The Science of Discworld

by Terry Pratchett

  Stories of people seeing ghosts may well have the same explanation. You've read the tales, you know what a ghost ought to look like (maybe you watched Ghostbusters or a Stephen King movie), and you're trying to sit up all night in the Haunted House. You're thinking about ghosts, about headless horsemen and Elizabethan ladies who walk through walls and go transparent, and then you start to doze off because it's 2 am and you've been up all night... The sleep paralysis circuit glitches ... Aaaaagh!

  OK, if you insist … Our favoured line here is 'hairy'. But hairs don’t fossilize, so how can you tell? If you have hair, you need grooming. All over the body. This requires flexible backbones, and you can tell how flexible they are from the shape of the vertebrae. Which do fossilize. (Sometimes scientists can be very ingenious.) Evolution crossed that line about 230 million years ago.

  How many recipe books do you have that tell you to boil water, but never specify the altitude at which this should be done? It matters: higher up, water boils at lower temperatures.

  FORTY-ONE

  DON'T PLAY GOD

  THE ARCHCHANCELLOR WAS RATHER QUIET OVER TEA.

  Eventually he said, 'Can we stop this project, Ponder?'

  'Er ... are you sure, sir?'

  'Well, what is it achieving? I mean, really? Y'know, I thought, all you had to do is get a world working, and before you could say "creation" there'd be some creature who'd stand up, getting a grip on its surroundings, gaze with a certain amount of intelligence and awe at the infinite sky and say —’

  ‘...that thing's getting bigger, I wonder if it's going to hit us,' said Rincewind.

  'Rincewind, that remark was extremely cynical and accurate.'

  'Sorry, Archchancellor.'

  Bonder's lips were moving quietly as he worked things out.

  'We could start running it down, yes. The thaumic reactor hasn't been putting so much into it in the last week. We've nearly used up the fuel.'

  'Really?'

  'The squash court will have a rather high thaumic index, sir, so whoever goes in to pull the switch will suffer a certain amount of —’

  There was the sound of something spinning. The wizards looked at Rincewind's chair, which finally fell over on to the flagstones. Of its former occupant there was no sign, although there was the distant sound of a slamming door.

  The Dean sniffed.

  'Strange behaviour,' he said.

  'I suggest we give it one more day of our time,' said Ridcully. 'I was hoping we might create a world, gentlemen, but instead it's clear to me that any life in this universe has to get used to living in ... in some kind of huge celestial snow globe. Fire and ice, ice and fire. Gentlemen, round worlds are intrinsically flawed. If there's any hidden gods on ours, they're pretty damn well hidden.'

  'The Omnians say "Don't play God. He always wins",' said the Senior Wrangler.

  'I dare say,' said Ridcully 'So ... one more day, gentlemen? And then we can get on with something sensible.'

  The red sun rose quickly over the parched veldt. The apes stirred in their cave, which was little more than a rocky overhang, and saw the big black rectangle looming over them.

  The Dean tapped it with his pointer

  'Do try to pay attention today, will you?' He turned and chalked rapidly across the blackboard. 'Here we have R ... O ... C ... K, rock. Can anyone tell me what you do with it? Anyone? Anyone? Look, stop doing that, will you?' He tried to hit an ape with his virtual pointer, and then flung it away in disgust. It vanished.

  'Filthy little devils,' he muttered.

  'Not getting anywhere, Dean?' said Ridcully, appearing beside him.

  'No, Archchancellor. I've tried to explain to them that they've probably got just a few million years, and that's pretty hard to do in sign language, let me tell you. But the only word they know is S-E-X, and they don't waste time spelling it, oh no! For this I skipped breakfast?'

  'Never mind. Let's see how the Senior Wrangler's getting on.'

  'They're just bad copies of humans, if you ask me —’

  The wizards vanished.

  One of the apes knuckled over to the blackboard, and watched it disappear from view as HEX completed the spell.

  He hadn't the faintest idea what had been happening, but he had been impressed by the stick that had been waved about. That seemed to have gone now. That didn't worry the ape, which knew about things vanishing — often, these days, a member of the clan would vanish overnight, with a lot of snarling in the shadows.

