'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 ..."

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 !Kung41 '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).

This kind of rapid differentiation is a standard evolutionary story, called 'adaptive radiation'.

'Radiation' means 'spread', and 'adaptive' means that the organisms change as they spread, adapting to new environments -and, especially, to the changes brought about by their own adaptive radiation. It happened to 'Darwin's finches', where one small group of finches of a single species arrived in the Galapagos islands, and within a few million years had radiated into

13 separate species, plus a 14th on the Cocos Islands. (We wonder what the legend of The Fourteenth Finch might be.) Another well-known example is the vast array of cichlid fish that diversified in Lake Victoria over the last half a million years or so. There they produced variants for the catfish niche, for the planktonic filter-feeder, for the general-detritus feeder; they evolved into species with big crushing teeth for eating mollusc-shells, species that specialised in scraping scales or fins off other fish, and species that specialised in eating the eyes of other fish. Yes, really: when fish from that species were caught, all they had in their stomachs was eyes.42 These cichlids ranged in size from a couple of centimetres to half a metre. The original river-dwelling species Haplochromis burtoni, whose descendants they all are, grows to a length of 10-12 centimetres.

Curiously, the range of genetics of these fishes was quite small, considering their morphological and behavioural ranges: about the same as out-of-Africa humans, but not as wide as in-Africa humans. At least, that's the case according to some reasonable ways to estimate genetic diversity.

The second part of this story nearly always involves extinction: just occasionally, one of the newly differentiated species has evolved a new and successful trick, and survives while all the others perish.

But the usual demise of these specialised, adaptively radiated fish happens when a professional specialist -a catfish perhaps, whose ancestors have been feeding on detritus for 20 million years

-comes in and takes over from the amateur cichlid catfish. Unfortunately, in this case, it wasn't an inoffensive catfish, but the Nile perch, a specialised carnivore from an ancient stock. The Nile perch has now cleaned out nearly all of the Lake Victoria cichlid explosion, which is why we wrote the previous passage in the past tense.43 The main remnants of that glorious radiation of the cichlids are now to be found in the homes of a few amateur hobbyists, who are keeping some of the odd cichlid species in aquaria, and the Geoffrye Museum in London, which by chance has one of the largest ranges of cichlids and is now sponsored by public bodies. We don't know yet if any of the cichlid variants in Lake Victoria has hit on a trick to survive even the Nile Perch.

It's difficult to know what Nile Perch is about to come in and prune Homo sapiens' current diversity. With luck, it will be our own propensity to miscegenation, aided and abetted by airlines, despite the contrary admonitions of our priests. Maybe we'll all be mixed up into one fairly diverse type. Or maybe it will be Independence Day aliens, out to conquer the galaxy. Or perhaps more competent ones, with elementary virus protection software.