Human Ancestors Went Out Of Africa And Then Came Back: Researchers Propose Controversial New Model For Evolution Of Humans And Apes

SUNY-Albany biologist Caro-Beth Stewart and NYU anthropologist Todd R. Disotell have proposed a controversial new model for the evolution of apes and humans, which together are called the hominoids. Stewart and Disotell argue that the ancestor of humans and the living African apes evolved in Eurasia, not Africa.

This controversial new model for the evolution of humans and apes is the cover story of the July 30th issue of Current Biology. Stewart and Disotell describe their theory in an article entitled "Primate evolution -- in and out of Africa."

Today, the lesser apes (gibbons and siamangs) and some great apes (orangutans) live in Southeastern Asia, while other great apes (gorillas and chimpanzees) live in Equatorial Africa. The fossil record indicates that apes were present in Europe and Western Asia during the Miocene Era, from about 8 to 17 million years ago. Ancestors of these ape species must have moved between the African and Eurasian land masses during their evolutionary history. According to the theory traditionally held by most paleoanthropologists, the hominoids evolved in Africa. The lesser apes and orangutans subsequently dispersed out of Africa to Eurasia at different times, leaving behind representatives of the lineage leading to the gorillas, chimpanzees and humans.

Based on a synthetic analysis of molecular, fossil, and biogeographical data for the primates, Stewart and Disotell propose instead that the lineage leading to the common ancestor of all living apes dispersed out of Africa about 20 million years ago (during the early Miocene) and then speciated into the greater and lesser Ape lineages in Eurasia. Within the past 10 million years, one of the great ape species dispersed back into Africa. This lineage eventually speciated into gorillas, chimpanzees and humans.

This theory marks a significant departure from the long-held view that the evolutionary history of the lineage leading to humans was confined to the African continent. A theory similar to Stewart and Disotell's was proposed more than 25 years ago by pioneering molecular anthropologistVincent Sarich, but he lacked the rigorous analytic methodology necessary to prove it.

Stewart and Disotell's research is based on parsimony analysis. That is, the model which involves the fewest evolutionary events to explain the data is the most plausible. The technique -- which was first developed by entomologists and has been used only recently by anthropologists -- uses computer technology to analyze large sets of data and identify the most parsimonious evolutionary model.

Using parsimony analysis, Disotell and Stewart find it more likely that the common ancestor of the "great" apes (orangutans, African apes and humans) evolved in Asia rather than in Africa. The problem with the traditional model, say Disotell and Stewart, is that it calls for at least six separate dispersal events out of Africa to account for all living and extinct hominoid species in Eurasia. Disotell and Stewart's model -- which requires only two hominoid migration events -- is a more parsimonious model; and therefore, they assert, a more plausible one.

Caro-Beth Stewart, an associate professor of biology at Albany, was a 1994 winner of the National Science Foundation Presidential Faculty Fellow Award. Her research into the molecular basis for adaptive evolution of higher organisms has also been funded by the National Institutes of Health.

Todd Disotell is about to become an associate professor of anthropology at NYU after recently being promoted. He has set-up and runs a molecular anthropology laboratory where he is investigating various aspects of ape and Old World monkey evolution supported by a National Science Foundation CAREER Award for young investigators. With additional NSF funding, he is applying the latest automated approaches to DNA sequencing and typing. He is particularly interested in applying new laboratory and analytical techniques to molecular evolutionary problems. He also teaches and lectures quite extensively about human variation and race. He received his Ph.D. in physical anthropology from Harvard University in 1992.