  There was probably something you could do with a stick, he thought. Hopefully, it might involve sex.

  He poked around in the debris and found not a stick but a dried-up thighbone, which had a sufficiently stick-like shape.

  He rattled it on the ground a few times. It didn't do anything much. Then he reluctantly decided it would probably be impossible to mate with at the moment, and hurled it high into the air.

  It rose, turning over and over.

  When it fell, it knocked him unconscious.

  The Senior Wrangler was sitting under a virtually-there beach umbrella when the other wizards arrived. He looked as downcast as the Dean.

  A group of apes was playing in the surf.

  'Worse than the lizards,' he said. 'They had some style, at least. When this lot pick up anything, they try to see if they can eat it. What's the point of that?'

  'Well, I suppose they can find out if it's edible,' said Ridcully.

  'But they just mess about,' said the Senior Wrangler. 'Oh, no ... here we go again ...'

  There was a raucous shrieking as the tribe rushed out of the waves and swung up into the nearby mangrove trees. A shadow sped beyond the surf and headed back into the blue water, to an unregarded chorus of simian catcalls and mangrove seeds.

  'Oh yes, and they like throwing things,' said the Senior Wrangler.

  'Seafood is good for the brain, my granny always said,' said Ridcully.

  'This lot couldn't eat too much of it, then. Yell, throw things, and prod stuff to see what it does, that's the extent of their capabilities. Oh, why didn't we discover the lizards earlier? They had class.’

  'Wouldn't have stopped the snowball,' said Ridcully.

  'No. You were right, Archchancellor. It's so pointless.'

  The three wizards stood looking gloomily out to sea. In the middle distance, dolphins stitched their way across the water.

  'Should be coming up to coffee time,' said the Dean, to break the silence.

  'Good thinking, that man.'

  Rincewind was wandering in the next bay, staring at the cliffs. Oh, things were killed off on the Discworld, but... well... sensibly. There were floods, fires and, of course, heroes. There was nothing like a hero for a species whose number was up. But at least some actual thought went into it.

  The cliff was a series of horizontal lines. They represented ancient surfaces, some of which Rincewind had virtually walked on. And in many of them were the bones of ancient creatures, turned into stone by a process Rincewind did not understand and rather distrusted. Life had some how come out of the rocks of this world, and here you could see it going back. There were whole layers of rock made out of life, millions of years of little skeletons. Faced with a natural wonder on that scale, you could only be overawed by the sheer chasms of time or else try to find someone to complain to.

  A few rocks fell out, halfway up the cliff. A couple of small legs waved uncertainly in the strata, and then the Luggage tumbled out, slid down the pile of debris at the foot of the cliff, and landed on its lid.

  Rincewind watched it struggle for a while, sighed, and pushed it the right way up. At least some things didn't change.

  FORTY-TWO

  ANTHILL INSIDE

  YOU KNOW WHAT'S GOING TO HAPPEN TO THE APES — they're going to turn into us.* But why do we have them playing in the surf? Because it's fun? Yes ... but more significantly, because the seashore is central to one of the two main theories about how our ape ancestors acquired big brains. The other, more orthodox t
heory places the evolution of the big brain out on the African savannahs, and we know that some of our ancestors lived on the savannahs because we've found fossils. Unfortunately, seashores aren't a good place to leave fossils. You often find them there, but that's because they were deposited when the area wasn't a seashore at all, and the sea has subsequently eroded the rocks to expose the fossils. In the absence of direct evidence of this kind, the surfing apes theory has to take second place ... but it does explain our brains rather neatly, whereas the savannah theory rather sidesteps this issue.

  Our closest living relatives are two species of chimpanzee: the standard boisterous 'zoo' chimp Pan troglodytes and its more slender cousin the bonobo (or pygmy) chimp Pan paniscus. Bonobos live in very inaccessible parts of Zaire, and weren't recognized as a separate species of chimpanzee until 1929. We can to some extent unravel the past evolutionary history of the great apes by comparing their DNA sequences. Human DNA differs from the DNA of either chimpanzee by a mere 1.6% — that is, we have 98.4% of our DNA sequences in common with theirs. (It is interesting to speculate on what the Victorians would have made of this.) The two species of chimpanzee have DNA that differs by only 0.7%. Gorillas differ from us, and from both chimps, by 2.3%. For orangutans, the difference from us is 3.6%.

