Humans Emerged "Out Of Africa" Again And Again

Templeton, Charles Rebstock Professor of Biology in Arts & Sciences at Washington University, combined evidence from many different populations and many different genes in an analysis to reconstruct their movement and history.

Africa has played a dominant role in shaping the modern gene pool through successive population expansions, he says in the March 7, 2002 issue of Nature. But these populations interbreeding with resident populations means that genetic interchange between populations has occurred everywhere throughout history.

Templeton analyzed human genetic trees for maternally inherited mitochondrial DNA, paternally inherited Y-chromosomal DNA, and eight other DNA regions , including two on the X chromosome, to reach his conclusions. He used a computer program called GEODIS, which he created in 1995 and later modified with the help of David Posada, Ph.D., and Keith Crandall, Ph.D. at Brigham Young University, to determine genetic relationships among and within populations based on an examination of specific haplotypes, clusters of genes that are inherited as a unit. Templeton’s study is based on 10 DNA regions, while most other genetic analyses focus on just one, mitochondrial DNA, for instance. It also differs from most approaches because it uses a statistical approach with a priori inference criteria but requires no prior model of human evolution. Most others have a model in mind, and then see if the data are compatible with it.

GEODIS analyses place an older expansion out of Africa between 420,000 and 840,000 years ago and a more recent one between 80,000 and 150,000 years ago. GEODIS analyses also show conclusively, Templeton states, that the most recent out-of-Africa expansion event was not a replacement event. Replacement means the new population wiped out an existing one in Europe or Asia, resulting in their complete genetic extinction.

"If it had been (a replacement event), the three significant genetic signatures of the older expansion event and the six significant genetic signatures of older recurrent gene flow would have been wiped away," Templeton writes.

It is likely that the earlier out-of-Africa expansion also was characterized by interbreeding rather than replacement, but Templeton emphasizes that the evidence for this is tentative because the probability of such old gene flow is not statistically high enough.

"Humans expanded again and again out of Africa," Templeton concludes, "but these expansions resulted in interbreeding, not replacement, and thereby strengthened the genetic ties between human populations throughout the world."

The work was supported in part by a Burroughs Wellcome Fund Innovation Award in Functional Genomics.

Templeton, who joined the Washington University faculty in 1977, is a renowned population and evolutionary biologist who has analyzed the genomes of many different species to better understand their evolution and their survival. Since 1984, he has been the head of the Evolutionary and Population Biology Program in Washington University’s Division of Biological Sciences.

Templeton’s contributions to the controversy of recent human evolution include dashing the popular ‘Eve Theory’ because of flaws he detected in researchers’ 1987 computer analyses. In 1998, he published a paper in American Anthropologist that explained humans as one race, instead of a species with subdivisions, or races. His study showed that, among people now categorized by race, everyone shares about 85 percent of the same genes. The 15 percent of variation is not enough difference to separate people biologically.