Out of Africa or Out of Asia? A bitter row is raging about the origin of our species. Ayala Ochert reports

Of the many lessons that could be learned from the O. J. Simpson trial, there is one we can all agree on - that evidence from genetic material, or DNA, cannot always settle things one way or another. So, when palaeontologist Chris Stringer declared two months ago that the first analysis of ancient Neanderthal DNA "might have ended the debate" over the origin of our species, he should have known better.

If anything, the controversy is now more bitter than ever. Stringer's arch-rival, anthropologist Milford Wolpoff of the University of Michigan, says that Stringer is so off the mark in his interpretation of the Neanderthal evidence that "he's not even wrong".

To an outsider, the debate is a subtle one. Both sides agree that a type of early human, Homo erectus, evolved in Africa anywhere between one and two million years ago, and then started to migrate to Europe and Asia. But they disagree over what happened next. Most go along with Stringer and his colleagues at the Natural History Museum in London and their theory that our own species, Homo sapiens, emerged in Africa only very recently, less than 200,000 years ago, and then spread across the world, replacing all other human species that had descended from the first wave of Homo erectus, including the Neanderthals.

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According to this so-called "Out of Africa" hypothesis, although we share a common ancestor with Neanderthals, they are not our direct ancestors. Instead, several separate human species are supposed to have lived alongside one another for many thousands of years, without interbreeding. "We're in this unique situation now of being the only human species on earth. And that's probably only been so for the past 25,000 to 30,000 years," notes Stringer. The recent analysis of ancient Neanderthal DNA, says Stringer, confirms that they were not our ancestors, and many others would agree.

But not the "multiregionalists", including Wolpoff. They say that there was only one great exodus from Africa, of Homo erectus more than one million years ago. In fact, they say, Homo erectus was simply an earlier incarnation of our own species. While different regional forms evolved - Neanderthals, Peking Man, Java Man - these different groups maintained genetic contact with one another, and so evolved as a single species, Homo sapiens. Neanderthals, they say, were the direct ancestors of modern Europeans, and they are not persuaded by the DNA evidence that now purports to show otherwise.

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The trouble is that you cannot reach conclusions about the genetics of a population on the basis of one individual, says geneticist Alan Templeton of Washington University, Missouri, referring to the recent DNA tests on Neanderthal bones. And even if it were true that modern humans replaced Neanderthals, "it doesn't have much bearing on the 'Out of Africa' theory," he says. "Even if there was a local replacement, that does not imply that there was global replacement."

If DNA evidence is what it takes to prove things either way, the multiregionalists have some of their own. If Chris Stringer's theory is correct, then the most ancient human genes should be African, but that is not what population geneticist Rosalind Harding finds. At her lab in Oxford's Institute of Molecular Medicine, she has been examining a gene involved in the manufacture of human haemoglobin, the red pigment of blood. "The oldest genes are found only in Asia, and they are around 200,000 years old," says Harding. "The only way to account for this in people living now is to say that their ancestors included the people who were archaic humans living in Asia around 200,000 years ago," she says.

Harding's research will be aired next Thursday in the controversially titled Out of Asia television programme on BBC2. The documentary represents producer Chris Hale's attempt to find a middle ground in the debate. Drawing together various strands of evidence - genetic, palaeontological and archaeological - the programme concedes one point to the multiregionalists. Homo erectus may well be our direct ancestor, as he was the "only kid on the block" in Asia at that time.

But Hale's hope of bringing the two sides to some agreement seems to miss what is at the heart of the disagreement - where other species end and ours begins. Stringer says Homo sapiens is a very new species that was incapable of intermingling with its older cousins. Multiregionalists, on the other hand, argue that we and our cousins are one species that has been "mixing up" our genes together for almost a million years, gradually evolving into our modern form. According to the view you adopt, there is always a suitable interpretation for the evidence, DNA or otherwise.

In fact, though she does not say it directly to the camera, Harding considers her data to be nothing more than a complicating factor in the "Out of Africa" story. "It means that our ancestral gene pool is not just in Africa, but that there is migration into as well as out of Africa over the last 200,000 years," she says. The old Asian genes could have hitched a lift back into Africa on Homo erectus, who later evolved into Homo sapiens, a new and - crucially for the "Out of Africa" hypothesis - different species.

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One reason why the two sides find it hard to meet, aside from these intellectual differences, is the debate's political dimension. Just as happened with the O. J. Simpson trial, the debate on human origins has become embroiled with issues of race. Shortly after the Neanderthal DNA data became public, Stringer accused those who still refused to subscribe to his hypothesis of following "narrow political agendas" and of deliberately "concocting divisive theories".

It is the multiregionalists who usually get accused of supporting racist ideas, but Wolpoff argues that they have been ruled guilty by association. Multiregionalism, he says, has become confused with the 19th-century polygenism - the idea that the different races of the world were different species. "People really believed that," says Wolpoff, "but today we're all monogenic. Multiregionalism says that we see similarities because people exchanged genes, but they also had local ancestors. All the important changes through evolution are the ones that make us human - the important parts are not the differences but the similarities."

In the politically delicate area of human origins, both sides are keen to distance themselves as much as possible from charges of racism, but the "Out of Africa" supporters have been more successful. "(They) have realised the marketing value of saying 'we all are one'," says Marek Kohn, author of The Race Gallery, who has followed the debate closely. "But both models can be invoked by "scientific racists", he says.

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Out of Asia pays little more than lip-service to the debate, focusing instead on a separate political angle - the way Aborigines have adopted archaeological signs of their own antiquity to support land claims. Some of the ancient religious sites still used by Aborigines, like Jimnium in the Northern Territory, contain some of the oldest rock art in the world. The controversy was fuelled in recent years by claims that some of that art dates to before the time that Homo sapiens was thought to have evolved in Africa.

Worse still for the "Out of Africa" idea has been the finding of one-million-year-old human remains on Flores, a volcanic island in Indonesia that has been completely surrounded by water for more than two million years. If pre-modern humans were capable of producing art and of navigating across sea, that throws up important questions of how much they differed from us. "It has lots of implications for the intellectual and linguistic capabilities of Homo erectus," says Mike Morwood, an Australian archaeologist at the University of New England. "The ability to organise a maritime craft indicates a degree of complexity requiring language."

Though sceptical of thepossibility that Homo erectus was capable of crossing water, Stringer says that we should not betoo surprised if we find that other human species had greatercapabilities than they are so far credited with. "But increasing their capabilities still doesn't prove they're our ancestors," he says. "This sort of behaviour is what we might expect of humans who have got large brains."

If these species were so smart, and even had language and art, then what caused their ultimate downfall and our rise? We may be uncomfortable with the idea that our own species was responsible for the complete eradication of all our human cousins, but when we look to our own more recent history of genocide and mass extinction, it is perhaps less hard to believe.

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Out of Asia, Horizon, BBC 2, September 25, 9.25pm.