  These differences may seem small, but you can pack an awful lot into a small percentage of an ape genome. A big chunk of what we have in common must surely consist of 'subroutines' that organize basic features of vertebrate and mammalian architecture, tell us how to be an ape, and tell us how to deal with things we've all got — like hair, fingers, internal organs, blood ... The mistake is to imagine that everything that makes us human and not a chimpanzee must live in that other 1.6% of 'special' DNA — but DNA doesn't work that way. For example, some of the genes in that 1.6% of the genome may organize the other 98.4% in a completely new way. If you look at the computer code for a wordprocessor and a spreadsheet, you'll find they have an awful lot in common — routines for reading the keyboard, printing to the screen, searching for a given text string, changing fonts to italic, responding to a click on the mouse ... but this doesn't mean that the only distinction between a spreadsheet and a wordprocessor lies in the relatively few routines that are different.

  Since evolution involves changes to DNA, we can use the sizes of those differences to estimate when various ape species diverged from each other. This method was introduced by Charles Sibley and Jon Ahlquist in 1973, and while it needs to be interpreted with caution, it works well here.

  A convenient unit of time for such discussions is the 'Grandfather', which we define to be 50 years. It's a good human length, being about the age difference between the child and the grandparent who says 'When I was young ...' and passes on a sense of history. In these terms, Christ lived 40 Grandfathers ago, and the Babylonians go back about 100 Grandfathers. That's not a lot of grandads, passing down through recorded human history recollections like '... we never had any of this modern cuneiform when I was a lad ...' and '... bronze was good enough for me'. Human time is not very deep. We've just been good at packing a lot into it.

  DNA studies indicate that the two chimp species diverged about 60,000 Grandfathers ago — three million years. Humans and chimps diverged 80,000 Grandfathers earlier — so a chain of only 140,000 grandfathers unites you and your chimplike ancestor. Who was also, we hasten to point out, a modern chimpanzee's manlike ancestor. Humans and gorillas diverged 200,000 Grandfathers ago; humans and orangutans diverged 300,000 Grandfathers ago. So among these animals, we are most closely related to a chimpanzee, and least closely related to the orangutan. This conclusion is borne out by physical appearance and habits, too. Bonobos really like sex.

  If those times seem rather short for all the necessary evolutionary changes, bear two things in mind. First, that they were estimated by using a realistic rate for DNA mutations; second, that according to Nilsson and Pelger an entire eye can evolve in a mere 8,000 Grandfathers — and lots of different changes can, should, and did evolve in parallel.

  The most striking feature of humans is the size of our brains: bigger, in comparison to body weight, than any other animal. Strikingly bigger. A detailed story of what makes us human must be extraordinarily complicated, but it's clear that big, powerful brains were the main invention that made it all possible. So we now have two obvious questions to think about: 'Why did we evolve big brains?' and 'How did we evolve big brains?'

  The standard theory addresses the 'why'. It maintains that we evolved out on the savannahs, surrounded by lots of big predators — lions, leopards, hyenas — and without much cover. We had to become smart in order to survive. Rincewind would instantly see one flaw in this theory: 'If we were so smart, why did we stay on the savannahs, surrounded by lots of big predators?' But, as we've said, it fits the fossil evidence. The unorthodox theory addresses the 'how'. Big brains need lots of brain cells, and brain cells need lots of chemicals known as 'essential fatty acids'. We have to get these from our food — we can't build them ourselves from anything simpler — and they're in short supply out on the savannahs. However, as Michael Crawford and David Marsh pointed out in 1991, they are abundant in seafood.

  Nine years earlier Elaine Morgan had developed Alister Hardy's theory of the 'aquatic ape': we evolved not on the savannahs, but on the seashore. The theory fits a number of human peculiarities: we like water (newborn babies can swim), we have a funny pattern of hair on our bodies, and we walk upright. Go to any Mediterranean resort and you see at once that an awful lot of naked apes think that the seashore is the place to hang out.

