind of catalogue-by-complexity of today's organisms. The other is a catalogue-by-time of today's organisms, yesterday's ancestors, the day before's ancestors-of-ancestors, and so on. Although today's cockroach may be primitive in the sense that it is simpler than an elephant, it is not primitive in the sense of being an ancient ancestral organism. It can't be: it's today's cockroach, a dynamic go-ahead cockroach that is ready to face the challenges of the new millennium.

  Although ancient fossil cockroaches have the same appearance as modern ones, they operated against a different backgrounds. What you needed to be a viable cockroach in the Cretaceous was probably rather different from what you need to be a viable cockroach today. In particular, the DNA of a Cretaceous cockroach was probably significantly different from the DNA of a modern cockroach. Your genes have to run very fast in order for your body to stand still.

  The general picture of evolution that theorists have homed in on resembles a branching tree, with time rising like the sap from the trunk at the bottom, four billion years in the past, to the tips of the topmost twigs, the present. Each bough, branch, or twig represents a species, and all branches point upwards. This 'Tree of Life' picture is faithful to one key feature of evolution — once a branch has split, it doesn't join up again. Species diverge, but they can't merge.*

  However, the tree image is misleading in several respects. There is, for instance, no relation between the thickness of a branch and the size of the corresponding population — the thick trunk at the bottom may represent fewer organisms, or less total organic mass, than the twig at the top. (Think about the human twig ...) The way branches split may also be misleading: it implies a kind of long-term continuity of species, even when new ones appear, because on a tree the new branches grow gradually out of the old ones. Darwin thought that speciation — the formation of new species — is generally gradual, but he may have been wrong. The theory of 'punctuated equilibrium' of Stephen Jay Gould and Niles Eldredge maintains the contrary: speciation is sudden. In fact there are excellent mathematical reasons for expecting speciation to have elements of both — sometimes sudden, sometimes gradual.

  Another problem with the Tree of Life image is that many of its branches are missing — many species go unrepresented in the fossil record. The most misleading feature of all is the way humans get placed right at the top. For psychological reasons we equate height with importance (as in the phrase 'your royal highness'), and we rather like the idea that we're the most important creature on the planet. However, the height of a species in the Tree of Life indicates when it flourished, so every modern organism, be it a cockroach, a bee, a tapeworm, or a cow, is just as exalted as we are.

  Gould, in Wonderful Life, objected to the 'tree' image for other reasons, and he based his objections on a remarkable series of fossils preserved in a layer of rock known as the Burgess Shale. These fossils, which date from the start of the Cambrian era,* are the remains of soft-bodied creatures living on mud-banks at the base of an algal reef, which became trapped under a mudslide. Very few fossils of soft-bodied creatures exist, because normally only the harder parts survive fossilization. However, the significance of the Burgess Shale fossils went unrecognized from their discovery by Charles Walcott in 1909, until Harry Whittington took a closer look at them in 1971. The organisms were all squashed flat, and it was virtually impossible to recognize what shape they'd been while alive. Then Simon Conway Morris teased the squished layers apart, and reconstructed the original forms using a computer — and the strange secret of the Burgess Shale was revealed to the world.

  Until that point, palaeontologists had classified the Burgess Shale organisms into various conventional types — worms, arthropods, whatever. But now it became clear that most of those assignments were mistaken. We knew, for example, just four conventional types of arthropod: trilobites (now extinct), chelicerates (spiders, scorpions), crustaceans (crabs, shrimp), and uniramians (insects and others). The Burgess Shale contains representatives of all of these — but it also contains twenty other radically different types. In that one mudslide, preserved in layers of shale like pressed flowers in the pages of a book, we find more diversity than in the whole of life today.

  Musing on this amazing discovery, Gould realized that most branches of the Tree of Life that grew from the Burgess beasts must have 'snapped off' by way of extinction. Long ago, 20 of those 24 arthropod body plans disappeared from the face of the Earth. The Grim Reaper was pruning the Tree of Life, and being heavy-handed with the shears. So Gould suggested that a better image than a tree would be something like scrubland. Here and there 'bushes' of species sprouted from the primal ground level. Most, however, ceased to grow, and were pruned to a standstill hundreds of millions of years ago. Other bushes grew to tall shrubs before stopping ... and one tall tree made it right up to the present day. Or maybe we've reconstructed it incorrectly, amalgamating several different trees into one.

