Gene implicated in apes' brain growth.

The brains of people who have had the misfortune of inheriting specific rare mutations in the ASPM gene are only one-third the normal size. That gene is the locus of beneficial alterations that began to accumulate as early as 8 million years ago in populations of now-extinct apes, according to a report in the May Public Library of Science Biology, an online journal.

At least some of those ancient DNA changes led to bigger brains in various ape species long before human ancestors experienced unprecedented brain growth, theorize Vladimir Larionov of the National Cancer Institute in Bethesda, Md., and his colleagues.

The scientists sequenced the ASPM gene in chimps, gorillas, orangutans, and rhesus monkeys. They then compared the gene's nucleotide sequences among these primate species. Comparable data from people were already available.

Segments of the gene displayed systematic nucleotide additions and repetitions consistent with the evolutionary spread of useful mutations, Larionov says. These DNA fingerprints of natural selection appeared most strongly in gorillas and people. Comparisons of DNA sequences across species enabled the researchers to estimate when favorable ASPM-gene mutations began to spread.

This gene is thought to play a key role in the division of cells that later become neurons and may also have a role in certain cancers, Larionov says.