Were bigger brains really smarter? | Science Blog:
bigger brains were not generally smarter brains,“ said neurobiologist William H. Calvin at the American Association for the Advancement of Science meeting in Washington, D.C. on Friday, Feb. 18. ”Thanks to the archaeologists, we know that our ancestors went through two periods, each lasting more than a million years, when toolmaking techniques didn't gradually improve, despite a lot of gradual brain size increase.“
There is no lack of other candidates for why a bigger brain would be better.
Fossil data and testable evolutionary hypotheses.