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Brains vs. brawn. Shrinking jaw muscles may have helped a gorillalike skull (right) evolve into a human one, making way for a bigger brain.

Weak Jaw, Big Brain

Our brain is three times the size of our ancestors' and a muscle protein may be behind its rapid expansion. The protein is a building block of jaw muscles, and the discovery of a mutant form in all humans studied to date--but in no other primates--suggests that it played a role in human evolution, resulting in smaller muscles around the skull that made it easier for the brain to get bigger.

Researchers from all walks of science have long wondered how humans evolved. Among other traits, our jaws are smaller, our skulls are less rigid, and our brains are larger. But finding the genes that underlie these differences from our ancestors has been difficult.

Hansell Stedman, a gastrointestinal surgeon at the University of Pennsylvania in Philadelphia and his colleagues came across what appears to be one such gene by accident in their studies of the genetics of muscle movement. The new gene, MYH16, codes for a myosin. When the researchers compared it to the same gene in gorillas, chimps and other non-human primates, they found the human one had a flaw that resulted in a shorter-than-usual MYH16 protein and relatively weak muscles, they reported in the 25 issue of Nature.

Stedman and colleagues dated the origins of the mutation by comparing difference between the human gene and that of other primates. This molecular analysis indicates that the mutation appeared 2.4 million years ago, about the same time that human evolution took off. Stedman proposes that because of this genetic change, the primates' massive jaw muscles shrank, making possible a threefold expansion of the brain.

The theory is both exciting and questionable to other researchers. "It's an extremely important step in defining what makes us human," says Peter Currie, an evolutionary developmental biologist at the Victor Chang Cardiac Research Institute in Sydney, Australia. And it is a first in tying a change in a gene to a change in a protein that made a relevant functional change in primates. But some experts in human origins scoff at Stedman's conclusions. "To suggest that the brain is constrained by chewing muscles is just rubbish," asserts Ralph Holloway, a physical anthropologist at Columbia University.

Related sites
A comprehensive overview of human origins and related species and fossils
Images and a discussion of primate skulls
Summary of human evolution