The Science of Discworld II - The Globe tsod-2 Page 13

  At the end of the twentieth century there was, for some years, a belief that Homo sapiens was a polyphyletic species. This word means that different groups of Homo sapiens evolved from different groups of Homo erectus in different places. This, it was thought, might account for the racial differences, especially differences in skin pigmentation, that seemed to fit geography pretty well. From DNA studies, we now know this theory can't be true. On the contrary, there was a bottleneck in our population as we came out of Africa -humanity was reduced to rather small numbers -and all of us living today, all of the out-of-Africa 'races', were extracted from that small population. All the Homo erectus died out. The evidence so far looks as if there was only one exodus, of a minimum of some 100,000 people. We were all there in potentia in that tiny population, Japanese and Eskimos and Norsemen and Sioux and Beaker people and Mandarin Chinese; Indians and Jews and Irishmen. In the same way, all the current kinds of dog were 'present' in the original domesticated wolf (assuming it was indeed a wolf) -that is, they were in the wolfs space of the adjacent possible -and we've pulled out Saint Bernards and chihuahuas and labradors and King Charles spaniels and poodles from that local region of organism-space.

  There was, about thirty years ago, a brief fashion for the concept of 'mitochondrial Eve', and many media reports seem to have picked up the idea that there was just one woman, a veritable Eve, in that ancestral bottleneck. This is nonsense, but the media reports were written up to encourage the belief. The real story, as always, was a little more complicated, and it goes like this. There are mitochondria in the cells of people, indeed of most animals and plants. These are the billions-of-generations, descendants of symbiotic bacteria, and they still have some of their ancient DNA heredity, called mitochondrial DNA. Mitochondria from the mother go into the embryo's cells, but those from father do not: they die, or go only into the placenta. In any event, mitochondrial inheritance is very nearly all maternal. The mitochondrial DNA accumulates mutations over time, with important genes changing less (presumably because the resulting babies, if any, were defective) and some DNA sequences changing quite quickly. That enables us to judge how far back it is to the common ancestor of any pair of women, from the accumulated differences in several DNA sequences. Surprisingly, nearly all such pairs from very different women converge on to a single consensus sequence, about 70,000 years ago.

  A single woman, the ancestor of us all.

  Eve?

  Well, that was the story that the media latched on to, and you can see why. However, it doesn't hang together. The occurrence of just one mitochondrial DNA sequence doesn't mean that there was just one woman with that sequence, or that she was the ancestress of all the other women whose DNA was sequenced. Evidence based on the current diversity of various genes shows that there were at least 50,000 women in the human population 70,000 years ago, and many of them will have had that particular DNA sequence, or one that cannot be distinguished from it with the evidence remaining today. The lineages of the women who did not have that sequence continued for some time, but eventually died out: their 'branch' of the human family tree doesn't reach all the way to the present day. We can't be certain why those lineages died out, but in mathematical models such effects are commonplace. Perhaps the women carrying sequences like today's sole survivor were more 'fit', or they simply came to outnumber the others by chance. It is even possible that the choice of the contemporary women to test was in some way biased, and that more than one mitochondrial DNA sequence is actually present in today's women.

  How do we know that there were at least 100,000 humans 70,000 years ago, and not, as in the stories, just two 6,000 years ago? Many (about 30 per cent) of the genes in the cell nucleus have several versions in today's human population. Like most 'wild' populations (not bred in the laboratory or for dog shows), each individual human has two versions of about 10 per cent of his or her genes, different versions received from father and mother in sperm and egg. Humans have roughly 30,000 genes, of which about 3,000 will be represented by two versions in the average person. For some genes, notably those of the immune system that give each of us a very specific lock-and-key individuality, making us susceptible to some ailments but resistant to others, there are hundreds of versions of each gene (of four important ones, anyway). The (common)

  chimpanzee has a set of these immune variants that is very like the human: in one list of 65 variants of one immune gene, only two were not exactly the same. We don't know about the DNA of enough bonobos yet to see if the story is the same for them, but the smart money says that it will be, possibly even more so. The gorilla set seems to be a little different again (but only about thirty gorillas have been tested).

