Woody Allen once described his brain as his “second favorite organ,” but for all of us it’s by far the most expensive one to run. It accounts for 2 percent of our body weight but consumes 20 percent of our energy. Human brains grow so large that human beings must be born prematurely18 (at least, compared to other mammals, who are born when their brains are more or less ready to control their bodies), and even then they can barely make it through the birth canal. Once out of the womb, these giant brains attached to helpless baby bodies require somebody to carry them around for a year or two.
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