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.
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.