  At any rate, all of these immune gene variants had to come out of Africa in that 'bottleneck'

  population that produced all the ex-African human populations. It is unreasonable to suppose that each individual inherited different versions of each variable gene from their parents: some will have carried only one version, the same from both parents, and no one can have carried more than two. The humans that came out of Africa have about 500 immune variants, at least, in common with chimpanzees, out of about 750 possibilities. The humans who stayed in Africa have more: they weren't subject to the bottleneck. There are many other genes where several ancient versions (ancient because they're common to us, chimpanzees, perhaps gorillas, maybe other species) have come through; 100,000 people is a reasonable minimum to carry all those. If you want to be critical and get that number down a bit, you could argue that a few variants from African populations may have been mixed in later, for example via slavery to the US, or to Mediterranean peoples and then via Phoenician sailors to the rest of us. Still, the evidence does not point to an Adam and an Eve, unless they came with a lot of servants, slaves or concubines.

  The Biblical stories don't mention those[39].

  11. THE SHELLFISH SCENE

  The wizards watched carefully.

  'There's five of them sitting there with him now.' said Ponder. 'And some children. He seems to be getting on well enough.'

  'They're very interested in his hat,' said the Dean. 'A pointy hat always commands respect in any culture,' said Ridcully. 'Then why have several of them tried to eat it?' said the Lecturer in Indefinite Studies.

  'At least they don't appear to be warlike,' said Ponder. 'Let's go and introduce ourselves, shall we?'

  And, again, when the wizards arrived at the little group around the fire there was the strange sensation of ... nothing. No surprise, no shock. The heavy people treated them as if they'd just returned from the bar; their curiosity level extended perhaps to the flavour of crisps they'd brought back, but no further.

  'Friendly souls, ain't they?' said Ridcully. 'Which one's the boss?' Rincewind looked up, and then turned and snatched his hat from a big fist.

  None of them,' he snapped. 'Stop pinching the sequins!'

  'Have you mastered their language?'

  'I can't! They don't have one! It's all point and kick! That's my hat, thank you so very very!'

  'We watched you walking around,' said Ponder. 'Surely you've learned something?'

  'Oh, yes,' said Rincewind. 'Follow me, and I'll show you - give me my hat'

  Holding his sequin-stripped hat firmly on his head with both hands, he led the wizards to a big lagoon on the other side of the village. An arm of the river flowed through it; the water was crystal clear.

  'See the shells?' said Rincewind, pointing to a large heap a little way from the beach.

  'Freshwater mussels,' said Ridcully. 'Very nutritious. Well?'

  'It's a big heap, right?'

  'And?' said Ridcully. 'I'm quite fond of mussels myself.'

  'You see that hill further along the bank? The one covered in grass? And the one behind that, with all the shrubs and trees? And the -well, see how the whole area is a lot higher than rest of the land around here? If you want to know why, just kick the soil away. It's mussel shells all the way down! These people have been h
ere for thousands and thousands of years!'

  The tiny clan had followed them and were watching with the uncomprehending interest that was their ground-state expression. Several of them waded in after mussels.

  'That's a lot of shellfish,' said the Dean. 'Obviously not a taboo animal.'

  'Yes, and that's surprising because frankly these people seem related to them,' said Rincewind wearily. 'Their stone tools are frankly rubbish and they can't build huts and they can't even make fire.'

  'But we saw a—'

  'Yes. They've got fire. They wait for lightning to strike a tree or set fire to grass,' said Rincewind.

  'Then they just keep it going for years and years. Believe me, it took a lot of grunting and pointing to work that one out. And they have no idea about art. I mean, you know, pictures? I drew a picture of a cow in the dirt and they seemed puzzled. 1 really think they were just seeing ... well, lines. Just lines.'

