The research, led by Bruce T. Lahn of the University of Chicago, focused on two genes called microcephalin and ASPM. When these genes malfunction, the result is a condition called primary microcephaly, in which brain size is severely diminished. Previous studies across millions of years of primate lineages indicated that these genes had undergone accelerated evolution at times after the chimp and human lines diverged, hinting they played a role in the emergence of our impressive brain size. Lahn's team analyzed the genes in an ethnically diverse sample of 90 individuals from around the world. They found that for both genes, one variant occurred much more frequently than would be expected by chance, suggesting that natural selection is at work. The microcephalin variant arose about 37,000 years ago; the ASPM one just 5,800 years ago, the team reports.
Exactly where these changes arose and how they spread through the population remains unclear. "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," Lahn remarks. "Here we have two microcephaly genes that show evidence of selection in the evolutionary history of the human species and that also show evidence of ongoing selection in humans." Indeed, Lahn predicts that if humans are still around in another million years, it is likely that their brains will exhibit significant structural differences from those seen today.
The researchers caution that the findings from two genes do not tell the whole story, however. Many other genes that have not yet been identified could influence brain size and development, Lahn notes, and further research could illuminate how natural selection has affected them as well. He explains: "We want to know how broad a trend these two genes represent."