So, why do chins exist?

There are no firm answers, which isn't for lack of effort. Evolutionary biologists have been proposing hypotheses for more than a century, and Pampush has recently reviewed all the major ideas, together with David Daegling. “We kept showing, for one reason or another, that these hypotheses are not very good,” he says.

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 join and not the outer one. In other words, you'd want the opposite of a chin.

Others have suggested that the chin is an adaptation for chinwags: It resists the forces we create when speaking. After all, speech is certainly a feature that separates us from other living animals. But there's no good evidence that the tongue exerts substantial enough forces to warrant a thick chunk of reinforcing bone. “And any mammal that also communicates vocally or suckles or engages in complex feeding behaviors that involve the tongue are probably experiencing similar stresses and strains, and they're not getting chins,” says Pampush.

Maybe it's about sex, then? Men typically have bigger chins than women, and stronger chins are often equated with attractiveness. Perhaps the chin is a sexual ornament, the human equivalent of a stag's antlers or a peacock's tail, a way of attracting mates or perhaps even signaling one's health and quality. “But if that's the case, we'd be the only mammal ever where both sexes have selected for the exact same ornament,” says Pampush. In other words, women have chins, too. Chin shape may well be relevant to sex, but that doesn't explain chin presence. “They must have been there for some other reason before we started looking at the shape of them.”

Then, there are hypotheses that “stretch the concept of natural selection,” says Pampush. For example, one century-old idea says that chins are adaptations for deflecting punches to the face. That is, they helped early humans to take one on the chin. “That would require humans to hit each other so often, and to suffer such dire consequences from being hit without a chin ... it's unrealistic,” says Pampush. Also, chins are terrible for deflecting blows. They don't disperse the incoming forces very evenly, which results in broken jaws. Even if our ancestors were constantly pummeling each other in the face, they would have fared better by reinforcing their jaws all the way round.