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 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."
Recent African origin of modern humans
The historical alternative to the recent origin model is the multiregional origin of modern humans, initially proposed by Milford Wolpoff in the 1980s. This view proposes that the derivation of anatomically modern human populations from Homo erectus at the beginning of the Pleistocene 1.8 million years BP, has taken place within a continuous world population. The hypothesis necessarily rejects the assumption of an infertility barrier between ancient Eurasian and African populations of Homo. The hypothesis was controversially debated during the late 1980s and the 1990s. The now-current terminology of "recent-origin" and "Out of Africa" became current in the context of this debate in the 1990s. Originally seen as an antithetical alternative to the recent origin model, the multiregional hypothesis in its original "strong" form is obsolete, while its various modified weaker variants have become variants of a view of "recent origin" combined with archaic admixture. Stringer (2014) distinguishes the original or "classic" Multiregional model as having existed from 1984 (its formulation) until 2003, to a "weak" post-2003 variant that has "shifted close to that of the Assimilation Model".
Multiregional origin hypothesis: Main article: Multiregional origin of modern humans
The multiregional hypothesis, multiregional evolution (MRE), or polycentric hypothesis is a scientific model that provides an alternative explanation to the more widely accepted "Out of Africa" model of monogenesis for the pattern of human evolution. Multiregional evolution holds that the human species first arose around two million years ago and subsequent human evolution has been within a single, continuous human species. This species encompasses all archaic human forms such as Homo erectus and Neanderthals as well as modern forms, and evolved worldwide to the diverse populations of anatomically modern humans (Homo sapiens).
The hypothesis contends that the mechanism of clinal variation through a model of "Centre and Edge" allowed for the necessary balance between genetic drift, gene flow and selection throughout the Pleistocene, as well as overall evolution as a global species, but while retaining regional differences in certain morphological features. Proponents of multiregionalism point to fossil and genomic data and continuity of archaeological cultures as support for their hypothesis. The multiregional hypothesis was first proposed in 1984, and then revised in 2003. In its revised form, it is similar to the Assimilation Model.
Recent analyses of DNA taken directly from Neanderthal specimens indicates that they or their ancestors contributed to the genome of all humans outside of Africa, indicating there was some degree of interbreeding with Neanderthals before their replacement. It has also been shown that Denisova hominins contributed to the DNA of Melanesians and Australians through interbreeding.
By 2006, extraction of DNA directly from some archaic human samples was becoming possible. The earliest analyses were of Neanderthal DNA, and indicated that the Neanderthal contribution to modern human genetic diversity was no more than 20%, with a most likely value of 0%. By 2010, however, detailed DNA sequencing of the Neanderthal specimens from Europe indicated that the contribution was nonzero, with Neanderthals sharing 1-4% more genetic variants with living non-Africans than with living humans in sub-Saharan Africa. In late 2010, a recently discovered non-Neanderthal archaic human, the Denisova hominin from south-western Siberia, was found to share 4-6% more of its genome with living Melanesian humans than with any other living group, supporting admixture between two regions outside of Africa. In August 2011, human leukocyte antigen (HLA) alleles from the archaic Denisovan and Neanderthal genomes were found to show patterns in the modern human population demonstrating origins from these non-African populations; the ancestry from these archaic alleles at the HLA-A site was more than 50% for modern Europeans, 70% for Asians, and 95% for Papua New Guineans. Proponents of the multiregional hypothesis believe the combination of regional continuity inside and outside of Africa and lateral gene transfer between various regions around the world supports the multiregional hypothesis. However, "Out of Africa" Theory proponents also explain this with the fact that genetic changes occur on a regional basis rather than a continental basis, and populations close to each other are likely to share certain specific regional SNPs while sharing most other genes in common. Migration Matrix theory (A=Mt) indicates that, dependent upon the potential contribution of Neanderthal ancestry, we would be able to calculate the percentage of Neanderthal mtDNA contribution to the human species. As we do not know the specific migration matrix, we are unable to input the exact data, which would answer these questions irrefutably.
The primary competing scientific hypothesis is currently recent African origin of modern humans, which proposes that modern humans arose as a new species in Africa around 100,000-200,000 years ago, moving out of Africa around 50,000-60,000 years ago to replace existing human species such as Homo erectus and the Neanderthals with limited interbreeding: at least once with Neanderthals and once with Denisovans. This differs from the multiregional hypothesis in that the multiregional model predicts interbreeding with local human populations in any such migration.