A new study points the finger at a small number of genes associated with brain development which are also have some connections to common developmental disorders and mental health conditions, and in particular to a gene called SRGAP2 which is one of the several dozen genes that seem to be most plausible candidates for genes that distinguish hominins from other great apes.
[L]oss of SRGAP2 function accelerates neurons' migration in the developing brain, potentially helping them reach their final destination more efficiently. Moreover, neurons that have decreased SRGAP2 function, due to expression of the human-specific SRGAP2 display more knob-like extensions or spines on their surfaces, making the neurons appear much more like those found in the human brain. These spines enable connections between neurons to form.The authors of the study think the effect of this mutation would have been dramatic as soon as it emerged. "If this gene duplication did indeed produce an immediate effect during evolution . . . there must have been a fascinating period in human history characterized by "huge variation" in human cognition and behavior."