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Skull Changes Show Time of Human-Neandertal Split

Scott Norris
for National Geographic News
March 17, 2008
 
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].)

Genetic Drift

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.

(Get the basics on DNA.)

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.

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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