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 seven 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.
And, of course, there was our kind.
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?
More than once, one variety may have done in another either by murdering its rivals outright or out-competing 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 one and seven 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."
More than a million years ago, our direct ancestors found themselves in a real evolutionary pickle. On 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 favours 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.
In the 20th century, a handful of scientists and evolutionary thinkers observed that infant apes bore a striking resemblance to adult humans, especially when it came to the shapes of their faces and heads. Naturally this raised a few questions: Was this simply a coincidence? Why would we resemble baby apes? And did this have anything to do with our own evolution?
Louis Bolk, a professor of anatomy in Amsterdam, became somewhat obsessed with those questions. Between 1915 and 1929, he penned six detailed scientific papers and one pamphlet on the subject. In them, he argued that a surprisingly high number of human physical traits "have all one feature in common, they are fetal conditions [seen in apes] that have become permanent."
Bolk enumerated 25 specific fetal or juvenile features that disappear entirely in apes as they grow to adulthood but persist in humans. Flatter faces and high foreheads, for example, and a lack of body hair. The shape of our ears, the absence of large brow ridges over our eyes, a skull that sits facing forward on our necks, a straight rather than thumblike big toe, and the large size of our heads compared with the rest of our bodies. You can find every one of these traits in fetal, infant, or toddling apes and modern human adults.
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 two-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.
We are different. During those six critical years, our brains furiously wire and rewire themselves, capturing experience, encoding and applying it to the needs of our particular life. Our extended childhood essentially enables our brains to better match our experience and environment. It is the foundation of the thing we call our personalities, the attributes that make you you and me me. Without it, you would be far more similar to everyone else, far less quirky and creative and less, well . . . you. Our childhood also helps explain how chimpanzees, remarkable as they are, can have 99 percent of our DNA but nothing like the same level of diversity, complexity or inventiveness.
Walter is the author of Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived