One of the first traits that differentiated humans from our ancestors was upright gait. There are several hypotheses about the emergence of this trait, but it seems to have offered a way to move more efficiently in open environments such as the savanna. Although our earliest human ancestors were very apelike in terms of their brains, their upright gait had changed their pelvis to look much like our modern one. This reshaped pelvis came with a narrower birth canal, making childbirth more difficult.
Meanwhile the new roaming grounds afforded advantages in acquiring resources and negotiating social relationships to those with flexible, problem-solving behavior. Over time, natural selection increased brain size in these early humans. But at some point, the selection for bigger and bigger brains collided head-on, so to speak, with the narrow pelvis. If babies’ heads got any bigger, they would get stuck in the birth canal and kill both mother and child. Although natural selection worked to maximize what could be done—for instance, babies’ heads compress as they twist their way around the bones in the pelvis—there simply is not enough room for a big, mature brain to pass through.
Therefore, Bock explains, human baby brains continue to develop substantially after birth and it takes longer for them to learn how to walk.
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