Handsome by Chance

August 1, 2007

Chance, not natural selection, best explains why the modern human skull looks so different from its Neanderthal predecessor, according to a new study by Timothy Weaver, assistant professor of anthropology at UC Davis.

The study appears in the August issue of the Journal of Human Evolution.

"For 150 years, scientists have tried to decipher why Neanderthal skulls are different from those of modern humans," Weaver said. "Most accounts have emphasized natural selection and the possible adaptive value of either Neanderthal or modern human traits. We show that instead, random changes over the past 500,000 years or so -- since Neanderthals and modern humans became isolated from each other -- can better explain these differences."

Weaver and his colleagues compared cranial measurements of 2,524 modern human skulls and 20 Neanderthal specimens, then contrasted those results with genetic information from a separate sample of 1,056 modern humans.

The scientists concluded that Neanderthals probably did not develop their prominent brow ridges and pronounced overbites as adaptations to icy Pleistocene weather or the demands of using teeth as tools, as some anthropologists have proposed.

Instead, random "genetic drift" is the likeliest reason that humans acquired smooth foreheads and strong lower jaws.

Weaver conducted the research with Charles Roseman, an anthropologist at the University of Illinois at Urbana-Champaign, and Chris Stringer, a paleontologist at the Natural History Museum in London.