the accepted theory of human development is that human beings arose out of Africa about 50,000 years ago. One of the questions was whether human beings interbred with other humanoids in other other parts of the world. A new study of human skulls supports both that human beings came out of Africa more than 50,000 years ago and that there was no interbreeding. The Scientific American has this report:
The shapes of skulls from around the world may have opened a new window onto the exodus of the first humans from Africa. According to a new report, groups of skulls from local populations are less diverse the farther those populations settled from the ancestral continent.
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.
Read it all here.
One exciting aspect of this study is that the variation in skulls can be used to deal with other interesting issues of human origins, such as the pathway of human expansion.