In the September 9 issue of Science, a team lead by human geneticist Bruce Lahn of the University of Chicago published two papers claiming that genes thought to be involved in regulation of brain growth continued to be under natural selection long after the rise of modern humans--and may even be under selection today. The genes in question are microcephalin and ASPM, which, when mutated, lead to primary microcephaly in humans. This is a condition in which the adult brain is only about 400 cubic centimeters in volume, compared to a normal average of about 1350 cc. It is accompanied by mild to moderate mental retardation.

Lahn's team found that variant versions, or alleles, of these two genes arose about 37,000 years ago in the case of microcephalin and 5,800 years ago in the case of ASPM. They then spread rapidly among humans, in what might have been a "selective sweep" due to some evolutionary advantage they conferred on those who had them. These dates correspond to the Upper Paleolithic "symbolic explosion" in Europe and the rise of cities in the Near East, respectively, although the margins of error for these dates are fairly broad. However, in a story in the same issue accompanying the papers, I quote other researchers expressing skepticism about any direct connection between the rise of these alleles and these key events in human cultural evolution.

The findings also raise social and ethical issues, because not everyone has the favored alleles. In the case of the microcephalin allele, for example, the prevalence is quite high in Europe, Asia, and the Americas, but fairly low in sub-Saharan Africa. Lahn's group, along with other collaborators, are now exploring whether the alleles confer a cognitive advantage, and the University of Chicago has taken out a patent that covers any test for the alleles. But a number of researchers, quoted in my story, point out that there is no clear proof that the alleles were selected because of their function in the brain; they may have provided an advantage in a non-brain related trait.

This story received a great deal of press coverage, which is no surprise. If anyone would like a pdf of my Science news story, please get in touch at mbalter@michaelbalter.com.