The result supports the popular scientific theory that modern humans swept "out of Africa" some 50,000 years ago and supplanted earlier species such as Neandertals. It may also help researchers better pinpoint where in Africa modern humans came from and how messy the exodus could have been, says evolutionary geneticist William Amos of the University of Cambridge in England.
Amos, Cambridge evolutionary biologist Andrea Manica and their colleagues analyzed shape data from 4,666 male skulls, all less than 2,000 years old, collected from 105 places around the world. For each location, they compared the variation in 37 different measurements with the distance the population's ancestors would have had to travel to get there from Africa. (Cairo and New York City appear relatively close on a map, but early humans would have had to hoof it across Asia and the Bering Strait to reach North America from Africa.)
As smaller bands broke off from larger settlements, they would have carried with them a less diverse subset of the bigger group's genes, which partly translate into anatomical features such as skull shape. So the farther early Homo sapiens trod from their homeland, the less variable their skulls should become. Unless, that is, they bred with previously established populations of Neandertal or other early humans, which would have injected new genes and boosted variability.
The researchers found no signs of interbreeding, they report online today in Nature. "What you find is a very nice linear decline of variability as you move farther away from Africa," Amos says. Prior studies had identified an identical trend in the diversity of simple genetic sequences or markers.
"The beauty of the skulls," Amos says, "is there are so many of them and they come from populations that are not very well represented genetically," such as aboriginal Americans and Australians. He notes that additional skulls from African populations could help map early human migrations from or to Ethiopia, where the oldest known human remains originate.
Physical anthropologist Erik Trinkaus of Washington University in St. Louis says few experts doubt the out-of-Africa scenario in broad terms. "The issue is how much modern humans spreading out of Africa after 50,000 years ago interbred with regional groups of archaic humans, where and when"—something we may never know, he says. The new study, he contends, only "reinforce[s] the idea of a special isolation of modern humanity from anything less 'pure.'"
Amos says that even very limited interbreeding would have disrupted the skull trend if the offspring had survived and propagated—although the extent of that limit has yet to be worked out. The big unknown, he says, is the messiness of the African exodus, including its timing among different groups. Mathematical models of migration could shed light on that problem, he says, adding that the skull data "open up lots of nice testable hypotheses."