Don't get too cocky about your soda-bottle-sized brain. Humans 5,000 years ago had brains that were even larger.

"We do know from archaeological data that pretty much everywhere we can measure — Europe, China, South Africa, Australia — that brains have shrunk about 9 cubic inches (150 cubic centimeters), from an average of about 82 in³ (1,350 cm³). That's roughly 10 percent," University of Wisconsin at Madison paleoanthropologist John Hawks told LiveScience in 2009.

Researchers don't know why brains might be shrinking, but some theorize that they're evolving to be more efficient. Others think our skulls are getting smaller because our diets include more easily chewable foods and so large, strong jaws are no longer required.

Whatever the reason, brain size doesn't directly correlate with intellect, so there's no evidence that ancient man was brainier than humans of today.

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