This TED Talk does a phenomenal job of explaining brain size of primates through mathematics. The part in the video at about 8:50 where she begins explaining the amount of energy needed to sustain a brain with 86 billion neurons and the size of a body is really interesting in comparison with the great apes. She draws the conclusion that humans are set aside from other primates through the act of cooking.
An article that was published on NationalGeographic.com seconds her theory of why humans are humans citing cooking as the reason. http://news.nationalgeographic.com/news/2012/10/121026-human-cooking-evolution-raw-food-health-science/?rptregcta=reg_free_np&rptregcampaign=20131016_rw_membership_r1p_us_se_w#finished