The Science of Discworld Revised Edition

by Terry Pratchett

  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.1

  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,2 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. (Some good deposits are now known in China, too.) 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.
3 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.

  1 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.

  2 According to the most recent dating methods, the Cambrian began 543 million years ago. The Burgess shale was deposited about 530–520 million years ago.

  3 In the words of Discworld’s God of Evolution: ‘The purpose of the whole thing is to be the whole thing.’

  THIRTY-ONE

  GREAT LEAP SIDEWAYS

  RINCEWIND WAS IN his new office, filing rocks. He’d worked out quite a good system, based on size, shape, colour and twenty-seven other qualities including whether or not he felt that it was a friendly sort of rock.

  With careful attention to cross-referencing, he reckoned that dealing with just those rocks in this room would take him at least three quiet, blessed years.

  And he was therefore surprised to find himself picked up bodily and virtually carried towards the High Energy Magic building holding, in one hand, a hard square light grey rock and, in the other hand, a rock that appeared to be well disposed to people.

  ‘Is this yours?’ roared Ridcully, stepping side to reveal the omniscope.

  The Luggage was now bobbing contently a few metres offshore.

  ‘Er …’ said Rincewind. ‘Sort of mine.’

  ‘So how did it get there?’

  ‘Er … it’s probably looking for me,’ said Rincewind. ‘Sometimes it loses track.’

  ‘But that’s another universe!’ said the Dean.

  ‘Sorry.’

  ‘Can you call it back?’

  ‘Good heavens, no. If I could call it back, I’d send it away.’

  ‘Sapient pearwood is meta-magical and will track its owner absolutely anywhere in time and space,’ said Ponder.

  ‘Yes, but not this bit!’ said Ridcully.

  ‘I don’t recall “not this bit” ever being recorded as a valid subset of “time and space”, sir,’ said Ponder. ‘In fact, “not this bit” has never even been accepted as a valid part of any magical invocation, ever since the late Funnit the Foregetful tried to use it as a last-minute addition to his famously successful spell to destroy the entire tree he was sitting in.’

  ‘The Luggage may consist of a subset of at least n dimensions which may co-exist with any other set of >n dimensions,’ said the Bursar.

  ‘Don’t pay any attention, Stibbons,’ said Ridcully wearily. ‘He’s been spouting this stuff ever since he tried to understand HEX’s write-out. It’s completely gibberish. What’s “n”, then, old chap?’

  ‘Umpt,’ said the Bursar.

  ‘Ah, imaginary numbers again,’ said the Dean. ‘That’s the one he says should come between three and four.’

  ‘There isn’t a number between three and four,’ said Ridcully.

  ‘He imagines there is,’ said the Dean.

  ‘Could we get inside the Luggage in order to physically go into the project universe?’ said Ponder.

  ‘You could try,’ said Rincewind. ‘I personally would rather saw my own nose off.’

  ‘Ah. Really?’

  ‘But the thought occurs,’ said Ridcully, ‘that we can use it to bring things back. Eh?’

  Down under the warm water, the strange creature’s stone structure collapsed for the umpteenth time.

  A week went past. On Tuesday a left-over snowball collided with the planet, causing considerable vexation to the wizards and destroying an entire species of net-weaving jellyfish of which the Senior Wrangler had professed great hopes. But at least the Luggage could be used to bring back any specimens stupid enough to swim into something sitting underwater with its lid open, and this included practically everything in the sea at the moment.

  Life in the round world seemed to possess a quality so prevalent that the wizards even discussed the idea that it was some conceptual element, which was perhaps trying to fill the gap left by the non-existent deitygen.

  ‘However,’ Ridcully announced, ‘Bloodimindium is not a good name.’

  ‘Perhaps if we change the accent slightly,’ said the Lecturer in Recent Runes. ‘Blod-di-min-dium, do you think?’

  ‘They’ve certainly got a lot of it, whatever we call it,’ said the Dean. ‘It’s not a world to let a complete catastrophe get it down.’

  Things turned up. Shellfish suddenly seemed very popular. A theory gaining ground was that the world itself was generating them in some sort of automatic way.

  ‘Obviously, if you have too many rabbits, you need to invent foxes,’ said the Dean, at one of the regular meetings. ‘If you’ve got fish, and you want phosphates, you need seabirds.’

  ‘That only works if you have narrativium,’ said Ponder. ‘We’ve got no evidence, sir, that anything on the planet has any concept of causality. Things just live and die.’

  And then, on Thursday, the Sen
ior Wrangler spotted a fish. A real, swimming fish.

  ‘There you are,’ he said triumphantly. ‘The seas are the natural home of life. Look at the land. It’s just rubbish, quite frankly.’

  ‘But the sea’s not getting anywhere,’ said Ridcully. ‘Look at those tentacled shellfish you were trying to educate yesterday. Even if you so much as made a sudden movement they just squirted ink at you and swam away.’

  ‘No, no, they were trying to communicate,’ the Senior Wrangler insisted. ‘Ink is a natural medium, after all. Don’t you get the impression that everything is striving? Look at them. You can see them thinking, can’t you?’

  There were a couple of the things in a tank behind him, peering out of their big spiral shells. The Senior Wrangler had the idea that they could be taught simple tasks, which they would then pass on to the other ammonites. They were turning out to be rather a disappointment. They might be good at thinking, ran the general view, but they were pants at actually doing anything about it.

  ‘That’s because here’s no point in being able to think if you haven’t got much to think about,’ said the Dean. ‘Damn all to think about in the sea. Tide comes in, tide goes out, everything’s damp, end of philosophical discourse.’

  ‘Now these are the chaps,’ he went on, strolling along to another tank. The Luggage had been quite good as a collector, provided the specimens didn’t appear to be threatening Rincewind.

  ‘Hmph,’ sniffed the Senior Wrangler. ‘Underwater woodlice.’

  ‘But there’s a lot of them,’ said the Dean. ‘And they have legs. I’ve seen them on the seashore.’

  ‘By accident. And they haven’t got anything to use as hands.’

  ‘Ah, well, I’m glad you’ve pointed that out …’ said the Dean, walking along to the next aquarium.