  This new image changes our view of human evolution. One animal in the Burgess Shale, named Pikaia, is a chordate. This is the group that evolved into all of today's animals that have a spinal cord, including fishes, amphibians, reptiles, birds, and mammals. Pikaia is our distant ancestor. Another creature in the Burgess Shale, Nectocaris, has an arthropod-like front end but a chordate back, and it has left no surviving progeny. Yet they both shared the same environment, and neither is more obviously 'fit' to survive than the other. Indeed, if one had been less evolutionarily fit, it would almost certainly have died out long before the fossils were formed. So what determined which branch survived and which didn't? Gould's suggestion was: chance.

  The Burgess Shale formed on a major geological boundary: at the end of the Precambrian era and the start of the Palaeozoic. The early part of the Palaeozoic is known as the Cambrian period, and it is a time of enormous biological diversity — the 'Cambrian explosion'. The Earth's creatures were recovering from the mass extinction of the Ediacarans, and evolution took the opportunity to play new games, because for a while it didn't matter much if it played them badly. The 'selection pressure' on new body-plans was small because life hadn't fully recovered from the big die-back. In these circumstances, said Gould, what survives and what does not is mostly a matter of luck — mudslide or no mudslide, dry climate or wet. If you were to re-run evolution past this point, it's quite likely that totally different organisms would survive, different branches of the Tree of Life would be snipped off.

  Second time round, it could easily be our branch that got pruned.

  This vision of evolution as a 'contingent' process, one with a lot of random chance involved, has a certain appeal. It is a very strong way to make the point that humans are not the pinnacle of creation, not the purpose of the whole enterprise.* How could we be, if a few random glitches could have swept us from the board altogether? However, Gould rather overplayed his hand (and he backed off a bit in subsequent writings). One minor problem is that more recent reconstructions of the Burgess Shale beasts suggests that their diversity may have been somewhat overrated — though they were still very diverse.

  But the main hole in the argument is convergence. Evolution settles on solutions to problems of survival, and often the range of solutions is small. Our present world is littered with examples of 'convergent evolution', in which creatures have very similar forms but very different evolutionary histories. The shark and the dolphin, for instance, have the same streamlined shape, pointed snout, and triangular dorsal fin. But the shark is a fish and the dolphin is a mammal.

  We can divide features of organisms into two broad classes: universals and parochials. Universals are general solutions to survival problems — methods that are widely applicable and which evolved independently on several occasions. Wings, for instance, are universals for flight: they evolved separately in insects, birds, bats, even flying fish. Parochials happen by accident, and there's no reason for them to be repeated. Our foodway crosses our
airway, leading to lots of coughs and splutters when 'something goes down the wrong way'. This isn't a universal: we have it because it so happened that our distant ancestor who first crawled out of the ocean had it. It's not even a terribly sensible arrangement — it just works well enough for its flaws not to count against us when combined with everything else that makes us human. Its deficiencies were tolerated from the first fish-out-of-water, through amphibians and dinosaurs, to modern birds, and from amphibians through mammal-like reptiles to mammals like us. Because evolution can't easily 'un-evolve' major features of body-plans, we're stuck with it.

  If our distant ancestors had got themselves killed off by accident, would anything like us still be around? It seems very unlikely that creatures exactly like us would have turned up, because a lot of what makes us tick is parochials. But intelligence looks like a clear case of a universal — cephalopods evolved intelligence independently of mammals, and anyway, intelligence is such a generic trick. So it seems likely that some other form of intelligent life would have evolved instead, though not necessarily adhering to the same timetable. On an alternative Earth, intelligent crabs might invent a fantasy world shaped like a shallow bowl that rides on six sponges on the back of a giant sea urchin. Three of them could at this very moment be writing The Science of Dishworld.

  Sorry. But it is true. But for a fall of rock here, a tidal pattern there, we wouldn't have been us. The interesting thing is that we almost certainly would have been something else.

  There’s a silly reason for this, and a sensible one. The silly reason is that species are usually defined to be different if they don’t interbreed. If two separate species don’t interbreed, it’s difficult to put them back together again. The sensible one is that evolution occurs by random mutations — changes to the DNA code — followed by selection. Once a change has occurred, it’s unlikely for it to be undone by further random mutations. It’s like driving along country roads at random, reaching some particular place, and then continuing at random. What you don’t expect is to reverse your previous path and end up back where you started.