Here's a heady claim: A molecular biologist says that a gene that causes a rare deformity may also have spurred the vast expansion of hominid brains over the past 2 million years. But others are skeptical.
Mutations in the ASPM gene cause autosomal recessive primary microcephaly (MCPH), a congenital reduction in the size of the head and brain. That suggests that the normal ASPM gene might have boosted brain expansion in human ancestors, reasoned Jianzhi Zhang of the University of Michigan, Ann Arbor. To test that hypothesis, he looked for evidence that the gene had provided an evolutionary advantage to humans by comparing the human version of ASPM to that of close cousins such as chimpanzees and orangutans. The human gene was notably different, Zhang reports in the December issue of Genetics: It had undergone a much higher proportion of mutations that changed the amino acid sequence of the protein the gene produces. That's a clear sign that the gene had responded to Darwinian natural selection and enabled the individuals who possessed it to get ahead, he says.
However, other genes were likely involved in brain expansion, says neuroscientist Todd Preuss of Emory University in Atlanta: "Nobody should conclude that ASPM is the brain-size gene."
