Terry Pratchett - The Science of Discworld

by Terry Pratchett

  UNIVERSALSAND 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 evolutionary 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.[41]

  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,[42] 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 ani T mal 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 r
e-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.[43] 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.

  THIRTY-ONE

  THE FUTURE IS NEWT

  HEX WAS THINKING HARD AGAIN. Running the little universe was taking much less time than k had expected. It more or less ran itself now, in fact. The gravity operated without much attention, rainclouds formed with no major interference and rained every day. Balls went around one another.

  HEX didn’t think it was a shame about the crabs going. HEX had­n’t thought it was marvellous that the crabs had turned up. HEX thought about the crabs as something that had happened. But it had been interesting to eavesdrop on Crabbity - the way the crabs named themselves, thought about the universe (in terms of crabs), had legends of the Great Crab clearly visible in the Moon, passed on in curious marks the thoughts of great crabs, and wrote down poetry about the nobility and frailty of crab life, being totally accu­rate, as it turned out, on this last point.

  HEX wondered: if you have life, then intelligence will arise some­where. If you have intelligence, then extelligence will arise somewhere. If it doesn’t, intelligence hasn’t got much to be intelli­gent about. It was the difference between one little oceanic crustacean and an entire wall of chalk.

  The machine also wondered if it should pass on these insights to the wizards, especially since they actually lived in one of the world’s more interesting outcrops of extelligence. But HEX knew that its creators were infinitely cleverer than it was. And great masters of disguise, obviously.

  The Lecturer in Recent Runes had designed a creature.

  ’Really, all we need is a basic limpet or whelk, to begin with,’ he said, as they looked at the blackboard. ’We bring it back here where proper magic works, try a few growth spells, and then let Nature take its course. And, since these extinctions seem to be wiping out everything, it’ll gradually become the dominant feature.’

  ’What’s the scale again?’ said Ridcully, critically.

  ’About two miles to the tip of the cone,’ said the Lecturer. ’About four miles across the base.’

  ’Not very mobile, then,’ said the Dean.

  ’The weight of the shell will certainly hamper it, but I imagine it should be able to move its own length in a year, perhaps two.’

  ’What’ll it eat, then?’

  ’Everything else.’

  ’Such as...’

  ’Everything. I’d advise suction holes around the base here so that it can filter seawater for useful things like plankton.’

  ’Plankton being-?’

  ’Oh, whales, shoals offish and so on.’

  The wizards looked long and hard at the huge

  cone-shaped object.

  ’Intelligence?’ said Ridcully

  ’What for?’ said the Lecturer in Recent Runes.

  ’Ah.’

  ’It will withstand anything except a direct hit with a

  comet, and I estimate it’ll have a lifespan of about 500,000 years.’

  ’And then it’ll die?’ said Ridcully. ’Yes. I estimate it will, by then, take it twenty-four hours and one second to absorb enough food to last it for twenty-four hours.’

  ’So after that it will be dead?’

  ’Yes.’

  ’Will it know?’

  ’Probably not.’

  ’Back to the drawing board, Senior Lecturer.’

  Ponder sighed.

  ’It’s no good ducking,’ he said. ’That won’t help. We’re

  paying special attention to comets. We’ll let you know

  in plenty of time.’

  ’You’ve got no idea what it was like!’ said Rincewind,

  creeping along the beach. ’And the noise!’

  ’Have you seen the Luggage?’

  ’It certainly made my ears ring, I can tell you!’

  ’And the Luggage?’

  ’What? Oh ... gone. Have you looked at that side of the

  planet? There’s a whole new set of mountain ranges!’

  The wizards had let time run forward for a while after the strike. It made such a depressing mess of everything. Now, drawing on its bottomless reserves of bloodimindium, life was returning in strength. Crabs were already back although, here, at least, they did­n’t seem inclined to make even simple structures. Perhaps something in their souls told them it’d be a waste of time in the long run.

  Rincewind mentally crossed them off the list. Look for signs of intelligence, the Archchancellor had said. As far as Rincewind was concerned, anything really intelligent would be keeping out of the way of the wizards. If you saw a wizard looking at you, Rincewind would advise, then you should walk into a tree or say ’dur?’.

  All along the beaches, and out below the surf, everything was acting with commendable stupidity.

  A soft sound made him look down. He’d almost stepped on a fish.

  It was some way from the water line, and squirming across the mud towards
a pool of brackish water.

  A kind man by nature, Rincewind picked it up gingerly and car­ried it back to the sea. It flopped around in the shallows for a while and then, to his amazement, inched its way back on to the mud.

  He put it back again, in deeper water this time.

  Thirty seconds kter, it was back on the beach.

  Rincewind crouched down, as the thing wiggled determinedly onwards.

  ’Would it help to talk to someone?’ he said. ’I mean, you’ve got a good life out there in the sea, no sense in throwing it all away, is there? There’s always a silver lining if you know where to look. Okay, okay, life’s a beach. And you’re a pretty ugly fish. But, you know, beauty is only sk- scale deep, and -’

  ’What’s happening?’ said Ponder’s voice in his ear.

  ’I was talking to this fish,’ said Rincewind.

  ’Why?’

  ’It keeps coming out of the water. It seems to want to go for whatever is the opposite of a paddle.’

  ’Well?’

  ’You told me to keep a look out for anything interesting.’

  ’The consensus here is that fish aren’t interesting,’ said Ponder. ’Fish are dull.’

  ’I can see bigger fish in the shallows,’ said Rincewind. Terhaps it’s trying to keep away from them?’

  ’Rincewind, fish are designed for living in water That’s why they’re fish. Go and find some crabs. And put the poor freakish thing back in the sea, for goodness’ sake.’

  ’Perhaps a rethink is in order here,’ said Ridcully.

  ’About the newts,’ said Ponder.

  ’Newts is going far too far,’ said the Dean. ’I’ve seen more shapely things in the privy.’

  ’I want the person who put the newts on this continent to own up right now,’ said Ridcully.

  ’No one could,’ said the Senior Wrangler. ’No one’s seen the Luggage since the last comet. We couldn’t get anything in there.’