Re-analysis of gene studies and new fossil evidence cast doubts on a popular theory of human origins.
Anthropologists have long debated the origins of modern humanity, and by the mid-1980s two main competing theories emerged. One, Multiregional Evolution, posits that humans arose in Africa some two million years ago, evolved as a single species spread across the Old World and were linked through interbreeding and cultural exchange. The Out-of-Africa hypothesis, in contrast, proposes a much more recent African origin for modern humans - a new species, distinct from Neanderthals and other archaic humans, whom they then replaced. Emphatic support for Out-of-Africa came in 1987, when molecular biologists declared that all living peoples could trace a piece of their genetic legacy back to a woman dubbed "Eve," who lived in Africa 200,000 years ago. Although that original Eve study was later shown to contain fatal flaws, Out-of-Africa has continued to enjoy much molecular affirmation, as researchers have increasingly turned to DNA to decipher the history of our species.
But a closer look at these genetic studies has led some researchers to question whether the molecular data really do bolster the Out-of-Africa model. And striking new fossil data from Portugal and Australia appear to fit much more neatly with the theory of Multiregional Evolution.
The DNA from mitochondria, the cell's energy-producing organelles, has been key Out-of-Africa evidence. Mitochondria are maternally inherited, so genetic variation arises largely from mutation alone. And because mutations have generally been thought to occur randomly and to accumulate at a constant rate, the date for the common mitochondrial DNA (mtDNA) ancestor can theoretically be calculated. This "molecular clock" indicates that the mtDNA ancestor lived a a mere 200,000 years ago, and the root of the gene tree traces to Africa. These results, along with the observation that variation is highest in Africa (indicating that modern humans had been in Africa the longest), seemed to offer unambiguous support to a recent African origin for all modern humans.
Out-of-Africa theory posits that modern humans arose in Africa and replaced other human species across the globe
But the significance of each finding has been questioned. The date is suspect because the molecular clock depends on problematic assumptions, such as the calibration date and mutation rate. And if natural selection has shaped mtDNA, as some studies suggest, then the rate of mutation accumulation may have differed at different times. The African root for the mtDNA gene tree is compatible with Out-of-Africa, but it does not exclude Multiregionalism, which predicts that the common ancestor lived somewhere in the Old World, probably Africa. And neither does the high mtDNA variation in African populations as compared with non-Africans uniquely support Out-of-Africa, according to anthropologist John H. Relethford of the State University of New York College at Oneonta. "You could get the same result if Africa just had more people living there, which makes sense ecologically," he asserts.
Another problem plaguing the genetic analyses, says geneticist Alan R. Templeton of Washington University, lies in a tendency for researchers to draw conclusions based on the particular genetic system under study. "Very few people try to look across all the systems to see the pattern," he observes. Some nuclear genes indicate that archaic Asian populations contributed to the modern human gene pool, and Templeton's own analyses of multiple genetic systems reveal the genetic exchange between populations predieted by Multiregionalism.
Still, Relethford and Templeton's arguments haven't convinced everyone. Henry C. Harpending, a population geneticist at the University of Utah, finds Multiregionalism difficult to swallow because several studies put the prehistoric effective population size - that is, the number of breeding adults - at around 10,000. "There's no way you can get a species going from Peking to Cape Town that's only got 10,000 members," he remarks. (Other researchers counter that this number, based on genetic diversity, may be much smaller than the census size of the population - perhaps by several orders of magnitude). And many geneticists, such as Kenneth K. Kidd of Yale University, insist that "the overwhelming majority of the data is incompatible with any ancient continuity".
But those who believe that Out-of-Africa's genetic fortress is crumbling find confirmation in fresh fossil data that pose new difficulties for the theory's bony underpinnings. Last December researchers unearthed in western Portugal's Lapedo Valley a fossil that preserves in exquisite detail the skeleton of a four-year-old child buried some 24,000 years ago. According to Erik Trinkaus, a Washington University paleoanthropologist who examined the specimen, the team fully expected the remains to represent a modern human, based on its date and the style of the burial. But subsequent analysis, published in the June 22 Proceedings of the National Academy of Sciences USA, revealed a surprising combination of features, such as a modern-looking chin and Neanderthal limb proportions. After reviewing scientific literature on primate hybrids, Trinkaus concluded that this child resulted from interbreeding between Neanderthals and modern humans.
Not everyone is persuaded. Christopher B. Stringer of London's Natural History Museum, lead proponent of the Out-of-Africa model, wonders whether the fossil might simply represent a cold-adapted modern human, because Portugal then was colder than it is today. In any case, Stringer maintains that his model does not exclude occasional interbreeding.
Yet Trinkaus notes that because the fossil is dated to thousands of years after these groups came into contact, "we're looking at populations admixing." Furthermore, adult fossils from central and eastern Europe show the effects of mixing, too, states paleoanthropologist David W. Frayer of the University of Kansas. And if the groups were interbreeding across Europe, asserts University of Michigan multiregionalist Milford H. Wolpoff, "that would mean you could make a strong case that [contemporary] Europeans are the result of the mixture of these different groups." Another name for that, he says, is Multiregional evolution.
Multiregionalism also best explains the surprising new date for a previously known fossil from western New South Wales, according to paleoanthropologist Alan Thorne of the Australian National University. In the June Journal of Human Evolution Thorne and his colleagues report that the fossil, known as Lake Mungo 3, now looks to be some 60,000 years old - nearly twice as old as previously thought - and unlike the other early Australian remains (all of which date to less than 20,000 years ago), this one bears delicate, modern features. To Stringer, this gracile form indicates the arrival of modern humans from Africa, albeit an early one. Over time, he reasons, selection could have led to the robust morphology seen 40,000 years later.
But Thorne argues that such dramatic change is unlikely over such a short period and that fossils from the only environmentally comparable region - southern Africa - show that people have remained gracile over the past 100,000 years. Moreover, Thorne maintains, "there is nothing in the evidence from Australia which says Africa" - not even the Mungo fossil's modern features, which he believes look much more like those of contemporaneous Chinese fossils. And Thorne observes that living indigenous Australians share a special suite of skeletal and dental features with humans who inhabited Indonesia at least 100,000 years ago.
Therefore, he offers, a simpler explanation is that the two populations arrived in Australia at different times - one from China and the other from Indonesia - and mixed, much like what has been proposed for Neanderthals and moderns in Europe. Exactly the same pattern exists in recent history, Thorne adds, pointing to the interbreeding that took place when Europeans arriving in North America and Australia encountered indigenous peoples. "That's what humans do." The mystery of human origins is far from solved, but because DNA may not be as diagnostic as it once seemed, Thorne says, "we're back to the bones." University of Oxford geneticist Rosalind M. Harding agrees. "It's really good that there are things coming from the fossil side that are making people worry about other possibilities," she muses. "It's their time at the moment, and the DNA studies can just take the back seat."