Genes tied to recent brain evolution.

Two genes already known to influence brain size have undergone relatively recent, survival-enhancing modifications in people and appear to be still evolving, a research team reports.

Specific variants of these genes have spread quickly by natural selection, say Bruce T. Lahn of the University of Chicago and his colleagues, who published separate reports on each gene in the Sept. 9 Science.

The researchers examined DNA from 1,186 adults representing 59 populations worldwide and determined the frequency of specific variants of the two genes called microcephalin and ASPM.

A variant of microcephalin originated roughly 37,000 years ago and now appears in 7 of 10 people, the scientists conclude from comparisons of the gene's sequence for the different groups. Populations outside of sub-Saharan Africa most frequently possess this modified gene.

A distinctive ASPM variant arose approximately 5,800 years ago and now shows up in 3 of 10 people. It occurs most often in Europeans, North Africans, Middle Easterners, and South Asians.

The functions of these particular DNA alterations, including any potential influence on intelligence or reasoning, remain unknown.--B.B.