The Science of Discworld II

plains. Giant hyenas, massive wild dogs, lions and cheetahs made a good living from the vast herds of herbivores that lived on the productive savannah grasses; the giant pythons were probably originally savannah animals, too.
    The story has been told many times, in many versions. And that’s just the point: we understand our ancestry through story. We wouldn’t be able to work out our ancestry from the fossils that we have discovered unless we’d learned just what clues to look for, especially since few fossil sites have enough evidence left.
    The new ancestral plains ape saw the world differently. Judging from the behaviour of today’s chimpanzees, especially bonobos, it was a highly intelligent animal. We call their fossils southern apes, australopithecines, and there are hundreds of books that tell stories about them. They may have sojourned by the sea, doing clever things on beaches. Some certainly lived on lake margins. Today’s chimpanzees use stones to smash hard nuts open, and sticks to extract ants from nests; the australopithecines also used stones and sticks as tools, rather more so than their cousins the chimpanzees now do. They may have killed small game, as chimpanzees do. They probably used sexual behaviour to hang much of their pleasure on, like today’s bonobos, but most likely they were more gender-conscious and male-dominant. Like previous apes, they diverged into gracile and robust lines. The robust ones, called Anthropithecus boisi , or even a different genus Zinjanthropus (‘nutcracker man’) and other defamatory names, were vegetarians like today’s gorillas, and probably left no descendants in modern times.
    This kind of split into gracile and robust forms, by the way, seems to be one of the standard patterns of evolution. Mathematical models suggest that it probably happens when a mixed population of big and small creatures can exploit the environment more effectively than a single population of medium-sized ones, but this idea has to be considered highly speculative until more evidence comes in. The zoological world was recently given a reminder of how common such a split is, and of how little we really know about the creatures of ourown planet.
    The animal involved could not have been better known, nor more appropriate to Discworld: the elephant. 1 As every child learns at an early age, there are two kinds of elephant, two distinct species: the African elephant and the Indian elephant.
    Not so. There are three species. Zoologists have been arguing for at least a century about what they thought was at most a subspecies of ‘the’ African elephant Loxodonta africana . The typical big, burly African elephant lives on the savannah. The elephants that live in the forest are shy, and difficult to spot: there is just one of them in the Paris zoo, for example. Biologists had assumed that because the forest elephants and the savannah elephants can interbreed at the edges of the forest, they could not be separate species. After all, the standard definition of a species, promoted by the evolutionary biologist Ernst Mayr, is ‘able to interbreed’. So they either insisted that there was just one species, or that ‘African elephant’ had a distinguished subspecies, the forest elephant Loxodonta africana cyclotis . On the other hand, zoologists who have had the good fortune to see forest elephants are in no doubt that they look very different from the savannah ones: they are smaller, with straighter, longer tusks, and round ears, not pointed ones. Nicholas Georgiadis, a biologist at theMpala Research Centre in Kenya, has said: ‘If you see a forest elephant for the first time, you think, “Wow, what is that?”’ But because biologists knew , on theoretical grounds, that the animals had to be all the same species, the observational evidence was rejected as inconclusive.
    However, in August 2001 a team of four biologists – Georgiadis, Alfred Roca, Jill Pecon-Slattery and Stephen O’Brien – reported in the journal Science their ‘Genetic evidence for two species of elephant in Africa’. Their DNA analysis makes it absolutely clear that the African elephant really does come in two distinct forms: the usual robust form, and a separate gracile form. Moreover, the gracile African elephants really are a different species from the robust ones. As different, in fact, as either African species is from the Indian one.