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On the Origin of Chins

Chin up.jpg

Here's an evolutionary puzzle I had never thought about. Why do we have chins?

That bony protuberance at the end of your jaw may be hidden by a beard or a fleshy throat, but it is still there. Chimpanzees don't have chins, neither do gorillas, orangutans, spider monkeys, dogs, horses, frogs, or fish. Did you know that only humans have chins? I didn't, but then I never studied vertebrate anatomy. It seems that having a chin is diagnostic of being a modern human. Even Neanderthals may have lacked chins, though this is a matter of dispute.

So the question of the origin of chins quite naturally arises. It turns out that a number of scientists have thought about it. Writing in the journal Evolutionary Anthropology, James Pampush and David Daegling discuss all the ways researchers have tried to come up with an explanation for why we have chins. The article is tellingly titled, "The Enduring Puzzle of the Human Chin."

The story has caught the attention of science journalists. The funniest is Ed Yong over at The Atlantic. Unfortunately he has taken all the chin jokes that I can think of. Melissa Hogenboom also wrote a piece for BBC Earth.

All those scientists cannot find a good evolutionary answer to the question. A chin doesn't make our jaws stronger for chewing. Or for taking punches -- that's another theory. It doesn't aid with speech. It isn't involved with sexual selection since both males and females have chins (thank goodness). It doesn't open up our airways.

It isn't the result of living in groups and needing to reduce testosterone. That's right -- the hypothesis is that less testosterone allows us to live in social groups. It's called self-domestication. Reduced testosterone pulls back the mid face and exposes the chin, so the theory goes. Except that men have higher levels of testosterone than females and have larger chins, not smaller ones.

The last hypothesis: having a chin is a spandrel -- a side effect of another adaptive change. The idea is that when we started eating soft food our jaws became weaker. The top part -- the maxilla -- shrank, and the teeth moved accordingly, but the mandible didn't shrink proportionately, and voil√°, we have a chin! The trouble with this story is just that. It's a story, with no way of testing it. Well, there were some animal experiments, unpleasant ones, but those yielded no conclusive evidence.

The authors conclude:

Each of the proposals we have discussed falter either empirically or theoretically; some fail, to a degree, on both accounts... This should serve as motivation, not discouragement, for researchers to continue investigating this modern human peculiarity... perhaps understanding the chin will reveal some unexpected insight into what it means to be human.

Perhaps it does reveal something important about being human. Perhaps no adaptive explanation for chins exists, because they aren't adaptations -- they arise from aesthetic considerations. Without a chin there would be no delicate curve of the neck in Swan Lake, no graceful oval shape to the face, no balance or proportion in portraits. It's not just that we are used to chins. We respond to proportion in all things, be it architecture, landscapes, or the spiral petals of a rose. The chin balances the face.

Now I've gone all serious when all I intended was light-hearted comments on looking for adaptations in all the wrong places. Chin up. I'm done.

Image credit: © YakobchukOlena / Dollar Photo Club.