http://www.livescience.com/culture/080811-brain-evolution.html

The conjecture goes like this: because we learned to cook our food, humans were able to get more and better calories than our raw food eating cousins thus spurring a leap of cognitive ability.

In most animals, the gut needs a lot of energy to grind out nourishment from food sources. But cooking, by breaking down fibers and making nutrients more readily available, is a way of processing food outside the body. Eating (mostly) cooked meals would have lessened the energy needs of our digestion systems, Khaitovich explained, thereby freeing up calories for our brains.

Makes intuitive sense.