Last month, a team of scientists at the University of Pennsylvania announced that it might have discovered a crucial event in the evolution of our species. The news was surprising and not a little deflating. The mutation that lifted us above the other apes had nothing directly to do with our brain. Rather, it had to do with our jaw. What led to the emergence of modern humans, in all of our culture-creating, science-discovering, globe-dominating glory, was a chance defect in a gene responsible for the formation of our jaw muscles. So, at least, the scientists speculated.

Why should the size of our jaw make us the intellectual species par excellence? The next time you are chewing a mouthful of food, feel the side of your head with your fingers. You'll notice the muscles there moving as your jaw opens and closes. The jaw muscles are attached to the top of the skull. The larger those muscles are, the thicker the skull bone needs to be to anchor them. That is why apes, with their powerful, jutting jaws, have a bony crest running across the top of their heads. Anything that caused the jaw muscles to become smaller would also allow this thick bone to be shaved away over time. That, in turn, would free up space for the brain to expand into -- ''room for thought,'' as one of the scientists whimsically put it.

The mutation in the jaw-muscle gene, genetic analysis indicates, occurred around 2.4 million years ago. Not coincidentally, the fossil record shows that hominid skulls started looking more like modern human skulls shortly thereafter. Our protohuman kin became not only smarter than the apes but also, with their flatter faces and more graceful brows, rather better looking.

Did a jaw mutation really ''cause'' the enlarged brains that make us human? Darwin himself, curiously enough, suggested just the opposite: first we acquired our bigger brains, then our smaller jaws. The expansion of the brain, he theorized, occurred in tandem with the spread of hunting and tool-making. After all, a meal of meat takes intelligence to obtain, and it also provides rich protein for hungry brain tissue. And once we became meat eaters, we could afford smaller jaws, since meat needs less chewing than nuts and plants, especially if you're clever enough to tenderize it over a fire and cut it with a blade. Moreover, a knack for fashioning weapons makes a powerful muzzle less important. As our ancestors ''gradually acquired the habit of using stones, clubs or other weapons for fighting with their enemies,'' Darwin observed in ''The Descent of Man,'' ''they would use their jaws and teeth less and less. In this case, the jaws, together with the teeth, would become reduced in size.''

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Since those words were written, evolutionary theorists have come up with a bewildering variety of explanations for how natural selection might have produced human intelligence. Steven Pinker agrees with Darwin that hunting and tool-making had a lot to do with it, but he also cites the fact that our hominid ancestors were social creatures. Living in groups, they had to compete with one another with increasing shrewdness to further their interests. Such competition, the idea goes, might have set off a ''cognitive arms race'' that led to rapid growth in brain size. Another view, advanced by Geoffrey Miller, is that sexual selection explains the evolutionary push toward intelligence. The human brain, according to this hypothesis, is rather like the peacock's tail: a courtship device to attract and retain sexual mates. Wit, virtuosity and inventiveness are turn-ons, and those that have them end up producing more offspring.

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The late Stephen Jay Gould, by contrast, said he believed that our lurch into intelligence wasn't really driven by anything at all. He held that random genetic drift caused a slowing-down in the emergence of adult features in each individual. In fact, humans, with their relatively big brain cases, small jaws and hairless skin, look like baby apes. The prolonged period of development before adulthood gives our brains a chance to grow to three times the size of an ape's. Much of this growth necessarily takes place outside the womb, since the female pelvis can barely accommodate the newborn's enormous head as it is. (The pain women undergo in childbirth is part of the price we pay for our big brains.)