Using our Heads
The new study is based on a series of 37 measurements of the modern human and Neandertal 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.
Different human populations today have, on average, slight differences in cranial features. Our heads are not all shaped exactly the same—and neither are our genes.
Scientists can use changes in DNA sequences to estimate the amount of time that has passed since present-day human populations began to diverge.
With this information, 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 Neandertals backward in time and determine when the two groups separated from one another.
"The striking finding is that we obtained such similar dates to those from DNA sequences by assuming that [skull form] diverged by genetic drift," Weaver said.
Erik Trinkaus is a Neandertal 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," he said.
"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."
(Related story: "Modern Humans Came Out of Africa, "Definitive" Study Says" [July 18, 2007].)
Not so Random
Not all changes in the human skull have been random.
The large heads and brains of both humans and Neandertals result from natural selection that occurred earlier in our evolutionary history, experts say.
Once these features evolved, however, the main role of natural selection on the skull has been to constrain its dimensions to a range of variation necessary to accommodate and protect the brain, Weaver said.
"Within this range, cranial form may have virtually no influence on which individuals survive and reproduce," he noted.
"Under these circumstances, changes in cranial form will be due to chance."
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