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 of 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: Available to subscribers at Genes tied to recent brain evolution). 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 1