  'Perhaps you're not very good at cow pictures?' said Ridcully.

  'Look around,' said Rincewind. 'No beads, no face paint, no decoration. You don't have to be very advanced to knock out a bear claw necklace. Even people who live in caves know how to draw. Ever seen those caves up in Ubergigle? Buffaloes and mammoths as far as the eye can see.'

  'I must say you've seemed to strike up a rapport with them very quickly, Rincewind,' said Ponder.

  'Well, I've always been good at understanding other people enough to get an inkling of when to start running,' said Rincewind.

  'You don't always have to run, do you?'

  'Yes. Of course. The important thing is to know when it's the appropriate moment, though. Ah, this one's Ug,' said Rincewind, as a white-haired man prodded him with a thick finger. 'So are all the others.'

  The current Ug pointed towards the Shell Midden foothills.

  'He appears to want us to go with him,' said Ponder.

  'He might,' said Rincewind. 'Or he might be pointing out where he last had a really satisfying bowel movement. See them all watching us?'

  'Yes.'

  'See that strange expression they have?'

  'Yes.'

  'You wonder what they're thinking?'

  'Yes.'

  'Nothing. Believe me. That expression means that they're waiting for the next thought to turn up.'

  Beyond the Shell Midden Mountains was a thicket of willows, and in the centre of the thicket was a much older tree, or what remained of one. It had been split in two, was now dead, and at some point had been burned.

  The clan hung back, but the white-haired Ug followed them into it a little way.

  Something crackled under Rincewind's foot. He looked down, saw a yellowing bone, and nearly experienced an appropriate moment. Then he spotted the faint hummocks around the clearing, many of them overgrown.

  'And here's the tree that fire came from,' said Ridcully, who had noticed them as well. 'Sacred ground, gentlemen. And they bury their dead.'

  'Not exactly buried,' said Rincewind. 'More just left, I think you'll find. I think they just want to show me where they got fire.'

  Ridcully reached for his pipe.

  'These people really don't make it?' he said.

  'They didn't understand the question,' said Rincewind. 'Well, I say question ... they didn't understand what I hope was the question. We're not talking progressive thinkers here. It must have been a big step when they invented the idea of taking the skins off animals before wearing them. I've never met any people quite so ... well, dull. I can't work them out. They're not exactly stupid, but their idea of repartee is an answer within ten minutes.'

  'Well, this'll buck their ideas up,' said Ridcully, and lit his pipe. T expect they'll be impressed!'

  The Ugs looked at one another. They watched the Archchancellor blowing smoke. And then they attacked.

  On the Discworld the only tribe known to have absolutely no imaginations whatsoever are the N'tuiftif, although they are gifted with great powers of observation and deduction. They just never invent anything. They were the first tribe ever to borrow fire. Being surrounded by other tribes who were as imaginative as anything, they are also very good at hiding; when you are surrounded by tribes to whom a stick means club, prod, lever, world domination, you are at a natural disadvantages when, to you, a stick means 'stick'.

  To someone else a stick currently meant 'pole'.

  A figure vaulted across the clearing and landed in front of the Ugs.

  Orangutans do not enter the boxing ring, being too intelligent. If they did, however, the fact that they could knock out the opponent without getting up off their stool would quite make up for lack of finesse in the footwork.

  Most of the tribe turned to run, and would have come face to face with the Luggage if it had a face. They rocked when it butted them, and tried to wonder what it was. And by then the Librarian was on top of them.

  Those that worked out this was a good time to flee, fled. Those that didn't, stayed on the ground where they had been put.

  The astonished Archchancellor was still holding the burning match when the Librarian advanced on him, screaming loudly.

  'What say?' he said.

  'There's a lot about him being in a library and the next minute being in the river over there,'

  Ponder supplied. 'That all? Sounded more.' 'The rest was swearing, sir.' 'Apes swear?' 'Yes, sir.

  All the time.'

  There was another burst from the Librarian, accompanied by a pounding of knuckles on the ground. 'More swearing?' said Ridcully.

