African connection

In the big question of where man came from, there are still no easy answers. Despite DNA evidence, the debate continues about whether all modern humans originated in Africa.
"Every morning I see a Neanderthal in my shaving mirror", the American palaeontologist Milford Wolpoff is quoted as saying. Except that he didn't. "I never said that," Wolpoff insists. "It was made up. I have a round head and a Slav face - Romans look more like Neanderthals than I do."

Why should it matter? Because these personal remarks stem from a debate on the origins of modern humans in which Wolpoff, at Michigan University, and Chris Stringer at the Natural History Museum in London represent two extreme and opposed camps.

The idea that some modern Europeans have ancestors among the thick-set and heavily built Neanderthals who lived in the harsh European climate between about 300,000 and 30,000 years ago is a feature of Wolpoff's model of "multi-regional evolution".

His view is that although we probably had pre-modern African ancestors around 2 million years ago, modern humans evolved at slightly different times and places during the past 250,000 years.

Morphological or "racial" differences between populations are reflections of their relative isolation during pre-history. Modern Australian aboriginals, for instance, share the cranial features of pre-modern South-east Asian hominid fossils, suggesting a distinct South-east Asian lineage. "But regional lineages were not like different species," Wolpoff cautions, "because there were also genetic and cultural exchanges between them."

Wolpoff is careful to distinguish his model of multi-regional evolution from a racist tradition suggesting that some races might be "less evolved", or closer to chimps, than others. He argues that the image of independently evolving races is an anathema to his version of multi-regional evolution. "The human population is best seen as a single, constantly evolving, interlinked species - like a moving stream linked by flows both in and out, of both genes and cultures," he says, rather than the image of a branching tree, which goes back 100 years.

But Dr Stringer proposes a radically different story - of a single, recent African origin for modern humanity. "The fossil record suggests that modern humans originated in Africa, because modern human fossils appear in Africa earlier than elsewhere. Around 100,000 years ago the first modern people emerged from north-east Africa and swept round the globe, replacing the existing archaic human populations.

According to this hypothesis, "racial" characteristics associated with particular regions of the world are only skin-deep environmental adaptations of the incoming moderns. The apparently Neanderthal-like features of modern Europeans are independently derived adaptations to cold made over the past 100,000 years - not evidence of Neanderthal ancestry. And Australian aboriginal skulls resemble archaic aboriginal fossils only because their ancestors, who migrated to Australia from Africa during the past 80,000 years, happened to have evolved the same local environmental adaptation.

Last month, the analysis of DNA extracted from a 30,000-year-old Neanderthal bone from a museum in Bonn, and published in the scientific journal Cell, appeared to clinch the issue in Dr Stringer's favour - at least as far as the idea of a distinctly European lineage to modern humans is concerned. The report was hailed in the British press as a "terrific achievement", a "breakthrough in genetic analysis", the "coming of age" of ancient DNA research - "the equivalent", in Dr Stringer's words, "of landing Pathfinder on Mars".

A research team in Munich found the Neanderthal DNA sequence to be so very different from modern humans that they concluded that Neanderthals cannot be ancestral to modern humans, but must have been a distinct species.

But is this ancient DNA work really so definitive? Despite the media hype, the Neanderthal finding may be wrong. In recent years, reputable scientific journals have reported that DNA could be recovered from 20 million-year-old plants, from ancient insects preserved in amber, and possibly even from dinosaurs.

Not surprisingly there was much media interest. But the subsequent retraction of these claims attracted much less publicity. DNA degrades over time (more quickly in warm climates), with exposure to oxygen and water, and even when preserved in amber (which is gas-permeable). All previous attempts at sequencing ancient DNA have been plagued by the fact that it is easily contaminated by modern DNA when handled.

Although the Neanderthal specimen was unusual because it had been preserved by the now outmoded practice of varnishing bones, thus reducing contamination with modern human DNA, without independent replication the result must be treated with scepticism.

This is not the first time that molecular genetics has been hailed as the tool for resolving the "Out of Africa" versus "multi-regionalism" debate. Dr Stringer says: "By 1985 the fossil evidence pointed strongly to Africa, but the debate was not closed. The fossil and archaeological evidence could always be interpreted in other ways."

