The human brain is still evolving.
Both genes went through strong positive selection in the hominid lineage leading to humans, Lahn says, "suggesting that the evolution of these two genes might have contributed to human brain enlargement." Now, analyses across roughly 90 ethnically diverse human samples show abnormally high frequencies favoring a specific haplotype for each gene. The high-frequency variant appears to have arisen 37,000 years ago for
Aside from the association with microcephaly, however, the idea that the selection on these two genes was due to brain-size adaptation "is all speculation," said Morris Goodman of Wayne State University in Detroit. "Although, if the ideas aren't thrown forward, people will not be stimulated to try to figure out why this thing was positively selected."