I quite like Geoffrey Miller's theory that the human brain is primarily the result of sexual selection rather than natural selection - the hominid equivalent of the peacock's tail.
i find that theory to be unlikely to be true, because its usually not the smart guys but the buff guy that gets the prettiest girls (at least, that is the cultural belief).
Product of our culture more than anything. We'd all be buff if we barely ate and were outdoors/hunting all the time.