The idea of neoteny predates even Darwin’s On the Origin of the Species. It was noted as far back as 1836, when French naturalist Etienne Geoffroy Saint-Hilaire, a compatriot of Napoleon, first pointed out how young orangutans that had recently arrived from Asia at the Paris Zoo resembled “the childlike and gracious features of man.”
In the 20th century, a handful of other 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 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.
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.
Our long childhood has allowed us to collectively engage in ever broadening conversations as we keep finding new ways to communicate; we jabber and bristle with invention and pool together waves of fresh ideas, good and bad, into that elaborate, rambling edifice we call human civilization. Without all of this variety, all of these interlocked notions and accomplishments, the world, for better or worse, would not be as it is, brimming with this species of self-aware conflicted apes, ingenious enough to rocket rovers off to Mars and construct the Internet, wage wars on international scales, invent both WMDs and symphonies. If not for our long childhoods, we would not be here at all, the last apes standing. Can we remain standing? Possibly. I’m counting on the child in us, the part that loves to meander and play, go down blind alleys, wonder why and fancy the impossible.