As a species, humans are incredibly smart. We tell stories, create magnificent art and astounding technology, build cities, and explore space. We haven’t been around nearly as long as many other species, but in many respects we’ve accomplished more than any have before us. We eat them and they don’t eat us. We even run scientific studies on them—and are thinking about re-creating some of those that have gone extinct. But our intelligence comes with a curious caveat: our babies are among the dumbest—or, rather, the most helpless—that exist. A baby giraffe can stand within an hour of birth, and can even potentially flee predators on its first day of life. A monkey can grasp its mother and hang on for protection and nourishment. A human infant can’t even hold up its own head.
The evolution of human intelligence isn’t something that Celeste Kidd had ever pondered. A developmental cognitive scientist who currently works at the University of Rochester, her work had focussed mostly on learning and decision-making in children. Over years of observing young children, she became impressed with the average child’s level of sophistication. But when she looked at the infants she encountered, she saw a baffling degree of helplessness: How could they be so incompetent one second and so bright so soon thereafter? One day, she posed the question to her colleague Steven Piantadosi. “Both of us wondered what could possibly justify the degree of helplessness human infants exhibit,” she told me recently. “Even other primate babies, like baby chimps, which are close in evolutionary terms, can cling onto their moms.” She began to see a contradiction: humans are born quite helpless, far more so than any other primate, but, fairly early on, we start becoming quite smart, again far more so than any other primate. What if this weren’t a contradiction so much as a causal pathway?
That’s the argument that Kidd and Piantadosi make in their new paper, published in the June issue of PNAS. Humans become so intelligent becausehuman infants are so incredibly helpless, they argue; the one necessitates the other. The theory is startling, but it isn’t entirely new. Researchers have been pondering the peculiarities of our birth and its evolutionary significance for quite some time. Humans belong to the subset of mammals, called viviparous mammals, that give live birth to their young. This means that infants must grow to a mature enough state inside the body to be born, but they can’t be so big that they are unable to come out. This leads to a trade-off: the more intelligent an animal is, the larger its head generally is, but the birth canal imposes an upper limit on just how large that head can be before it gets stuck. The brain, therefore, must keep maturing, and the head must continue growing, long after birth. The more intelligent an animal will eventually be, the more relatively immature its brain is at birth.
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