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