New work done by the University of Washington's Evan Eichler and Franck Polleux at the Scripps Institute shows that when modern human diverged from australopithecines 2.5 million years ago, a single gene made copies of itself and changed the way human brains would develop, reports New Scientist's Sara Reardon. The extra copies of SRGAP2— which drives development of the neocortex — slowed down the development of individual brains, leaving more time for neurons to develop and create connections with each other, Reardon says. "Eichler's group discovered that SRGAP2 duplicated itself 3.5 million years ago, well after humans and chimps diverged," she adds. "One million years later, this 'daughter' of the original gene underwent its own duplication and created a 'granddaughter' copy. All three coexist in modern humans."
When Polleux put human copies of the duplicated gene into mice, the resulting proteins bound to the original copy of SRGAP2 and prevented it from functioning. "The effect of this genetic sabotage, however, was that the brain had more time to develop," Reardon says. "Although the mouse's brain itself didn't grow larger, the neurons in the neocortex changed to look like human brain cells, growing thick spines to exchange information with other cells." Now, the researchers plan to put the duplicated genes in a marmoset to see if the animal's behavior will be altered with an increasingly complex brain.