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Gene, fossil data back diverse human roots

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12:01pm, January 12, 2001
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Just how modern humans first proliferated remains a great mystery of anthropology. Several recent fossil and genetic studies support the influential notion that a small population of Homo sapiens in Africa around 100,000 years ago began to spread, replacing humanlike species such as Neandertals.

Two new investigations raise doubts about this out-of-Africa scenario. One, slated to appear in the Proceedings of the National Academy of Sciences, finds that ancient mitochondrial DNA retrieved from a 62,000-year-old Australian H. sapiens fossil differs greatly from the DNA of living people. This evidence is consistent with the multiregional theory of human evolution. Different mitochondrial DNA lineages could have flourished and then disappeared as modern H. sapiens evolved simultaneously in two or more parts of the world over the past 1 million to 2 million years.

The second study, published in the Jan. 12 Science, compares the anatomy of H. sapiens skulls from different regions and times. Led by Milford Wolpoff of the University of Michigan in Ann Arbor, the study's authors conclude that modern humans evolved separately in Central Europe and Australia, each lineage arising as a regional variation on an anatomical theme that originated in Africa nearly 2 million years ago.

"These two studies dovetail nicely in their support of multiregional evolution," says Wolpoff.

Some of the most striking evidence cited by out-of-Africa supporters comes from Neandertals' mitochondrial DNA, which contains nucleotide sequences that differ considerably from those of modern humans (SN: 7/19/97, p. 37). These scientists argue that the disparities in mitochondrial DNA—which is inherited from the mother—underscore Neandertals' evolutionary status as a separate, dead-end species.

The mitochondrial DNA extracted from early H. sapiens fossils in Australia may usher Neandertals back into the modern human fold, according to a team led by Gregory J. Adcock of Pierre and Marie Curie University in Paris.

He and his colleagues analyzed mitochondrial DNA from 10 modern-human fossils ranging in age from about 2,000 to 62,000 years. Genetic material from the oldest specimen, which was found at Lake Mungo in southeastern Australia, differs more from that of living people than do the previously isolated Neandertal sequences, the researchers contend. Mitochondrial DNA from the other, younger Australian fossils closely resembles that of humans today, they find.

"If the mitochondrial DNA sequences present in a modern human [the Lake Mungo individual] can become extinct, then perhaps something similar happened to the mitochondrial DNA of Neandertals," proposes John H. Relethford of the State University of New York at Oneonta in a comment published with Adcock's research. If so, a lack of Neandertals' mitochondrial DNA in modern samples may not rule out a contribution to human evolution.

Mark Stoneking of the Max Planck Institute for Evolutionary Genetics in Leipzig, Germany, questions the Australian data. Adcock's team needs to check for contamination of its ancient DNA and then have the results confirmed by an independent laboratory, Stoneking holds.

Wolpoff's group assessed the extent to which samples of early modern-human fossil crania from Central Europe and Australia—all between 20,000 and 30,000 years old—share anatomical features with older H. sapiens crania from Indonesia, the Middle East, and Africa, as well as European Neandertals.

In line with the multiregional origins theory, the fossil crania of European

H. sapiens showed distinctive links to older European Neandertal crania, whereas Australian specimens shared certain traits only with Indonesian H. sapiens fossils. Still, Central European and Australian H. sapiens shared a common core of cranial features also found in Middle Eastern and African predecessors, Wolpoff says. For the past 2 million years, such far-flung populations have interbred enough to maintain their collective status as H. sapiens, he asserts.

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