Then molecular geneticists entered the debate, confident that they would provide definitive answers. In 1987, a study of mitochondrial DNA (which lies outside the cell nucleus, and is passed down from mother to child) identified a single African ancestress who lived about 200,000 years ago. She was heralded as "mitochondrial Eve", the mother of modern humanity. This was radical evidence in favour of "Out of Africa". The "mitochondrial Eve" research was the first of a series of studies using patterns of present- day genetic diversity to give insights into the past.

Particular DNA sequences can vary from one person to another through mutation, which occurs when alterations appear in the sequence of bases that constitute our DNA and are passed from one generation to the next. Geneticists assume that mutations occur at a constant rate, like a ticking clock. The clock is calibrated by two pieces of data: the (separately estimated) date, 5 million years ago, when humans diverged from chimpanzees; and the fact that chimps differ from us in just 2 per cent of their DNA.

A population with more mutations in a particular fossil DNA sequence is likely to be older than one with fewer; it has had more time to generate mutations. From that, a family tree for the DNA sequence can be created, giving a date when the last common ancestor for that sequence was living.

"Mitochondrial Eve" was followed by an "African Adam" who lived about 200,000 years ago. He was pinpointed in 1995 through analysis of genes inherited on the Y chromosome, which is passed only from father to son. Then last year a study of diversity in a section of chromosome 12 - inherited from both parents - identified a single African ancestor about 200,000 years ago. All this pointed firmly towards Africa rather than multi-regionalism, to the extent that what was in the 1970s a controversial (though not entirely new) idea is now virtually established dogma.

Yet if you look more closely at the studies and talk to the researchers, it is clear that the genetic studies have not settled the question of human origins. Many of the grandiose claims for "mitochondrial Eve" were, with less publicity, later retracted or qualified. There is more genetic diversity in Africa - as would be expected if Africa is the origin of humankind - but a more diverse population may simply have been a larger rather than an older one.

Furthermore, genetic exchange between populations, natural selection and environmental changes are all likely to have affected estimates of population differentiation, yet are rarely taken into account, if at all. The genetic findings have plausible alternative explanations: greater African variation could be due to an early environmental catastrophe which reduced genetic diversity in populations outside Africa.

Analyses that use other ways of looking at the data come up with surprisingly different answers. Rosalind Harding, at the Institute of Molecular Medicine in Oxford, uses a method which, rather than treating different human populations as if they were separate species, goes to the other extreme and assumes that the whole world is one interbreeding species.

When she put all her DNA sequence data into one sample, assuming an interbreeding world population and that the most common variants were the oldest, she found that the date of the African exodus is pushed back to 800,000, rather than 200,000, years ago. Her work also questions a basic tenet of the extreme "Out of Africa" model - that modern humans replaced archaic populations as they spread around the world. It indicates that some modern humans have ancient Asian ancestry.

"There must have been some interbreeding - how much is a moot point. You do not need much to make a significant difference to the gene pool, but it certainly puts paid to the idea of total replacement," she says.

Another challenge to the complete replacement idea comes from a study by Michael Hammer of the University of Arizona, published in March this year. He compared the diversity of a region of the Y chromosome in different populations, and found that one sequence is more diverse, and shows up more often in Asians than in Africans and may therefore have ancient Asian origins. The fact that this sequence also shows up in some Africans may be a result of migration back to Africa from Asia, some time after the original African Exodus.

So if modern humans emerged from Africa and spread around the world, they may have done so in several stages, with some interbreeding with populations elsewhere, and with some migration back to Africa.

But even this scenario is not definitive, for each genetic study takes us back to the last common ancestor for a particular DNA sequence, not for a whole population or species. Molecular genetics provides only some pieces of the puzzle.

"Ten years ago, with the arrogance of our powerful molecular techniques, we expected to answer the questions about human evolution," says John Clegg, a molecular geneticist at the Institute of Molecular Medicine. "We thought, we don't need pre-history; archaeology and palaeontology are disciplines which have been floundering for years. We are trained to come out with yes or no answers, not maybes. But this issue - this is a maybe." Out of Africa or back to Africa, and then out again? The answer is still elusive