It’s sweaty and dusty business extracting clues from the past. Ask any paleoanthropologist. Which may explain why it’s taken scientists the better part of 180 years to find that at least 27 different human species (probably more) have walked our planet over the past seven million years, give or take a few hundred millennia.
This surprises most people because it is a whopping departure from the way we used to see our passage from the past to present. Not long ago we imagined that we Homo sapiens evolved serially, up from a single line of gifted ancestors, each replacing the previous model once natural selection had managed to get it right. (We all know the image of the hunched simian that slowly morphs in the perfect upright walking man.)
The story of our emergence, however, turns out to be much more interesting (and messy) than that. Rather than consisting of a single stalk, the human family tree is bushy with a startling assortment of other human species that struggled and evolved side by side, competing, even mating from time to time. These creatures initially had diverged after our lineage split from a common ancestor we shared with chimpanzees seven million years ago. Populations ebbed and flowed in tight little tribes, at first on the expanding savannas of Africa, later throughout Europe and Asia, all the way to the archipelagos of Indonesia.
As recently as 100,000 years ago, there were still several human species sharing the planet: Neanderthals in Europe and West Asia, the mysterious Denisovan people of Siberia, the so-called Hobbits of Indonesia (Homo floresiensis), and other yet unknown descendants of Homo erectus who left smoking gun evidence 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 and primal fingernails. Yet somehow, out of all of them, our particular brand of human emerged as the sole survivor, and then went on, with rather alarming speed, to materially rearrange an entire planet.
Surprising and illuminating as all of this information is, it still begs the question: if there were so many other human species roaming planet Earth all of this time, why is only one still standing? And why is it us?
It turns out the reason we, and we alone, are still here to contemplate our success while so many others found their way to the genetic trash bin is because we are all big babies, scientifically speaking at least. And we are because of a rare, and largely forgotten, evolutionary phenomenon called neoteny, defined as “the retention of juvenile features in the adult animal.”
Originally this term had nothing to do with us. A German embryologist named Julius Kollman, a contemporary of Charles Darwin, coined it to describe the behavior of salamanders like the Mexican axolotl and mud puppy, animals that stubbornly refuse to fully grow up and out of their larval stage, even in their adulthood. (This would be something like a toddler behaving in every way like a full-grown, sexually mature twenty-five-year-old). In us neoteny isn’t quite that pronounced (probably a good thing), but still, it is remarkable, and when its Darwinian hand descended upon our ancestors, it set in motion events that endowed us with cerebral powers no other animal possesses.
Its first effect was to lead to our predecessors being born “earlier,” or more accurately, less physically mature, than their cousin primates. If, for example, you came into the world as physically mature as a baby gorilla, you would have had to pass 20 months in the womb—clearly unacceptable to your mother! This makes us, basically, fetal apes, born entirely helpless.
Why did this happen? And why should we care? More than a million and half years ago small bands of our direct ancestors were barely managing to scrape out a living on Africa’s savannas. As if they didn’t have enough to contend with, two key trends were placing them in an even nastier evolutionary pickle. Partly because they were eating a meatier diet, their brains were growing larger. Generally speaking bigger brains are a good thing when trying to survive, but they had also taken to walking fully upright. And while by itself this was also good, striding along on two legs requires realigning the pelvis, which in turn narrows the birth canal. That, coupled with larger heads began to make it increasingly difficult to deliver a full-term baby.
So our ancestors developed into a species of “preemie” primates that were not only born younger, but began to remain that way longer. While other apes abandon their fetal and childlike traits as they grow up, we Homo sapiens steadfastly hang on to many of ours. We remain relatively hairless even in adulthood, for example. Our jaws stay square like a baby chimp’s, and our foreheads remain high and flat rather than developing a simian slope as we grow older. At the same time in the early years after birth our neurons continue to multiply as exuberantly as if we were still in the womb. In short, youth was amplified and elongated. We needed more time to grow up and that led to something entirely new: childhood.
We take childhood for granted because we all live through one. But childhoods are rare in nature. Other mammals pass through a brief infancy then leap directly to a juvenile stage just before they reach childbearing age. Inside of six months of their birth, dogs are ready to mate. For rabbits and mice the time is even shorter. Chimpanzees and gorillas reach adulthood at age 11, about six years earlier than we do.
The odd thing about neoteny is that it makes no evolutionary sense. Why insert this new stage of life that delays getting on with the pressing business of having babies and continuing the species? If you were a betting primate a million-and-a-half years ago, wagering against early births and elongated childhoods would have looked especially astute. Helpless offspring wouldn’t seem likely to survive the harsh realities of the African savanna, or for that matter much help their parents stay among the living either. Nevertheless, the line of savanna apes that led to us managed to make it, and then some.
The reason neoteny, risky as it was, succeeded was because by bringing us into the world prematurely, it forced our brains to develop after we were born. A monkey comes into the world with 70 percent of its cerebral development already behind it. The rest is completed within six months. A chimpanzee manages all of its brain development 12 months after its birthday. Ours is just gathering steam as we exit the birth canal. At birth your brain is a paltry 23 percent of its adult weight, but in the first three years of life it triples in size, continues to grow through age six while furiously rewiring itself, undergoes another growth spurt in adolescence and finally completes most, but not all, of its development by the time you reach your mid-twenties. Even then our brains continue to rewire themselves in reaction to our experience right to the bitter end.
These circumstances are so extraordinarily strange and rare that they are unique in nature. It means that unlike any other primate, your brain, the seat and foundation of your reality and personality, is shaped not in the quiet safety of mother’s womb, but the boisterous, complicated world outside it. And that is what makes you you and me me,
Everything from trauma to exposure to music can shape the people we grow up to be. This helps explain how you can be born in Fargo, North Dakota, learn to speak fluent French in Paris, develop wit like Woody Allen or become as reclusive as Howard Hughes, all while devotedly plumbing the intricacies of subjects as wildly different as calculus, Mozart and baseball. It’s how you get Albert Einstein and Marilyn Monroe, Lady Gaga and Aaron Judge, all in the same species..
So while being born “young” delayed the time when we could bring new versions of ourselves into the world, it has also endowed us with powers that make us the most intelligent, adaptable, collaborative species to have yet come down the evolutionary pike, ingenious enough to paint the “Mona Lisa,” rocket rovers to Mars, construct the Internet, wage war, and invent both WMDs and symphonies. In other words it is the reason we are the last ape standing.
Can we remain that way? I think so. I’m betting on the child in us. The part that loves to meander and play, go down blind alleys, wonder why and fancy the impossible. After all, it’s has gotten us this far, hasn’t it?