The idea is that cooking food makes it easier to digest and allows our body to extract more useful energy out of a given amount of food. This leads humans to having a smaller digestive system and thus more body mass available for a larger brain & so forth.
All of this got me thinking about eggs. In popular culture we have tough guys showing how committed--to whatever quest they are on--by drinking a glass full of raw eggs. Setting aside the risk of salmonella poisoning, raw eggs actually remove an essential nutrient from your body. Egg is, or I should say would be a really great place for bacteria to grow but birds have found a way to prevent this. There is a protein called Avidin in egg whites which binds very strongly to Biotin (which is one of the B vitamins. The complete absence of Biotin makes eggs a lot less attractive to bacteria. When you cook eggs, the proteins become denatured and loose their ability to bind Biotin.