One of the biggest “human” questions is “where did we come from?”. While the mechanisms of evolution are well established, the route humanity took to get to its present state is not as well detemined. It’s the difference between knowing the rules of chess and being able to figure out the personality and play style of a grandmaster from a few snapshots of a very long game in progress.

One proposed mechanism for the evolution of humans from primates is neoteny, where juvenile traits are retained and adult adaptations lost. This has been observed in foxes subject to behavioural selection. For instance, look at this young chimpanzee.


This picture is from a 1926 study by the German anthropologist Adolf Naef. He describes it as “the the most human-like picture of an animal, of any that is known to me.” The little guy does seem to have a rather regal and refined air about him, but we can’t just wave our hands and call it case closed at this point. Can we look at the development of a chimpanzee and see if there are any quantifiable parallels?

Bone structure is a great place to start. Chimpanzees, like humans, have a skeleton that changes shape and size as the organism matures.


The two skulls on the far left are those of an infant chimpanzee (top) and an infant human (bottom). Bone structure and shape are very similar, with the classic huge head and tiny cute face we seem programmed to love. The two skulls in the middle are of a adolescent chimpanzee (top) and an adult human (bottom). We can see the jaw start to lengthen in both, and their overall similarity. The final picture on the top right is of an adult chimpanzee, who has a significantly larger and more powerful bite than any adult human.

So what does this show us? Well, humans and chimpanzees appear to have very similar development in terms of bone structure as they grow up, except that humans just seem to… stop at a certain point. There are a multitude of theories as to why this happens, but they all seem to follow the pattern of certain behaviours being selected for which affect the balance of hormones in the body that control the development of adult features. This is called neoteny.

Now neoteny doesn’t mean that every single part of the entire animal becomes more juvenile, or that the animal becomes less complex overall. It’s a selective reduction in complexity – traits that appear later in the animals development (ie adolescence) become less likely to appear.

So how did humans get their unique features? It’s very difficult to select for traits like a bigger brain or hairlessness when those traits don’t appear in the wild in any real frequency to begin with. Viewing human evolution through this lens seems to indicate that change would be very slow, and very hard to do.


But what if instead of selecting for a simple trait, we (or the species as a whole) selects for a behaviour? The neat thing about selecting for this is that hormones have a strong influence on behaviour. So we are partly selecting for certain hormone levels or actions. These hormones also share logical relationships with other hormones, and act in many different parts of the body, not just the parts of the brain influencing behaviour.

If we put significant selection pressure on a species, we are effectively increasing the mutation rate (ie “mutant” creatures tend to be selected more). Increases in mutation rates would be more likely to affect more logically complex proteins arising later in life involved in the development of adolescent features (due to more references to more parts of the mutating DNA) rather than less logically complex proteins that would be involved in juvenile features.

As a result, we now have a mechanism for how these bizarre traits that we simply don’t see in the wild can become so common, so quickly, and also a predicted side effect – neoteny.

But how could this end up as an advantage? It seems that mutations are destroying those adult adaptations that made the organism successful in the first place. But what if the world changes simply because you and others like you live in it? We like to think of physical strength as the be all and end all of “dominance”, but I think this is only true if you’re “one chimp against the world”. A chimp who can more accurately figure out social structure and how to manipulate his place in it could be far more successful in breeding than a chimp who is simply stronger than average.

A chimpanzee’s ability to learn is drastically reduced upon reaching maturity. But baby chimps…


Baby chimps will eagerly mimic a human caretaker – sticking out their tongues, opening their mouth wide, or making their best effort at a kissy face. Not only is the basic mechanism of learning there (imitation), it appears to be very focused on social relationship. And this ability decreases with age! It seems that the retention of juvenile traits is not the burden it appears at first.

So the origin of humanity? Well, it’s still up in the air. But I think it’s incredibly likely that we literally changed ourselves – that living together created environmental pressures (namely social ones) that selected for behaviour in an incredibly complex manner, where the ability to learn and social skills were valued and led to reproductive success. All too often we look for outside pressures in evolution, when some of the most magnificent examples (like the plumage and mating rituals of birds of paradise) are simply a result of everyone agreeing to play an elaborate game.

7 thoughts on “Chimpanzees and Neoteny

  1. Starting in the late ’70s Steve Gould dealt with neoteny as key to understanding human evolution leading to upright posture, retention of juvenile traits, and long life spans.

    Just check under Gould, Steven Jay. He left us too soon.


  2. Even before Stephen J., Desmond Morris, in “The Naked Ape”, spends quite a bit of time talking about neoteny and its possible place in human evoluation. There was probably someone before him…

  3. dying and reviving hypotheses

    Sure . . . you can turn to Aldous Huxley, After Many a Summer Dies the Swan (1939) for the idea that neoteny was caused by alterations to “glandular secretions.” When the ‘secretions’ hypothesis failed, so did neoteny.

    But, this really was a case of baby and bath water — as Gould points out in 1977. Retention of juvenile characteristics happens frequently enough to consider it a fact for several animals. Gould who in 1977 was writing just before the explosion in biotechnology treated it as a then unexplained phenomenon.


  4. Except Gould I will propose you Ashley Montagu’s “Growing Young” 1981, which is one step further than Gould in the behavioral aspect of Neoteny, and has an extended history of Neoteny! the only one I have found up to now.

  5. I’m still at “meh” on this one.

    1) Primates also exist in complex social structures. They pass on knowledge about tools to descendants, communicate organized hunts (often against other primates, e.g. chimps)…the only thing missing is language. Maybe that’s somehow a piece (like I have any idea). maybe it put an increased focus on communication in general–face and body cues (so less hair), access (social relationships), and so forth. Again, like I know my ass from my hand on this topic…

    2) The loss of brain “plasticity” (ability to learn or change) after the early years would seem to me a typical feature of social organisms rather than something sparking it. Selecting for a brain more apt or able to learn is like selecting for a brain more intelligent–good, I guess, but not an explanation for our “smarter” species. Intelligence and ability to learn are synonymous with smart, so it doesn’t explain much. Similarly, if it were found that human babies spend more of their life in that “plastic” phase, that could also be explained by just being in more demanding contexts where language and relationships are stressed. Without evidence that it occurred first, it’s just speculation–good, again, but just “meh” for me.

    And CREATIONISM = #1!! LOL just kidding guys.

    Cool to read your website. Thanks!

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