Our brains grew so big that prehumans began delivering their bigger-brained babies prematurely, at nine months instead of twelve months, like other primates. Mothers carried their “premature” babies by strapping them to their bodies with animal bladders and skins. Those technologies effectively allowed babies to finish what is sometimes called their “fourth trimester” outside the womb. The final outcome was the human brain. It demands two to three times as much energy by mass as the brains of other primates.40
