In the late Eighties, many scientists were persuaded by an idea called the 'African Eve' hypothesis, which held that modern humans arose in Africa relatively recently - about 250,000 years ago. A new collection of papers in the journal American Anthropologist has cast doubt on this, but Eve's expulsion from the African Garden of Eden may not yet be final.
Even before the Eve hypothesis, some anthropologists had already become convinced that there were two human migrations out of Africa: the earliest, more than a million years ago, was followed by a more recent exodus of truly modern humans about 100,000 years ago. These 'children of Eve' replaced all populations from the previous migration.
Other anthropologists were just as certain there had been no second migration and that modern humans developed from these widespread ancestral populations over a million years, without any sudden replacement by incoming groups of 'moderns'. Each camp based its conclusions on the same scanty fossil record.
Then the geneticists announced they had broken the deadlock by analysing the mitochondrial (mt) DNA (which, unlike nuclear DNA, is inherited only down the maternal line) of people alive today. By comparing mtDNA samples from members of different ethnic groups, Rebecca Cann and Mark Stoneking, two researchers at University of California, Berkeley, concluded in the mid- Eighties that all living people were descended from a woman who lived in Africa 200,000 years ago. They called her Mitochondrial Eve.
Her existence rested on a number of controversial assumptions, notably that changes or mutations in mtDNA occur at a constant rate and can therefore be used to date stages in genetic development. The multiregionalists - anthropologists who believe that modern humans developed simultaneously in different parts of the globe from the pre-existing human-like populations - insisted that fossils were the only direct evidence of ancient humans and what really counted. But the tide seemed to be against them.
Over the past year, however, the Eve hypothesis has unravelled spectacularly. First, critics revealed flaws in Cann and Stoneking's procedures. Now, in his contribution to American Anthropologist, the geneticist Alan Templeton argues that the mtDNA data actually support the multiregionalist case. Whether it is bones or DNA, it seems that where human origins are in question something impels scientists to draw opposite conclusions from the same evidence.
The theory of Eve emerged from the laboratory at Berkeley headed by the late Allan Wilson, whose reputation as a molecular biologist underwrote the findings. But Professor Templeton argues that specialist expertise was what was needed. 'They weren't
population geneticists,' he says. 'They simply didn't have the training for this kind of analysis.'
Cann and Stoneking had compared the differences between mtDNA types to create a 'family tree' that arose in Africa. At Washington University in St Louis, Missouri, Professor Templeton found that thousands of equally plausible trees could be generated, some arising in Africa and some not. More important, he challenged the presumption that the mtDNA tree corresponded to movements of human populations, rather than the diffusion of genes by interbreeding. 'My mitochondrial DNA came from Germany, but
most of my nuclear DNA is Scottish,' he notes.
The data have led him to take a multiregionalist perspective, in which the various features of the modern human type arose separately and spread throughout the world's population by interbreeding. This aligns him with the anthropologists who, in their own contribution to the American Anthropologist forum, review the fossils once again and declare the Eve hypothesis disproved.
The most combative of the multiregionalists is Milford Wolpoff, of the University of Michigan, Ann Arbor. His principal adversary is Chris Stringer, of the Natural History Museum in London. 'They get at me for not being specific about things, and those are exactly the criticisms I've aimed at them,' says Dr Stringer.
He plans to reply to American Anthropologist, and his ideas about modern human origins will also be aired in his book In Search of the Neanderthals. He rejects Professor Wolpoff's claim that compromise between the hypotheses is logically impossible. Like other anthropologists, he believes the truth may lie somewhere between the two extremes.
ANOTHER contributor to the journal, Lesley Aiello, of University College London, has also reviewed the fossil evidence and has concluded that both multiregionalism and the out-of-Africa school of thought are too simplistic. But unlike Professor Templeton, she thinks the modern human type must have arisen in one place, because separate features would have taken too long to spread around the Old World and merge.
One lesson of the Eve episode is that some geneticists overestimated their ability to come up with a solution to a complicated anthropological problem. 'This is something I've seen in a lot of students and older people working in this area,' says Professor Templeton. 'They think the molecular data is just so wonderful that you don't need to analyse it; the answers just fall out automatically.'
He stresses that critical analyses such as his own have actually proved the richness of the evidence from the DNA and looks forward to the generation of more and more genetic data in the search for the origins of modern humanity. Eve has turned outto be the harbinger of a new synthesis between the evidence of fossils and that of molecules.
One critic of Cann and Stoneking's original work, Maryellen Ruvolo, of Harvard, has now obtained results that broadly support their estimates. She believes that Eve lived between 200,000 and 400,000 years ago and announced her findings at a meeting organised by Professor Wolpoff. Dr Stringer says other recent research supports the out-of-Africa case. 'I still say it hasn't been shown that they were wrong,' he maintains. 'It was just shown they were premature.'
'In Search of the Neanderthals' is published by Thames & Hudson, pounds 18.95.