Why Are Human Babies Born So Early?

David Geary says human babies are born so early because a pregnancy long enough for the infant brain to match a chimpanzee infant would last almost two years, and nobody wants to birth a two-year-old. Babies arrive premature in that sense, more dependent on care, with a longer growth period for the brain and the rest of the body.

Better tools, hunting, and pulling resources out of the environment also meant children had to learn the skills. Studies of hunter-gatherer groups, he says, show it takes a long time to become a skilled hunter or skilled at foraging, and for some of the work a person may not be an expert until about 30. The delayed stretch is the time spent learning how to succeed in the cultural niche. He argued 25 years ago that australopithecine ancestors were more like gorillas, a single male with a few females and the offspring, and that the males stayed, played with the offspring, and, most importantly, protected them. The arrangement is the germ of a human family and of paternal investment, going back 5 million years or more.