  Whether the Aquatic Ape story of human origins will displace the 'savannah' theory remains to be seen, but the savannah story is in trouble from a very different direction. Phillip Tobias was challenged not the fossil record, but its interpretation. He asked a question so simple that it seems to have eluded most other workers in the field. Yes, many areas where fossils of apelike human ancestors have been found are savannahs now. But were they savannahs then? When our distant great-great-… grandparents got themselves fossilised, 2.7 million years ago, could the vegetation have been different from what it is today?

  Given that the whole point is that the animals definitely were different — our ancestors, not us — it is a little surprising that this question seems no to have occurred to anyone earlier. Unfortunately, science is often like that. People specialise. Experts on prehistoric apes may not be very interested in botany.

  It turns out that Sterkfontein, one the places where fossil apes supporting the savannah theory are found, wasn’t a savannah back then at all. Fossil pollen suggested it was a wooded area, and fossilised lianas clinched it. Other areas of South Africa and Ethiopia (where the famous 'Lucy' was found) show that these areas were forests when the apes lived there. The 'killer ape of the savannahs,' says Tobias, is nonsense.

  And there may be some new evidence in favour of watery origins for humanity, though not necessarily the full-blooded Aquatic Ape. A common feature of fossil hominid sites is that they are all near water. This makes sense, because Homo sapiens needs to drink a lot, and sweats and urinates a lot. If we had evolved on savannahs, we would have annoyed the hell out of all the other animals there with our incessant peeing. And it looks like we were excellent swimmers at least a million years ago. There is evidence of human migrations to islands such as Flores, separated from Bali by a deep underwater valley. Even allowing for the lower sea-levels in the past, the new arrivals must have swum or rafted or otherwise made their way across at least 20 miles of open water.

  We may not have been the Aquatic Ape, but we surely were the Damp Forest Ape. Just as bonobos, one of our two closest living relatives, are today.

  Brains are fascinating. They are the physical vehicle for minds, which are even more interesting. Minds are (or, at least, give their owners the vivid impression that they are) conscious, and they have (or, at least, give their owners the vivid impression that they have) free will. Minds operate in a world of
'qualia' — vivid sense impressions like red, hot, sexy. Qualia aren't abstractions: they are 'feelings'. We all know what it's like to experience them. Science has no idea what makes them the way they are.

  Brains, though ... we can make progress on brains. On one level, brains are a kind of computational device. Their most obvious physical components are nerve cells, arranged in complicated networks. Mathematicians have studied such networks, and they find that what networks do is to carry out interesting processes. Give them an input and they will produce an output. Allow their interconnections to evolve by selecting for specific associations of input and output — such as responding to an image of a banana but not to an image of a dead rat — and pretty soon you've got a really effective banana-detector.

  What makes the human brain unique, as far as we can tell, is that it has become recursive. As well as detecting a banana, it can think about detecting a banana. It can think thoughts about its own thought processes. It is a pattern-recognition device that has turned its attention to its own patterns. This ability is what lies behind human intelligence. It probably underpins consciousness, too: one of the patterns that the pattern-recognition device has learned to recognize is itself. It has become 'self-aware'.

  As a result, brains operate on at least two levels. On a reductionist level they are networks of nerve cells sending each other incredibly complex but ultimately meaningless messages — like ants scurrying around inside an anthill. On another level, they are an integrated self — the anthill as a personality in its own right. Douglas Hofstadter's Gödel, Escher, Bach includes a sequence where Aunt Hillary (who is an anthill — use the American pronunciation of 'Aunt') has a meeting with Dr Anteater. When Dr Anteater arrives, the ants go into a panic — they change their actions. To Aunt Hillary, who operates on the emergent level, this change represents the knowledge that Dr Anteater has arrived. She is entirely happy to watch Dr Anteater consuming a meal of 'her' ants. Ants are a virtually inexhaustible resource, she can always breed new ones to take the place of the ones that got eaten.