This is from a Nova science documentary (Becoming Human): some scientists believe that humans evolved larger brains in response to a rapidly changing ecosystem.

This is from a Nova science documentary (Becoming Human): some scientists believe that humans evolved larger brains in response to a rapidly changing ecosystem. They've found evidence of a ginormous lake appearing and disappearing many times repeatedly over a few thousand years, in the Rift Valley, near where early hominids started evolving larger brains. The theory is that the rapid adaptability afforded by a larger brain was what selected for it.