From 
MSNBC:
A gene known to be important for brain development is more active in humans than in apes, a discovery that might have played a key role in human evolution. The gene is used by cells to make an opiatelike protein found in apes and humans called “prodynorphin,” or PDYN. In humans, PDYN is believed to be important for perception, memory and susceptibility to drug dependence. People who don’t make enough of the protein are vulnerable to drug addiction, schizophrenia, bipolar disorder and a form of epilepsy, studies have found.
More here.