The Science of Discworld II

nobody’s yet done the research: either theAustralian ‘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. 1 We had diversified in Africa, too, from the tall willowy Masai and Zulus to the !Kung 2 ‘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 Galápagos 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. 3 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