DNA data yield new human-origins view.

The genetic story of human evolution has taken a surprising turn. Anatomically modern Homo sapiens sprang from a population of so-called archaic H. sapiens that lived in Africa around 200,000 years ago, a new DNA investigation suggests. That ancestral group then split into African and Asian branches, which interbred to some extent as they evolved into today's humans.

This scenario differs from the two theories of human evolution that have been vying for dominance. Out-of-Africa supporters hold that a single African population of H. sapiens emerged between 100,000 and 200,000 years ago and then spread elsewhere. In contrast, multiregional enthusiasts argue that, starting as early as 2 million years ago, human groups in Africa, Europe, and Asia interbred enough to evolve collectively into modern H. sapiens.

"Our data fall between the predictions of current human-origins theories," says geneticist Jody Hey of Rutgers University in Piscataway, N.J., who conducted the new study with Rutgers anthropologist Eugene E. Harris. "Researchers may need to rethink their inferences about human evolution."

Harris and Hey examined a nucleotide sequence on the X chromosome of 16 African and 19 non-African men, as well as 2 chimpanzees. The sequence is part of a gene that regulates the chemical breakdown of glucose in the body.

Samples from the African and non-African groups displayed marked genetic differences, the scientists report in the March 16 Proceedings of the National Academy of Sciences. The variation exceeds that typically seen at other chromosomal sites when populations are compared.

One particular nucleotide sequence occurred in all the Asian men and none of their African counterparts. Comparison of the chimp data with the human allowed the researchers to estimate that this mutation, marking the evolutionary split between the groups, arose about 200,000 years ago.

The oldest sequences, which occurred in the African group, originated approximately 1.9 million years ago, Harris and Hey contend.

The X-chromosome sequence varied greatly among the Africans but little among the Asians. This pattern suggests that natural selection affected this particular gene differently in the two groups, according to Hey.

Since modern H. sapiens fossils date at most to 130,000 years ago, the X-chromosome findings support the notion that an early form of H. sapiens split into separate African populations around 200,000 years ago, Hey says. One group then migrated into Asia, he theorizes, although its members probably didn't trek far into that continent.

A modest amount of interbreeding between African and Asian populations then would have fostered the concurrent evolution of modern human traits, in Hey's view.

"These new DNA data show surprisingly ancient separations in our species," comments anthropologist Henry C. Harpending of the University of Utah in Salt Lake City. The results contradict mitochondrial DNA studies, including some coauthored by Harpending, which place modern humanity's roots in a single African population that greatly expanded, its numbers around 50,000 years ago as it spread to other regions.

Suspicions have arisen that natural selection can rapidly reshape mitochondrial DNA structure, casting doubt on the ability of that genetic material to clarify the time and place of modern human origins (SN: 2/6/99, p. 88).