The chin is one of our most unique human features, unseen in our Neanderthal ancestors. “The presence of a chin is something we use to define modern humans,” Nathan Holton, who studies craniofacial biology at the University of Iowa tells mental_floss. “It’s a diagnostic feature for our species.” And yet, its origins remain a mystery. Why do we have it? Does it serve some evolutionary purpose or was it just a random byproduct of our development over time? 

“Any feature of the modern human skull that’s unique to us becomes very interesting for us to understand why it developed,” Holton says. To get some answers, he and his team studied the facial growth of 40 people as they aged from toddlers to adults, and concluded that our chins developed as our faces evolved to be smaller than those of our ancestors -- about 15 percent smaller, to be precise. Here’s how Holton explains it: 

“If you look at Neanderthal for example, they have big prognathic face. Their mid-face sticks out much farther than ours, as a result what happens is the upper-face and mid-face grow forward. In a sense, it pulls that whole mid-face and the lower teeth forward resulting in more of a sloping chin region. In humans, we have a very short mid-face so the lower part of our mandible (aka the jaw) grows forward more.” 

Sounds complicated, but it really just means that as the upper and middle parts of the human face shrank backwards, the chin became more prominent.

So why did the face shrink in the first place? UI anthropologist Robert Franciscus thinks it has everything to do with human self-domestication. As we transformed from isolated hunter-gatherers to a network of communities, we no longer needed to fight as aggressively over land, and individual relationships flourished. Our hormones shifted. Specifically, testosterone levels dropped and as a result, our bodies (and faces) became smaller. 

"What we're arguing is that modern humans had an advantage at some point to have a well-connected social network, they can exchange information, and mates, more readily, there's innovation," says Franciscus, "and for that to happen, males have to tolerate each other. There had to be more curiosity and inquisitiveness than aggression, and the evidence of that lies in facial architecture."

Dietary changes and the advent of cooking may also have played a role in our small faces and pronounced chins. “We do a lot less chewing and we don’t chew foods that are as tough,” Holton says. “Regardless of why it happened, when we look at things like a chin or the absence of brow ridges or other unique features in modern humans, a lot of it seems to be tied to just a smaller face.”