“Little pig, little pig, let me come in,” says the big, bad wolf. “No, no, not by the hair on my chinny chin chin,” say the three little pigs. This scene is deeply unrealistic and not just because of the pigs' architectural competence, the wolf's implausible lung capacity, and everyone's ability to talk.

The thing is: Pigs don't have chins. Nor do any animals, except for us.

The lower jaw of a chimpanzee or gorilla slopes backwards from the front teeth. So did 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 really 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. Sure, some people have less pronounced ones than others, perhaps because their lower jaws are small or they have more flesh around the area. But if you peeled back that flesh and exposed their jawbones—and maybe don't do that—you'd still see a chin.

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.

Pampush doubts that chins are adaptations at all. He thinks it's more likely that they are spandrels—incidental features that have no benefits in themselves, but are byproducts of evolution acting upon something else.

For example, during human evolution, our faces shortened and our posture straightened. These changes made our mouths more cramped. To give our tongues and soft tissues more room, and to avoid constricting our airways, the lower jaw developed a forward slope, of which the chin was a side effect. The problem with this idea is that the chin's outer face doesn't follow the contours of its inner face, and has an exceptionally thick knob of bone. None of that screams “space-saving measure.”

A different explanation portrays the chin as a bit of the jaw that got left behind while the rest shrunk back. As early humans started cooking and processing our food, we made fewer demands upon our teeth, which started shrinking as a result. They gradually retracted into the face, while the part of the lower jaw that held them did not (or, at least, did so more slowly). Hence: chin.

Stephen Jay Gould and Richard Lewontin, who coined the concept of evolutionary spandrels, liked this hypothesis. So does Nathan Holton from the University of Iowa, who studies facial evolution. “It seems that the appearance of the chin itself is probably related to patterns of facial reduction in humans during the Pleistocene,” he says. “In this sense, understanding why faces became smaller is important to explaining why we have chins.”  

“But why did the lower border of the jaw also not shrink?” Pampush asks. “What happened that left that last little bit sticking out?” This is the problem with spandrel hypotheses more generally: They're often very hard to test.

It may seem frustrating to have so many imperfect competing hypotheses, but that's part of the joy of chins: They reveal something about how scientists think about evolution. Some see the sculpting power of natural selection in everything, and view chins as surely some kind of adaptation. Others see natural selection as just one of many evolutionary forces, and so gravitate towards a spandrel-based explanation. “The chin is one of these rare phenomena in evolutionary biology that really exposes the deep philosophical differences between researchers in the field,” says Pampush.

And, indeed, between people outside the field. “I always get entertaining emails from lay people trying to help me so let me thank you in advance for what I'm about to receive,” he tells me.

Because if there's one trait that more universally human than the chin, it's having opinions.