Nature is nothing if not parsimonious, especially when it comes to the human body. There’s a reason we don’t have webbed feet or nut-cracking beaks like other species, and that’s because we don’t need them. The system isn’t perfect, of course. If you ever wind up having painful abdominal surgery, odds are pretty fair that it will be your good-for-nothing appendix that’s to blame. And wisdom teeth seem a lot less wise when you consider how often they fall down on the job and need to get yanked.
As it turns out, the same why-bother pointlessness is true of what you might consider one of your loveliest features: your chin.
Researchers have long wondered what the adaptive purpose of the chin could possibly be. Sexual selection seems like an obvious answer, since an attractive chin increases your chances of mating. But a feature needs a function before it can appear in the first place. Only then can it be assigned some aesthetic value.
The other, better answer is all about chewing. The jaw exerts enormous forces when it bites and chews—up to 70 lbs. per sq. in. (32 kg per 6.5 sq. cm) for the molars. Conscious clenching increases the figure, and people who grind their teeth in their sleep may exceed the average force 10-fold. What’s more, the jaw moves in more than just one axis, both chewing up and down and grinding side to side.
That, so the thinking went, might increase bone mass in the same way physical exercise builds muscle mass. And bone mass, in turn, may produce the chin. The problem with the theory, however, is that it doesn’t account for Neanderthals and other primates—including the great apes—which lack prominent chins but in many cases have far more powerful bites than we do.
To answer the riddle, Nathan Holton, a post-doctoral researcher who specializes in craniofacial structure in the University of Iowa school of orthodontics, selected 37 of the many subjects whose facial measurements have been taken regularly from age 3 to young adulthood, as past of the longstanding Iowa Facial Growth Study (yes, there is such a thing).
With the help of basic physics, it’s possible to determine how much force any one jaw exerts without the subjects’ ever having to be tested directly with a bite gauge. Measuring the geometry of what orthodontic researchers call the mandibular symphysis and what everyone else just calls the chin region, and comparing that to what is known as the bending moment arm—or the distance between where a force is initially applied (in this case the muscles in the jaw) and where that force is eventually felt (the chin)—yields a pretty good measure of force exerted.
“Think about removing the lug nuts from a wheel on your car,” Holton wrote in an e-mail to TIME. “The longer the wrench, the easier it is because the longer wrench increases the moment arm, allowing you to create more force.”
And more force, in this case, should mean more bone mass in the chin—but that’s not what the results of the new research showed. Not only did the two turn out to be unrelated in the 37 subjects studied, but Holton and his colleagues even found that as the face matures, the chin is less adept at resisting mechanical forces, which is the whole reason it was assumed to grow more pronounced in the first place.
So why did we grow chins at all? The answer is, we didn’t. Holton and his collaborator, University of Iowa anthropologist Robert Franciscus, instead suspect that the face shrank away from behind the chin as primitive and pre-humans became modern humans, making it appear larger relative to everything else. The reason, as with so many things in the human species, has to do with male behavior—specifically violent male behavior.
As humans migrated from Africa 20,000 years ago and settled down into societies, males had to become less competitive and more cooperative—giving an advantage to those with lower testosterone levels. And reduced testosterone softens and shrinks the craniofacial structure.
“What we are arguing is that modern humans had an advantage at some point to have a well-connected social network,” Franciscus said in a statement accompanying the study. “And for that to happen, males had to tolerate each other. There had to be more curiosity and inquisitiveness than aggression, and the evidence of that lies in facial architecture.”
It wasn’t until we had our chins that we set about assigning value to them—strong ones, weak ones, angular, round, cleft or dimpled, depending on your tastes. Those tastes—and the mating choices that arise from them—ensure that the chin will stay. It might be biomechanically useless, but you’d look awfully silly without one.
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