Thanks for that clarification, Wolfwitch
What I've read so far is that humans started developing larger brains (or a larger cerebral cortex, to be precise) when they started walking on two feet which freed up their hands for dextrous activities, which in turn activate certain parts of the brain.
Of course, most of such research about our ancestors is in fact quite based on theories. One of the intriguing things about life to me is that we will never know everything and that's probably a good thing.