How childhood let modern humans conquer the planet.

There’s a misconception among a lot of us Homo sapiens that we and our direct ancestors are the only humans ever to have walked the planet. It turns out that the emergence of our kind isn’t nearly that simple. The whole story of human evolution is messy, and the more we look into the matter, the messier it becomes.

Paleoanthropologists have discovered as many as 27 different human species (the experts tend to debate where to draw the line between groups). These hominids diverged after our lineage split from a common ancestor we shared with chimpanzees 7 million years ago, give or take a few hundred millennia.

Many of these species crossed paths, competed, and mated. Populations ebbed and flowed in tight little tribes, at first on the expanding savannahs of Africa, later throughout Europe, Asia, and all the way to Indonesia. Just 100,000 years ago, there were several human species sharing the planet, possibly more: Neanderthals in Europe and West Asia, the mysterious Denisovan people of Siberia, the recently discovered Red Deer Cave people living in southern China, Homo floresiensis (the Hobbits of Indonesia), and other yet unknown descendants of Homo erectus who left indications that they were around (the DNA of specialized body lice, to be specific). And, of course, there was our kind, Homo sapiens sapiens (the wise, wise ones), still living in Africa, not yet having departed the mother continent. At most, each species consisted of a few tens of thousands of people hanging on by their battered fingernails. Somehow, out of all of these struggles, our particular brand of human emerged as the sole survivor and then went on, rather rapidly, to materially rearrange the world.

If there once were so many other human species wandering the planet, why are we alone still standing? After all, couldn’t another version or two have survived and coexisted with us on a world as large as ours? Lions and tigers coexist; so do jaguars and cheetahs. Gorillas, orangutans, bonobos, and chimpanzees do as well (though barely). Two kinds of elephants and multiple versions of dolphins, sharks, bears, birds, and beetles—countless beetles—inhabit the planet. Yet only one kind of human? Why?

More than once, one variety may have done in another either by murdering its rivals outright or outcompeting them for limited resources. But the answer isn’t as simple or dramatic as a war of extermination with one species turning on the other in some prehistoric version of Planet of the Apes. The reason we are still here to ruminate on why we are still here is because, of all those other human species, only we evolved a long childhood.

Over the course of the past 1.5 million years, the forces of evolution inserted an extra six years between infancy and pre-adolescence—a childhood—into the life of our species. And that changed everything.

Why should adding a childhood help us escape extinction’s pitiless scythe? Looked at logically, it shouldn’t. All it would seem to do is lengthen the time between birth and mating, which would slow down the clamoring business of the species’ own continuance. But there was one game-changing side effect of a long childhood. Those six years of life between ages 1 and 7 are the time when we lay the groundwork for the people we grow up to become. Without childhood you and I would never have the opportunity to step away from the dictates of our genes and develop the talents, quirks, and foibles that make us all the devastatingly charming, adaptable, and distinctive individuals we are.

Childhood came into existence as the result of a peculiar evolutionary phenomenon known generally as neoteny. The term comes from two Greek words, neos meaning “new” (in the sense of “juvenile”) and teinein meaning to “extend,” and it means the retention of youthful traits. In the case of humans, it meant that our ancestors passed along to us a way to stretch youth farther into life.

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.