Earlier fossil record and genetic studies had clearly show that the evolution of higher cognition began sometime after the chimp and human lines split, some 5 million to 6 million years ago, and continued at least until the rise of Homo sapiens (modern humans) 200,000 years ago.

But two genes linked to brain size are rapidly evolving in humans, indicating that the evolution of the human brain may not have stopped after Homo sapiens first came on the scene, said the researchers at the University of Chicago. Their two related papers were published in the Sept. 9 issue of Science.

"Our studies indicate that the trend that is the defining characteristic of human evolution--the growth of brain size and complexity--is likely still going on," said lead researcher Bruce Lahn, an assistant professor at the University of Chicago.

"Meanwhile, our environment and the skills we need to survive in it are changing faster then we ever imagined. I would expect the human brain, which has done well by us so far, will continue to adapt to those changes."

Evolution doesn't occur at the species level. Rather, some individuals first acquire a specific genetic mutation, and because that variant confers on those who bear it a greater likelihood of survival, it then spreads in the population, according to Lahn.

Lahn previously showed that there was accelerated evolution in humans among numerous genes, including microcephalin and abnormal spindle-like microcephaly-associated (ASPM). Both of these genes regulate brain size.

In the new study, the researchers investigated variations of microcephalin and ASPM within modern humans. They surveyed gene samples of more than 1,000 individuals representing 59 ethnic populations worldwide to determine the variation frequency of the two genes.

They found evidence that the two genes have continued to evolve, and for each gene, one class of variants has arisen recently and has been spreading rapidly because it is favored by selection.

For microcephalin, the new variant class emerged about 37,000 years ago and now shows up in about 70 percent of present-day humans. For ASPM, the new variant class arose about 5,800 years ago and now shows up in approximately 30 percent of today's humans.

These time windows are extraordinarily short in evolutionary terms, indicating that the new variants were subject to very intense selection pressure that drove up their frequencies in a very brief period of time--both well after the emergence of modern humans about 200,000 years ago.

Each variant emerged around the same time as the advent of " cultural" behaviors, said the researchers.

The microcephalin variant appears along with the emergence of such traits as art and music, religious practices, and sophisticated tool-making techniques, which date back to about 50, 000 years ago. The ASPM variant coincides with the oldest-known civilization, Mesopotamia, which dates back to 7,000 BC.

"Microcephalin has continued its trend of adaptive evolution beyond the emergence of anatomically modern humans. If selection indeed acted on a brain-related phenotype, there could be several possibilities, including brain size, cognition, personality, motor control or susceptibility to neurological/psychiatric diseases," their one paper said.