After two tremendous growth spurts — one in size, followed by an even more important one in cognitive ability — the human brain is now a lot like a teenage boy.

It consumes huge amounts of calories, is rather temperamental and, when harnessed just right, exhibits incredible prowess. The brain's roaring metabolism, possibly stimulated by early man's invention of cooking, may be the main factor behind our most critical cognitive leap, new research suggests.

About 2 million years ago, the human brain rapidly increased its mass until it was double the size of other primate brains.

"This happened because we started to eat better food, like eating more meat," said researcher Philipp Khaitovich of the Partner Institute for Computational Biology in Shanghai.

But the increase in size, Khaitovich continued, "did not make humans as smart as they are today."

The early shift