Of all the features that distinguish us humans from other primates, the size of our brains is arguably the most profound. At 1,350 cubic centimeters on average, the human brain is three times larger than would be expected for a nonhuman primate of our body weight. But humankind hasn't always been so cerebral. Indeed, for most of our lineage's existence--which, by current estimates, spans some six million years--hominid brain size was decidedly apelike. Around two million years ago, however, the human brain began to balloon. Anthropologists have long puzzled over what prompted this gray matter growth. A new hypothesis proposes that a genetic mutation associated with the reduction of the jaw muscles played an important role.

Most primates--including chimpanzees and gorillas--have powerful chewing muscles for breaking down tough plant foods, and early hominids such as Australopithecus had comparable jaw power. Modern humans possess a far daintier food processor. The difference, according to a report published today in the journal Nature, may stem from a mutation in MYH16, a previously unrecognized member of a class of genes that encode proteins central to muscle contraction. Hansell H. Stedman of the University of Pennsylvania and his colleagues found that whereas all nonhuman primates surveyed carried intact copies of MYH16 and had high levels of the associated protein in their jaw muscles, humans have an inactivated version of the gene.

Further analysis enabled the team to date the origin of the mutation in a hominid ancestor at 2.4 million years ago, give or take a few hundred thousand years--just prior to the reduction in the human chewing apparatus evident in the fossil record. What's more, this is also roughly around the time that hominid brain size was beginning to swell considerably. The researchers submit that the mutation led to the shrinking of the jaw muscles, which in turn eliminated stress on the skull. Thus freed from its evolutionary shackles, so the idea goes, the hominid brain attained proportions never before seen in a primate.

"Stedman and colleagues identification of the first molecular difference between human and non-human primates, traceable to an anatomical difference in the fossil record, provides independent evidence and fresh ideas with which to describe the mechanistic basis of hominid evolution," writes Pete Currie of the Victor Chang Cardiac Research Institute in Sydney, Australia, in an accompanying commentary. Noting the pending completion of the sequencing of the chimp genome, which will enable whole-genome comparisons between humans and our closest living relative, Currie adds that this new study more than any other before it "suggests that the genetic basis of human evolution can and will be defined." --Kate Wong