The altered genes, associated with larger brain size, appeared at roughly the same times as the emergence of culture, agriculture, and written language in human populations, studies indicate.

The research team, led by Bruce T. Lahn of the Howard Hughes Medical Institute at the University of Chicago, published its findings in the September 9, 2005, issue of the journal Science.

Their analyses focused on detecting sequence changes in two genes -- microcephalin and abnormal spindle-like microcephaly associated (ASPM) -- across different human populations. In humans, mutations in either of these genes can render the gene nonfunctional and cause microcephaly -- a condition in which the brain does not develop to a normal size.

Survival of the Smartest?

Lahn and his colleagues conducted earlier studies that determined microcephalin and ASPM underwent an accelerated evolutionary response to the pressure of natural selection in the lineage that led to the human species.

"ASPM has evolved most rapidly in the last six million years of hominid evolution, after the divergence of humans and chimpanzees," says Lahn. The genetic changes presumably accumulate because they confer some competitive advantage.

"The next logical question was to ask whether the same process is still going on today, given that these genes have been under such strong selective pressure, leading to the accumulation of advantageous changes in the human lineage," suggests Lahn.

The researchers sequenced the two genes in an ethnically diverse selection of about 1,000 individuals. They also sequenced the genes in the chimpanzee to assess the extent of human-chimpanzee divergence.

Genes Linked to Cultural Advances

The current version of Microcephalin appeared about 37,000 years ago, and now occurs in about 70 percent of the present human population, while that of ASPM emerged about 5,800 years ago, and now shows up in approximately 30 percent of today's humans, according to the team's analysis.

Each variant apparently emerged concurrently with cultural advances in human populations. The appearance of the microcephalin variant occurred roughly at the same time as the emergence of such traits as art and music, religious practices and sophisticated tool-making techniques. The emergence of the ASPM variant first appeared at about the same time as the spread of agriculture, settled cities, and the first record of written language, the study indicates.

Larger brain size does not necessarily correlate to greater intellect, cautions Lahn. Very little is known about the impact of these variants. They may not have anything to do with cognition or intelligence, the researchers emphasize.

Cause or Coincidence?

These time windows are extraordinarily short in evolutionary terms, indicating that the new versions of the genes were subject to very intense natural selection pressure that drove up their frequencies in a very brief period of time, researchers point out.

A major question," says Lahn, "is whether the coincidence between the genetic evolution that we see and the cultural evolution of humans was causative, or did they synergize with each other?"

The human brain will likely continue to evolve under the pressure of natural selection, speculates Lahn. "What we can say is that our findings provide evidence that the human brain, the most important organ that distinguishes our species, is evolutionarily plastic.