Based on skulls found in Europe, Asia, and the Middle East, the human brain was about 1,500 cubic centimeters 30,000 years ago, and now it's down to about 1,359 cubic centimeters. That means that, as a species, humans now only need 90% the brain size that we did 30,000 years ago. That's an incredibly rapid change—30 millennia is practically no time at all in evolutionary terms—and one for which anthropologists have plenty of explanations. First up, there's the physiological answer. It's easy to equate brain size with cognitive ability, but it's only part of the story.