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Putting a Face on the Past

13 December 2004 (All day)
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Changing face. Natural selection may have helped shape this 2.5-million-year-old australopith skull, but chance probably played a greater role in shaping facial features in the Homo lineage.

Natural selection is a force to be reckoned with in the biological world, but it may not explain the diversity of facial features found in our recent ancestors. A new analysis suggests that around the time our Homo ancestors took up tools, random genetic changes overtook adaptive evolution as the major influence on their facial features.

About 2.7 million years ago, two lineages arose from our bipedal ancestors: the robust australopiths, distinguished by their big heads and teeth, and the more petite-skulled Homo line. Although australopiths ultimately petered out, the Homo lineage diversified into several species, including eventually Homo sapiens. Scientists often invoke adaptations to a changing climate and environment to explain morphological changes such as bone size and shape in hominids. But some scientists argued that random processes such as "genetic drift" could play a role as well.

So Rebecca Ackermann of the University of Cape Town, South Africa, and James Cheverud of Washington University Medical School in St. Louis, Missouri, decided to take a new look at the forces shaping our old relatives. The scientists measured certain facial landmarks of seven fossil skulls of hominids and took similar measurements of adult humans, gorillas, and chimpanzees. Rather than assume that adaptation shaped morphology as previous studies had done, the researchers assessed the pattern of variation both within and between the groups.

Their analyses suggest that natural selection probably crafted australopith features such as big bones and a crested skull that may have anchored strong cheek muscles for chewing. But the scattershot pattern of variation among facial features within the genus Homo suggests that genetic drift played more of a role than natural selection, they report online this week in the Proceedings of the National Academy of Sciences.

Technological advances such as chopping and cooking may have lessened selection pressure on the face, says Cheverud. That makes sense to anthropological geneticist Charles Rosemann of Stanford University. At one time, he says, facial features such as the condition of the teeth would have played a large role in determining an individual's overall vigor. But that may have changed with the advent of tools--which happened roughly when the genus Homo arose. "If you are preprocessing food with stone tools, some features may become functionally unimportant," he says.

Related sites
National Museum of Natural History Hall of Human Ancestors
Rebecca Ackermann's page
More about early tools

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