Evolving genes may not size up brain.

Two gene variants previously proposed as contributors to the evolution of human brain size exert no influence on brain volume in people today, a new report indicates. If these particular genes indeed spread quickly by natural selection, that process might have been spurred by the genes' effects on reproductive organs or other tissue outside the brain, say neurologist Roger P. Woods of the University pf California, Los Angeles and his colleagues.

Prior research had indicated that a now-common variant of a gene called microcephalin originated 37,000 years ago and that a variant of a gene known as ASPM arose about 5,800 years ago (SN: 9/24/05, p. 206). Mutations in both of these genes have been linked to microcephaly, a disease that causes unusually small head size and mental retardation.

Woods' team used magnetic resonance imaging to measure the brain volumes of 120 healthy men and women from a variety of racial and ethnic backgrounds. Analysis of blood samples from each participant identified those who had inherited the common microcephalin and ASPM variants and those with either of two other versions of each gene.

After accounting for brain-size differences between the sexes and among ethnic groups, the researchers found that no specific gene variant regularly appeared in individuals with especially large or small brain volumes. The team reports the findings in the June 15 Human Molecular Genetics.

--B.B.