for National Geographic News
Gradual changes in human skull size and shape suggest a split between humans and Neandertals (often spelled Neanderthals) about 300,000 to 400,000 years ago, according to a new study.
The work provides the first estimate of a divergence date for modern humans and Neandertals based on the rate of change of physical characteristics.
It also lends support to previous estimates that are based on DNA changes.
(Related: "Neandertal Gene Study Reveals Early Split With Humans" [October 26, 2006].)
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, researchers say.
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 Neandertal relatives.
If differences in human skulls are due to genetic drift, Weaver said, "then the amount of divergence will be proportional to the amount of time elapsed since the ancestors of Neandertals and modern humans [separated] from each other."
The study by Weaver's team appears this week in the journal Proceedings of the National Academy of Sciences.
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