Science Article

A chimpanzee's jaws are so powerful it can bite off a person's finger in one chomp. That is not a theoretical calculation; more than one primate researcher has lost a digit that way.

This finding, which came in 2004, caused a stir when the researchers argued that smaller jaw muscles could have allowed the growth of a bigger skull (Nature, vol 428, p 415). Primates with big jaw muscles have thickened supporting bone at the back of their skull, which arguably constrains skull expansion, and therefore that of the brain too. "We are suggesting this mutation is the cause of the decrease in muscle mass and hence the decrease in bone," says Hansell Stedman, a muscle researcher at the University of Pennsylvania in Philadelphia, who led the work. "Only then do you lift the evolutionary constraint that precludes other mutations that allow your brain to continue growing."

The team dated the mutation to 2.4 million years ago - just before our brain expansion took off. But another study, which sequenced a longer section of the muscle gene, came up with an earlier estimate for when the mutation occurred - 5.3 million years ago (Molecular Biology and Evolution, vol 22, p 379).

Whichever date is right, the mutation still happened after we split from our last common ancestor with chimps. Why would our ancestors switch to a weaker bite? Stedman speculates that rather than changes in diet being the catalyst, it could be that our ancestors no longer used biting as a form of attack. "At some point, perhaps through social organisation, this form of weaponry became more optional for our ancestors," he says.


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