Jaw-dropping theory of human evolution
            Did mankind trade chewing power for a bigger brain?
            25 March 2004
            MICHAEL HOPKIN


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


            © Nature News Service / Macmillan Magazines Ltd 2004



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