No animals have chins, except for us. The lower jaw of a chimpanzee or gorilla slopes backwards from the front teeth. So does the jaw of other hominids like Homo erectus. Even Neanderthal jaws ended in a flat vertical plane. Only in modern humans does the lower jaw end in a protruding strut of bone. A sticky-outy bit. A chin.
“It’s strange that only humans have chins,” says James Pampush from Duke University. “When we’re looking at things that are uniquely human, we can’t look to big brains or bipedalism because our extinct relatives had those. But they didn’t have chins. That makes this immediately relevant to everyone.” Indeed, except in rare cases involving birth defects, everyone has chins.
So, why do chins exist?
There are no firm answers. Evolutionary biologists have been proposing hypotheses for more than a century, and Pampush has recently reviewed all the major ideas, together with David Daegling.
The most heavily promoted explanation is that chins are adaptations for chewing — that they help to reduce the physical stresses acting upon a masticating jaw. But Pampush found that, if anything, the chin makes things worse. The lower jaw consists of two halves that are joined in the middle; when we chew, we compress the bone on the outer face of this join (near the lips) and pull on the bone on the inner face (near the tongue). Since bone is much stronger when compressed than pulled, you’d ideally want to reinforce the inner face of the joint and not the outer one. In other words, you’d want the opposite of a chin.
Source: www.theatlantic.com
