"Nearly three million years ago, our ancestors had brains about as big as modern chimps. Since then the brain that would become human grew steadily, tripling in size. But this extra cranium capacity may not have resulted in smarter hominids.
As far as tool-making is concerned, there is little evidence of improvement over much of the period that the brain was growing.
'Archaeology has found that brain size grew gradually, but cleverness took steps,' said William Calvin, a neurobiologist from the University of Washington."