Our brains:
As a general rule, mammal species with big bodies tend to have big brains. If you know the weight of a mammal’s body, you can make a fairly good guess about how large its brain will be. As far as scientists can tell, this rule derives from the fact that the more body there is, the more neurons needed to control it. But this body-to-brain rule isn’t perfect. Some species deviate a little from it. A few deviate a lot. We humans are particularly spectacular rule breakers. If we were an ordinary mammal species, our brains would be about one-sixth their actual size.
Carl Zimmer goes on to describe how our body has adapted to feed such a large demand.
The Dish 