  'Oh yes, sir. He's really quite upset. Hex has told him there are no longer any libraries whatsoever at any point in the planet's history.'

  'Ow!'

  'Quite, sir.'

  'I burned my fingers!' Ridcully sucked his thumb. 'Where is Hex, anyway?'

  'I was just wondering that, sir. After all, the crystal ball belonged in the city which isn't here any more ...'

  They turned and looked at the tree.

  It must have blazed furiously when the lightning struck. Probably it had been dead and dry anyway. There were only a few stumps of branches. The whole thing was black, and strangely ominous against the green of the willows.

  Rincewind was sitting at the top.

  'What the hell are you doing up there, man?' Ridcully bellowed.

  'I can't run across water, sir,' Rincewind called down. 'And ... I think I've found Hex. This tree talks ..."

  12. EDGE PEOPLE

  Rincewind's 'edge people' are a caricature of early hominids, and quite close to what anthropologists used to think Neanderthals were like. We now think that Neanderthals had a bit more going for them, quite apart from burying their dead. At least, it suits the mood of the times to desire to think that they did have something happening behind that big brow-ridge. A bone with holes in it, which some archaeologists believe to be a 43,000-year-old Neanderthal bone flute, has been found in Slovenia. But others dispute that it is a musical instrument. Francesco d'Errico and Philip Chase have studied the bone carefully, and they are certain that the holes were gnawed in it by animals, not bored by a musically minded Neanderthal. We do not know if it's been handed to a musician ...

  Whatever the status of the flute, it is clear that Neanderthal culture didn't change significantly over long periods of time. The culture that led to us did. It changed dramatically, and so far it's never stopped. What made us so different from the Neanderthals?

  According to the Out of Africa theory, our ancestors, and everybody else's, came from an original population that evolved in Africa. They migrated through the Middle East; the ones bound for Australia probably left from South Africa, but might have gone round through the Far East and Malaysia. If you've got boats, you can do either.

  In principle the immune-gene story that we discussed in Chapter 10 could tell us more, but nobody's yet done the research: either the Australian 'aborigines' have the same gene spectrum as the rest of us post-bottleneck humans, or they have their own small and characteristic selection instead. Whichever is
the case, it will tell us something interesting, but until someone gathers the genetic data, we have no idea which interesting thing it will tell us. A lot of science is like that, a win-win situation. But try explaining that to the bean-counters who control research funding.

  When we speak of 'migrations' in this context, you shouldn't think of the exodus of the Hebrews from Egypt. It wasn't a case of one group of humans taking forty years or whatever and conquering other hominids along the way. It was more likely the successive formation of small settlements, slowly getting further and further away from the original homeland. The people themselves didn't even know that they were migrating. It was just 'Hey, Alan, why don't you and Marilyn settle down to hunter-gathering a couple of valleys over, by that nice Euphrates river?'

  Then, after a hundred years, there would be a few settlements on the far side of the river, too.

  This isn't pure speculation: archaeologists have found some of the settlements.

  If humans formed new settlements a mile away every ten years, it would take only 50,000 years, a mere 1,000 grandfathers, for them to diffuse from Africa all the way to the frozen north. And they surely diffused faster than that. Hardly anybody actually went anywhere; it was just that the kids set up home a few hundred yards along the track, where there was a bit more room to bring up their kids.

  As we diffused, we diversified. It is impressive how diverse we are, physically and culturally.

  But perhaps, from the elvish viewpoint, we're all much the same, from Chinese to Inuit to Maya to Welshman. Our similarities are far greater than our differences[40]. We had diversified in Africa, too, from the tall willowy Masai and Zulus to the !Kung[41] 'pygmies' and the stout Yoruba. These peoples are really, anciently, different: they differ from us, and from each other, almost as much as wolves differ from jackals. The post-bottleneck humans differentiated quite recently, just as the breeds of dogs differentiated from one kind of wolf (or perhaps it was a jackal).