When it comes to explaining the emergence of modern humans, researchers generally subscribe to one of two hypotheses. The Out of Africa theory holds that Homo sapiens burst onto the scene as a new species around 150,000 to 200,000 years ago in Africa and subsequently replaced archaic humans such as the Neandertals. The other model, known as multiregional evolution or regional continuity, posits far more ancient, diverse roots for our kind. Proponents of this view believe that Homo sapiens arose in Africa some two million years ago and evolved as a single species spread across the Old World, with populations in different regions linked through genetic and cultural exchange. Of these two models, Out of Africa has found favor with the majority of human evolution scholars. Just this month, however, results of two studies have come out that appear to support multiregionalism, adding new fuel to the long-standing debate.
Image: KATE WONG NEANDERTALS and other archaic humans may have been among our ancestors. |
Although the Out of Africa model was originally developed based on fossil evidence, it has gained popularity in part because so much genetic research seems to back it. The vast majority of these studies have focused on DNA from living populations. But in 1997 ancient DNA expert Svante P¿¿bo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues reported in the journal Cell that they had retrieved and sequenced for the first time mitochondrial DNA (mtDNA) belonging to a Neandertal.
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The team found that the difference between the Neandertal and modern mtDNA was more than three times that observed between any two living humans. Moreover, the Neandertal DNA didn't show any special similarity to DNA from living Europeans, which one might expect if the Neandertals, who occupied Europe for more than 200,000 years, contributed to the modern human gene pool. For many researchers, the Cell study put a serious, if not fatal, dent in the multiregionalists' argument that Neandertals were among our ancestors. And last year DNA from two other Neandertal specimens yielded similar results, further strengthening the Out of Africa replacement case.
Critics of the Neandertal DNA data, however, noted that without equally ancient samples from anatomically modern humans for comparison, the exact significance of the differences between Neandertal DNA and DNA from living people could not be fully understood. Perhaps any human DNA that old would be rather different from contemporary human DNA, owing to microevolutionary changes over time, they argued. One of the new studies appears to bear that suspicion out.
Reporting in the January 16 Proceedings of the National Academy of Sciences, Gregory J. Adcock of the Pierre and Marie Curie University in Paris and his colleagues extracted and analyzed mtDNA from Australian fossils representing anatomically modern humans. One of these, a specimen known as Lake Mungo 3 (LM3), dates to approximately 62,000 years old. As such, the LM3 mtDNA is the oldest known for a modern. It is also older than the two Neandertal samples analyzed last year and perhaps older than the sample that formed the basis of the Cell report (the age of the fossil that yielded that mtDNA is unknown).
Importantly, Adcock's team discovered that the LM3 mtDNA differed from that of living people as much as the Neandertal mtDNA did. In contrast, the mtDNA from the younger Australian fossils closely resembles that of living humans. "If the mitochondrial DNA sequences present in a modern human (LM3) can become extinct, then perhaps something similar happened to the mitochondrial DNA of Neandertals," noted John H. Relethford of the State University of New York at Oneonta in a commentary accompanying the PNAS report. If so, he says, the absence of Neandertal mtDNA in living humans does not rule out the possibility that they contributed to our gene pool. Other researchers are reserving judgment until the results are replicated in an independent lab, citing the possibility of contamination.
In truth, the mtDNA studies are additionally problematic because the history of a single gene does not necessarily reflect the history of a population. Different genes can tell different stories, and mtDNA, as far as human evolution researchers are concerned, represents only one gene. Unfortunately, with regard to ancient DNA, the chances of recovering nuclear DNA (and thus other genes) from early human fossils with currently available techniques are quite slim. Fossils thus remain very much a part of the human origins debate.
Image: MILFORD H. WOLPOFF EARLY MODERN EUROPEAN(center) shares more features in common with a Neandertal (left) than with a modern from the Middle East (right). |
To that end, the second study published this month calling the Out of Africa replacement scenario into question focused on bones. According to their report in the January 12 Science, Milford H. Wolpoff of the University of Michigan and his colleagues set out to test the replacement theory by examining early modern human skulls from Central Europe and Australia dated to between 20,000 and 30,000 years old (above), searching for genetic input from more than one population. Both groups apparently exhibit traits seen in their Middle Eastern and African predecessors. But the early modern specimens from Central Europe also display Neandertal traits, and the early modern Australians showed affinities to archaic Homo from Indonesia. "These features amount to a smoking gun for continuity within these regions," says team member John Hawks of the University of Utah.
"Ancient humans shared genes and behaviors across wide regions of the world, and were not rendered extinct by one 'lucky group' that later evolved into us," Wolpoff asserts. "The fossils clearly show that more than one ancient group survived and thrived." Eventually, multiregionalists argue, Neandertals and other archaic humans as entities disappeared through interbreeding. (Other paleoanthropologists dispute the new data, noting that previous analyses of these same skulls have supported the replacement model.)
To be sure, the new findings will not end the decades-long debate over modern human origins. Where it will go from here, however, is anyone's guess.
