Compared with other animals, human babies take much longer to learn to walk. Does this have something to do with our big brains?
—Mahmoud Dhaouadi, via e-mail

John Bock, an anthropologist at California State University, Fullerton, provides a reply:

A horse can walk within an hour after birth. A newborn baboon baby can cling to its mother’s hair while she jumps through the trees. Even among  our closest evolutionary relatives—chimpanzees and bonobos—babies are far more agile than their human counterparts. That’s because humans are born with brains that are largely immature, leaving babies with little control over their movements. This uniquely human attribute is the result of a lengthy evolutionary battle between big brains and narrow pelvises.

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.

As it turned out, the evolutionary answer was to let the brain keep growing outside the womb before it matures. So in contrast to other mammals, humans have a good bit of development to do after birth. The result is a relatively undeveloped infant who needs lots of care and can do much less for itself than other newborn primates.

Note: This article was originally printed with the title, "Ask The Brains."

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