Published online 25 March 2004 | Nature | doi:10.1038/news040322-9

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Jaw-dropping theory of human evolution

Did mankind trade chewing power for a bigger brain?

A big jaw in chimps (top) could preclude the evolution of a large brain, as in humans (bottom).A big jaw in chimps (top) could preclude the evolution of a large brain, as in humans (bottom).© SPL

Researchers have proposed an answer to the vexing question of how the human brain grew so big. We may owe our superior intelligence to weak jaw muscles, they suggest.

A mutation 2.4 million years ago could have left us unable to produce one of the main proteins in primate jaw muscles, the team reports in this week's Nature1. Lacking the constraints of a bulky chewing apparatus, the human skull may have been free to grow, the researchers say.

The timing of the mutation is consistent with rampant brain growth seen in human fossils from around 2 million years ago, says Nancy Minugh-Purvis of the University of Pennsylvania in Philadelphia, who helped with the study. "Right at the point you lose power in these muscles, brain size evolution accelerates," she says.

Chewing it over

The story hinges on a protein called MYH16, a chief component of the powerful jaw muscles of many non-human primates such as chimpanzees and gorillas. When the researchers examined human DNA samples from across the world, they discovered that we all share a defect in the gene that creates this protein. Using estimates of evolution rates, they deduced the mutation's age.

The researchers then compared human skulls to those of other primates, and saw that even distantly related species, such as gorillas and macaques, share large crests on their skulls to which their heavy jaw muscles attach. Such structures are notably absent from human skulls despite our fairly close genetic kinship with gorillas.

Our ancestors may have lost their skull crests when our jaw muscles stopped exerting so much strain on the skull, suggests Minugh-Purvis's colleague Hansell Stedman, who led the study. "Muscle sculpts bone," he says. "The structure can be modified by the forces acting on it."

By doing away with large anchors for chewing muscles, our skull may have freed itself to grow into its modern, rounded shape, says Stedman. Powerful jaws may be incompatible with powerful brains, he suggests.

The theory is by no means proven. Daniel Lieberman, who studies human evolution at Harvard University in Cambridge, Massachusetts, points out that skull crests do not seem to limit the growth of other primates' brains. Chimpanzees' brains are fully grown by the time they are three years old, for example, while their skull crests do not develop until the age of eight or nine. "The brain itself is the major determinant of how the braincase grows," he argues.

Lieberman is also sceptical that our ancestors' brains blossomed immediately after the loss of jaw power. The early human Homo erectus had a small brain as recently as 1.8 million years ago, he says.

That could have left mankind with neither strong jaws nor a larger brain for several hundreds of thousands of years. But Stedman argues that a quick, if small, burst in brain size immediately after the mutation could have given early man some benefit in thinking power right away. "It is plausible that right out of the blocks they gained an advantage," Stedman says.

Humans may not have needed particularly strong jaws anyway, adds Minugh-Purvis. By then, our ancestors may have switched from eating chewy leaves all day long to snacking on smaller portions of meat, she says. 

  • References

    1. Stedman, H. H. et al. Nature, 428, 415 - 418, doi:10.1038/nature02358 (2004).  | Article | ISI | ChemPort |