Magazine article Science News

Gene Change Tied to Ancestral Brain Gains

Magazine article Science News

Gene Change Tied to Ancestral Brain Gains

Article excerpt

A genetic loss approximately 2.4 million years ago may have made cranial room for the bigger brains that characterize our direct evolutionary predecessors. That proposal comes from researchers who have discovered a DNA deletion that occurs in people but not in other primates.

In what started out as a search for genes linked to muscular dystrophy, a team led by surgeon Hansell H. Stedman of the University of Pennsylvania School of Medicine in Philadelphia found that only people possess an inactivated version of a gene involved in facial-muscle movements. As a result, the gene fails to produce a variant of the protein, called myosin, that powers muscles used in biting and chewing, the scientists report in the March 25 Nature.

This genetic mutation explains much about why "we're the odd men out among primates regarding head shape," Stedman says. As a person grows, he argues, genetically constrained chewing muscles lead to relatively small jaws, thus permitting the deposition of additional cranial bone to encase a large brain.

After examining a segment of the particular myosin gene in people, chimpanzees, orangutans, macaque monkeys, and dogs, the researchers estimated that the gene-inactivating mutation occurred between 2.7 and 2.1 million years ago. To make this calculation, the researchers assumed, on the basis of prior molecular evidence, that the last common ancestor of people and chimps lived 7 million to 6 million years ago.

"Massive muscles of mastication that were present before this myosin-gene mutation occurred could have been constraining brain size by limiting the expansion of [cranial] plates in the developing skull," says University of Pennsylvania School of Medicine anatomist Nancy Minugh-Purvis, a coauthor of the new study.

The myosin-gene mutation appeared uniformly in DNA samples obtained by the investigators from people living in Africa, South America, Western Europe, Iceland, Japan, and Russia. …

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