We may have the most complex brains of any primate, says Universityof Chicago geneticist Chung-I Wu, but chimpanzees’ brains are evolvingfaster. Wu and his colleagues compared DNA sequences of genes expressedin the brains of humans, chimpanzees, and Old World monkeys. They foundthat while other primates seem to be experiencing rapid changes,humans’ brain genes are surprisingly static. “I was expecting to findthat a few genes would be evolving rapidly, while probably the overalldistribution would be changing at about the same rate among all theprimates, but instead we saw that the brain’s gene evolution in thehuman lineage has actually slowed down,” Wu says.

The human brain’s astounding complexity may be at the root of thedecelerating genetic changes, he adds. Because genes in the brain codefor proteins that interact with many other molecules, the wiggle roomfor evolutionary tinkering is limited: Change a gene too much and itwill be unable to continue its existing functions.

Does this mean that other primates are gaining on us? Notnecessarily, Wu says. In fact, “the higher rate of evolution in chimpsprobably just reflects the fact that each gene in that species hasfewer other genes interacting with it than in humans,” says Wu. “Andsince the complexity of the interactive network likely determinesintelligence, the answer is probably the opposite.”