The scientific debate over the nature of human evolution has taken a new, genetically inspired twist.
Ancient humans migrated out of Africa in at least two major waves, and human groups in Africa, Asia, and Europe have interbred for the past 600,000 years, says geneticist Alan R. Templeton of Washington University in St. Louis.
Templeton’s conclusion clashes with the influential theory that modern Homo sapiens originated in Africa around 100,000 years ago and then colonized the rest of the world. In this scenario, humans replaced now-extinct European Neandertals but did not interbreed with them.
“Humans expanded again and again out of Africa, but these expansions resulted in interbreeding, not replacement, and thereby strengthened the genetic ties between human populations throughout the world,” Templeton says.
His analysis of geographic patterns in evolutionary trees constructed from DNA in current populations of Africa, Asia, and Europe appears in the March 7 Nature. He reanalyzed previously identified DNA sequences of cells’ mitochondria, which are inherited from the mother; of the Y chromosome, which is inherited from the father; and of eight regions of nuclear genetic material, which is inherited from both parents.
Most previous studies have reconstructed human evolution from one gene or one type of DNA. The resulting genetic disparities among populations have been interpreted to support a 100,000-year-old African origin for humanity or, occasionally, to argue for interbreeding among widespread H. sapiens groups over at least the past 1 million years (SN: 2/6/99, p. 88: http://www.sciencenews.org/sn_arc99/2_6_99/bob1.htm).
Templeton developed a computer program to test whether evolutionary trees, generated from variations in the 10 genetic sequences, exhibit geographic patterns over time. From the patterns that resulted, he calculated whether populations in different regions consistently interbred or had at some point severed their genetic ties. Templeton also modeled population movements that spread particular gene mutations from one area to another.
Interbreeding was a mainstay of human evolution well before 100,000 years ago, Templeton contends. The evolutionary trees that he devised indicate that four nuclear DNA sites first arose approximately 600,000 years ago, he says. Of the remaining six DNA regions in his study, four of them appeared between 500,000 and 200,000 years ago.
A major migration of people from Africa to Asia occurred between 840,000 and 420,000 years ago, Templeton estimates. A second large out-of-Africa migration followed at around 100,000 years ago. If that event had resulted in the replacement of non-African groups, it would have erased genetic evidence of the older expansion, he asserts.
“This new analysis is complicated, but it makes the most sense of any genetic study of evolution that I’ve seen,” comments anthropologist John H. Relethford of the State University of New York at Oneonta. He agrees with Templeton that human origins lie mostly but not completely in Africa and that dispersed populations interbred.
Templeton’s method is promising, adds geneticist Michael F. Hammer of the University of Arizona in Tucson. Still, he says, one difficulty for Templeton is that no one knows the exact distribution of Stone Age groups in Africa and elsewhere.
Stanford University geneticist Peter A. Underhill is more critical of Templeton’s approach. The number of people whose nuclear DNA sequences were analyzed in the new report was too small to provide convincing evidence, Underhill says.
No ancient gene sequences have been identified in living people that reflect their ancestors’ interbreeding with Neandertals or any other extinct Homo species, the Stanford researcher says.