Why Are Humans The Only Animals With Chins?

Human, complete with chin. Spyros Papaspyropoulos/Flickr. CC BY-NC-ND 2.0

Here’s a question for the ages: Why do humans have chins?

Firstly, it’s worth noting that the chin is not just that extra bit of face beneath your mouth. It is the piece of your lower jaw that juts out from your face. Even our close genetic relatives – the chimpanzee and the gorilla – don’t have a “proper” chin. While they do have an area below their jaw, if you look at their skulls, you can see the bone slopes backwards and away from their lower teeth. Why humans, then, have this unique feature has been a source of contention for evolutionary biologists since the 1800s.

James Pampush, a postdoctoral associate in evolutionary anthropology at Duke University, recently wrote about the puzzle of the human chin in the journal Evolutionary Anthropology.

Speaking to The Atlantic, Pampush explained: “The chin is one of these rare phenomena in evolutionary biology that really exposes the deep philosophical differences between researchers.”

 

Figure 1. Lateral views of hominoid mandibles, including ones from several extinct hominins (not to scale). The receding basal margin of the anterior symphysis in the extant apes ( Pan, Gorilla , and Pongo ) contrasts sharply with the projecting basilar margin in Homo sapiens . Neanderthals ( Homo neanderthalensis ) have a more vertical symphyseal orientation, but typically lack the basilar projection. As compared to the extant apes, the extinct hominins all tend to have more vertically oriented symphyses. 

 

Victorian evolutionary biologists proposed the idea that the chin developed because it is effective at deflecting punches. However, this would require a lot of early humans being punched in the face extremely frequently and with dire damage.

Other theories suggest the chin helps the jaw cope with the stress that comes with speech. All that tongue movement and mouth opening, some believe, can strain the jaw, so a larger area developed to help distribute the stress and reduce its impact. However, the strain of some gentle tongue flapping isn’t that large or strenuous, according to Pampush.

Some have suggested that the chin was a tool of sexual selection that showed a prospective partner some evolutionary advantage to encourage them to create offspring with them. However, speaking to NPR, Pampush explained that these kinds of features typically only develop in one of the genders, something known as sexual dimorphism.  

Pampush concludes that he thinks the chin is just a “spandrel” – a kind of evolutionary red herring that didn’t develop for any direct function, but as an indirect byproduct of another adaptation. It’s perhaps an inadvertent effect of our faces reducing in size as we began processing and refining our food. As we made less demands on our teeth for eating and biting, they started a process of shrinking and retracting into the face. 

While the theory sounds legitimate, it’s pretty hard to scientifically prove that a feature doesn’t give an evolutionary advantage. The puzzle of the chin seems to continue, but whatever the answer, the chin remains one of the few attributes that, bizarrely, make us human.

As Pampush concluded in his interview with NPR, “If you're looking across all of the hominids, which is the family tree after the split with chimpanzees, there's not really that many traits that we can point to that we can say are exclusively human... The one thing that really sticks out is the chin. Perhaps it will tell us really what gave us that last little step into becoming anatomically modern that left those other human-like creatures behind.”

[H/T: The Atlantic]

Main image credit: Spyros Papaspyropoulos/Flickr. (CC BY-NC-ND 2.0)

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