More than a million years ago, our direct ancestors found themselves in a real evolutionary pickle. One the one hand, their brains were growing larger than those of their rain forest cousins, and on the other, they had taken to walking upright because they spent most of their time in Africa’s expanding savannas. Both features would seem to have substantially increased the likelihood of their survival, and they did, except for one problem: Standing upright favors the evolution of narrow hips and therefore narrows the birth canal. And that made bringing larger-headed infants to full term before birth increasingly difficult.
If we were born as physically mature as, say, an infant gorilla, our mothers would be forced to carry us for 20 months! But if they did carry us that long, our larger heads wouldn’t make it through the birth canal. We would be, literally, unbearable. The solution: Our forerunners, as their brains expanded, began to arrive in the world sooner, essentially as fetuses, far less developed than other newborn primates, and considerably more helpless.
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In the nasty and brutish prehistoric world our ancestors inhabited, arriving prematurely could have been a very bad thing. But to see the advantages of being born helpless and fetal, all you have to do is watch a 2-year-old. Human children are the most voracious learners planet Earth has ever seen, and they are that way because their brains are still rapidly developing after birth. Neoteny, and the childhood it spawned, not only extended the time during which we grow up but ensured that we spent it developing not inside the safety of the womb but outside in the wide, convoluted, and unpredictable world.
The same neuronal networks that in other animals are largely set before or shortly after birth remain open and flexible in us. Other primates also exhibit “sensitive periods” for learning as their brains develop, but they pass quickly, and their brain circuitry is mostly established by their first birthday, leaving them far less touched by the experiences of their youth.
Based on the current fossil evidence, this was true to a lesser extent of the 26 other savanna apes and humans. Homo habilis, H. ergaster, H. erectus, even H. heidelbergensis (which is likely the common ancestor of Neanderthals, Denisovans, and us), all had prolonged childhoods compared with chimpanzees and gorillas, but none as long as ours. In fact, Harvard paleoanthropologist Tanya Smith and her colleagues have found that Neanderthals reversed the trend. By the time they met their end around 30,000 years ago, they were reaching childbearing age at about the age of 11 or 12, which is three to five years earlier than their Homo sapiens cousins. Was this in response to evolutionary pressure to accelerate childbearing to replenish the dwindling species? Maybe. But in the bargain, they traded away the flexibility that childhood delivers, and that may have ultimately led to their demise.