Here is something weird - if EDSC is to be believed, there was no selection pressure outside humans for that. It was basically all human groups fighting. http://en.wikipedia.org/wiki/Evolution_of_human_intelligence#Ecological_dominance-social_competition_model

Wait, it gets better. Ecological dominance also suggests there is probably enough food to go around. No need to compete for that. What else would a mammal compete for? Um, I guess, females. There is a significant chance that the evolution of human intelligence is based on kidnapping brides.

This is utterly horrifying and fascinating. I always assumed intelligence evolved as a way to deal with nature. Turns out, we could deal with nature all right while being dumb. Intelligence evolved because we could not simply decide who fucks whom - so we went to war over it. Gives me the shudders.