Washington, March 18 : A new study has determined that gradual changes in human skull size and shape suggest a split between humans and Neanderthals about 300,000 to 400,000 years ago.
According to a report in National Geographic News, the study, done by researchers from University of California Davis, provides the first estimate a divergence date for modern humans and Neanderthals based on the rate of change of physical characteristics. The researchers said that just as DNA changes accumulate over time and provide a kind of "molecular clock" by which the separation of closely related species can be dated, evolved differences in physical form can provide similar information.
"But that is true only if the differences are due to the random process of "genetic drift," and not driven by natural selection," said study lead author Tim Weaver of the University of California Davis.
During genetic drift, different traits accumulate in separate populations by the spread of chance mutations-not because the traits provide any individual advantage in survival or reproduction.
The new study builds on previous work by Weaver's team suggesting that such random genetic changes are the reason people no longer sport the low forehead and protruding brow of our Neanderthal relatives.
"If differences in human skulls are due to genetic drift, then the amount of divergence will be proportional to the amount of time elapsed since the ancestors of Neanderthals and modern humans separated from each other," said Weaver.
The new study is based on a series of 37 measurements of the modern human and Neanderthal skulls.
For instance, researchers studied the width of the jaw and eye sockets and the distance between the various bones that make up the cranium.
Weaver's team was able to measure the rate at which changes in skull form have occurred.
Knowing that rate allowed the researchers to plot the differences between humans and Neanderthals backward in time and determine when the two groups separated from one another.
According to Erik Trinkaus, Neanderthal expert at Washington University in St. Louis, the new study by Weaver's team is valid in indicating that those aspects of the human cranium that are likely to be governed by random processes, such as drift, are in agreement with genetic analyses."
"Both of them are also in general agreement with the fossil record, which indicates that you start getting divergent aspects of human anatomy in Africa and Europe 300,000 to 500,000 years ago," he said.