The most popular theory that tries to account for the extremely fast evolution of human brain is the “Machiavellian intelligence” hypothesis (also known as “social brain” hypothesis).
Human brain has evolved much faster than other mammals. In only 25 million years lots of mutations have taken place in many human genes. The Machiavellian intelligence hypothesis might explain this phenomenon, and could give us the reason why we have such a big and complex brain. According to this theory, the intense social competition was (and still is) the main reason why the human brain evolved to a highly complex organ consuming 20% of our energy. Natural selection supported those individuals whose social strategies provided them with social and reproductive success. Sophisticated “Machiavellian” strategies, involving social behaviors like lying, cunning or leadership were the means to be successful in the emerging complex society.
Sergey Gavrilets and Aaron Vose, from the University of Tenesse, have provided data that supports this hypothesis. They have designed a mathematical model to simulate the development of human brain according to the Machiavellian intelligence theory. In their model, the genes control brains that invent and learn social strategies (memes). These strategies are used by males in their competition for mates. The model suggests that cerebral capacity evolves faster that learning capacity, and the advantage of having a large brain decreases as the exposure to memes increase in modern societies.
Source: The dynamics of Machiavellian intelligence . Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0601428103. Abstract.