Why Are Humans The Only Animals With Chins?

[edd]

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.”

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.”

[Auspex]

I have always thought of it as an artifact of having our spinal column enter the base, rather than the rear, of our skull. Note how proportionally short our jaws are; this is a consequence of front-facing erect posture and all that.

[ynot]

If We did not have a chin then how could We do this.... ?

I think that is why Homosaipeans developed chins.

Tony

PS I really agree with Auspex's reasoning on this.

[Raggedy Man]

Actually, elephants have chins as well.

[engineeringstudent]

I have always thought of it as an artifact of having our spinal column enter the base, rather than the rear, of our skull. Note how proportionally short our jaws are; this is a consequence of front-facing erect posture and all that.

Huh?

[Auspex]

Huh?

To accommodate the evolution of an erect posture, the human skull "rotated" forward (else we would all be looking straight up).

Our spinal column attaches at the bottom, instead of the back.

[engineeringstudent]

To accommodate the evolution of an erect posture, the human skull "rotated" forward (else we would all be looking straight up).

Our spinal column attaches at the bottom, instead of the back.

I guess I'm imagining some weird looking hominids with spines connected to the back of their heads. I just don't feel like all the species listed in the image above were like that before getting to sapiens.

[Auspex]

Another consequence of erect posture comes from dexterous hands: our ancestors did not need long, pointy jaws to manipulate things; hands were better. Just look at the length-to-width ratios of various animal mandibles: a human's is very short.

[engineeringstudent]

Another consequence of erect posture comes from dexterous hands: our ancestors did not need long, pointy jaws to manipulate things; hands were better. Just look at the length-to-width ratios of various animal mandibles: a human's is very short.

But if this change came about in early humans then why would the chin only be represented in the last species? My confusion is why don't the neaderthal, heidelberg and erectus not have chins if they also have their spines exiting the base of their skulls as well?

[Auspex]

But if this change came about in early humans then why would the chin only be represented in the last species? My confusion is why don't the neaderthal, heidelberg and erectus not have chins if they also have their spines exiting the base of their skulls as well?

That's the $64,000 question, isn't it? What adaptive purpose could it have served that would've had selection value? My suspicion is that it had to do with mate-selection.

[drujd]

Years ago I remember reading in an old bio-anthropology text book, that one theory was that it acted as a buttress for the jaw, allowing it to reduce in size compared to earlier hominids.

Not sure how much water that idea holds though.

Daniel