The human brain is not a chimp brain. Sure, somewhere between five million and eight million years ago, before the two species diverged, there was no difference. But at this point, despite our bodies weighing only 20 percent more than those of chimps, the human brain weighs 250 percent more and contains 50 percent more neurons. Eight million years notwithstanding, that would seem to be a sign of rapid change.

But a team of researchers from the University of Chicago, as well as from institutes in Taiwan and Japan, says not so fast, at least with regards to the speed at which the human mind has evolved since its split with the chimpanzee.

"We found that genes expressed in the human brain have in fact slowed down in their evolution, contrary to some earlier reports," states Chung-I Wu, an evolutionary biologist at the University of Chicago and the co-author of a study appearing in the most recent issue of PLoS Biology. "The more complex the brain, it seems, the more difficult it becomes for brain genes to change."

Wu and his colleagues compared the number of substitutions in sequences coding for thousands of genes expressed the brain, liver or muscle within the genomes of mice, macaque monkeys, chimpanzees and humans. What the researchers discovered, unsurprisingly, is that the more sophisticated species had faster rates of overall evolution when compared to less sophisticated species.

However, when the team turned to genes expressed solely in the brain (or in brain tissue as well as other tissues), the trend reversed. Brain-expressed genes in the mouse evolved most quickly; those in humans most slowly.

Wu reasons that when compared to the brains of other animals, the myriad interactions between proteins in the human brain have slowed down its evolution. "We know that proteins with more interacting partners evolve more slowly," explains Wu. "Mutations that disrupt existing interactions aren't tolerated."

Rather than alterations to protein sequences, Wu hypothesizes that it is changes in gene expression that have set humans apart from their chimpanzee cousins. He cites previous reports that found that the number of genes expressed per neuron in the human brain is much greater than that in chimps.

The authors suggest that it is this increased transcription--from genetic sequence to active protein--that is responsible for the higher-order network that leads to the complexity of the human brain.

Stephen Dorus, a biologist at England's University of Bath, notes an important aspect not considered in the study: whether there was rapid evolution of brain genes during primate evolution compared to other mammalian species. " The morphological and cognitive advancement in primates has happened in the last 25 million years not simply since the divergence of chimps and humans," he says. "As such, although these genes may be evolutionarily conserved in recent human evolution, they may have undergone significant evolutionary changes in more ancestral primates."