Two extra copies of the SRGAP2 gene appear to have led to slower human brain development, but denser interconnection between neurons, possibly contributing to our current cognitive abilities. (Total brain interconnectivity correlates moderately well with IQ.)
Makes one wonder about the possibilities for further changes…
From New Scientist:
Eichler’s group discovered that SRGAP2 duplicated itself 3.5 million years ago, well after humans and chimps diverged. 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.
The effect of this genetic sabotage, however, was that the brain had more time to develop. Although the mouses brain itself didnt grow larger, the neurons in the neocortex changed to look like human brain cells, growing thick spines to exchange information with other cells. The neurons also formed 50 to 60 per cent more of these spines than normal mouse neurons do, which would likely increase the brains processing power.