The fight over modern human origins is heating up. A new study of thousands of human skulls claims to confirm genetic evidence that our species arose in Africa and then spread over the globe. But some researchers say that an alternative scenario has not been ruled out.
Researchers have long debated two opposing hypotheses for modern human origins. According to the Out of Africa hypothesis, our ancestors appeared in Africa about 200,000 years ago and then replaced all other human species, including Homo erectus and the Neandertals, with little or no interbreeding. The multiregional hypothesis holds that modern humans emerged from populations of "archaic" hominids in Africa, Europe, and Asia that evolved locally but also exchanged genes. Numerous genetic studies support the single-origin model, finding that the genetic diversity of today's human populations is greatest in Africa and decreases steadily with distance from that continent. The idea is that diversity declined because each group of migrants founded a new population, creating genetic bottlenecks. But some researchers see traces of mixing between moderns and archaics in the genetic data.
A team led by population biologist Andrea Manica of the University of Cambridge in the U.K. set out to test the two hypotheses with skulls rather than genes. The researchers analyzed 4666 male skulls from 105 worldwide populations. Based on 37 measurements--ranging from the length of the cranium to the height of the eye sockets--the team reports this week in Nature that the worldwide pattern of skull shapes closely matches the genetic data: The diversity of cranial shape within a population falls off the farther it is from Africa. Similar results came from a second study of 1579 female skulls. The researchers could find no evidence for multiple centers of diversity outside Africa, as might be predicted by the multiregional model. They concluded that their results strongly support the Out of Africa model.
"This is an important piece of work because it compares results from large sets of genetic and cranial data using similar analytical approaches," says paleoanthropologist Chris Stringer of the Natural History Museum in London, a primary advocate of the Out of Africa model. Yet Stringer cautions that the study cannot rule out the possibility of gene flow between Homo sapiens and other humans such as Neandertals. That exchange might not show up in the skulls because the authors used only crania that were no more than 2000 years old.
Charles Roseman, an anthropologist at the University of Illinois, Urbana-Champaign, says that he is not convinced that the Nature authors have adequately tested the Out of Africa model versus its multiregional rival. The researchers assumed that the multiregional model requires that modern humans arose more than once. "Proponents of the multiregional model have been very clear for some time that their models do not posit multiple origins, as suggested in the paper," Roseman says.