Evolutionary Notes: We are apes, whether we like it or not

CHARLES DARWIN shocked Victorian society by, as they thought, suggesting that "man is descended from the apes". What he really suggested, of course, is that humans and the hairy apes (the chimpanzee and gorilla) share a common ancestor, a creature more "ape-like" than "man-like". But now the argument has been turned on its head.

Both fossil evidence and studies of the molecules of heredity themselves - DNA - suggest not only that the common ancestor walked the Earth far more recently than Darwin could have imagined, but that it was more like a human being than like a chimp or gorilla. To use the same sloppy, but emotive, language used by those Victorians, the best evidence now is that "apes are descended from man".

The key word in that description of the common ancestor is "walked". Upright walking - proper upright walking - is a uniquely human characteristic. We know from the fossil evidence that our ancestors (including the famous Lucy) were walking upright by about five million years ago. Traditionally, this was regarded as long after the "man-ape split", which was set (on the basis of extremely limited fossil evidence, and a lot of wishful thinking) as about 15 million years ago. By the time Lucy walked the Earth, the story ran, humans had been evolving separately from other species for at least 10 million years.

But the molecular studies show that this is not the case. DNA analysis (just like the genetic fingerprinting that can be used to identify individual people) is now so accurate that it can tell us that we share 98.6 per cent of our genetic material with the hairy apes. This makes us extremely close relations - more closely related, for example, than the horse and the donkey, or sheep and goats. And because molecular biologists know how long it takes for changes to accumulate in the DNA, this also tells us that the man-ape split actually happened a bit less than five million years ago - crucially, after our ancestors on the human line had learned to walk upright.

Just after that time, the fossil evidence shows that the line that leads to us (Homo sapiens) shared the plains of Africa with two close relations, the Australo- pithecines, very similar species but with one larger than the other. On the traditional picture, they both vanished from the scene a couple of million years ago, leaving no descendants today. On the other hand, nobody has identified fossils ancestral to the modern chimpanzee and the gorilla - but they must have had ancestors! And they are very similar species, one larger than the other.

It scarcely takes a Sherlock Holmes to solve the mystery of what happened to the Australopithecines, once the evidence is presented like that. The new dating of the man-ape split matches up these fossils without descendants with the descendants without fossils. The Australopithecines, it seems, did not die out, but gave up the ability to walk upright and re-adapted to a life in the trees, becoming the chimpanzee and the gorilla. There were three closely related species around three to four million years ago, and there are three closely related species around today.

The techniques on which these conclusions are based are all well established, and entirely non-controversial. At least, they are not controversial as long as you don't apply them to human beings. But even today, more than a hundred years after Darwin, there are many people who still want to think that we are special, and somehow not subject to the same evolutionary rules as other animals. But they are wrong. Human beings are just one twig on the bush of evolution, growing right alongside the chimpanzee twig and the gorilla twig. It isn't that we are descended from the apes, or the apes from us. We are apes,whether we like it or not.

John Gribbin presents the Radio 4 series `Evolution After Darwin' at 9 pm on Wednesdays, starting 14 October