The genes that specify the architecture of the human brain seem to have started evolving faster some 20 million years ago, when the great apes split off from Old World monkeys.

The genes then doubled their speed of evolution after the human lineage parted ways with that of chimpanzees five million years ago.

The finding shows many different genes are involved in constructing the special features of the human brain, according to lead author Bruce T. Lahn of the University of Chicago in the report published in last month’s issue of Cell. This is what might be expected, he said, given that the human brain is characterized by its greater complexity, not just larger size. New neurons have to grow in the right place and connect in the right manner.