The Science of Discworld II

indirectly. For example, Ian’s cat ‘Ms Garfield’ usually comes out to the front of the house to greet the arrival of the family car, but her pleasure at the car’s appearance is heavily disguised as a strident harangue: ‘Where the bloody hell have you lot been?’ After absences on holiday or overseas, family members find that whenever they are in the garden, the cat coincidentally is in the same part of the garden – but either asleep or apparently just passing through. It looks as though house-cats are slowly losing the domestication battle, but putting up a strong fight. Feral cats are another matter, and real working cats like farm cats are often genuinely independent. These days, though, many farm cats are treated much like house-cats. At any rate, there are some good research projects still to be carried out on the co-evolution of ancient humans and their livestock and pets.
    In another instance of this co-evolution, the horse made chivalry a possible culture (hence the name, of course: compare the French ‘cheval’) and enabled the Mongols to achieve one of the largest, best-controlled empires in human history. Under the Khans it was said that a virgin could walk unmolested from Seville to Hang Chou. Only in the twentieth century was that again achievable, with luck and possibly a harder search for the virgin. The Spanish took horses to America, where humans had killed off several equine species some 13,000 years before, 5 and changed the lives of all the North American Indian tribes – and the cowboys, of course. And, a little later, Hollywood.
    The horse did wonders for the genetics of humans, too. Just as they say that the invention of the bicycle saved East Anglia from anincest implosion, so the people that had come out of Africa were only a tiny part of early Homo sapiens ’s genetic diversity. All recent studies of the DNA genetics of human populations agree that the genetic diversity outside Africa is only a tiny fraction of the diversity that is still found on that continent. Those who left, to go as far as Australia or China, to Western Europe or via the high Arctic to America, are less diverse in total than many small indigenous African peoples. With the arrival of the horse, it became possible for traders to carry goods – and gene alleles – for very long distances, very effectively. So the out-of-Africa humans have inherited a relatively small part of the African gene-pool: they are genetically impoverished, but well stirred.
    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 wolf’s 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 thatancestral 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