Many scientists have stroked their chins in puzzlement over...well, the human chin. The bony nub that juts out from the bottom of the lower jaw is unique in the animal kingdom, and although researchers have proposed several theories over the years as to why, the chin remains a mystery.

The chin isn’t just the lower part of your face: It’s a specific term for that little piece of bone extending from the jaw. While it may seem odd, humans are in fact the only animals that have one. Even chimpanzees and gorillas, our closest genetic cousins, lack chins. Instead of poking forward, their lower jaws slope down and back from their front teeth. Even other ancient hominids, like the Neanderthals, didn’t have chins —their faces simply ended in a flat plane, Ed Yong writes for The Atlantic.

“If you're looking across all of the hominids, which is the family tree after the split with chimpanzees, there [are] not really that many traits that we can point to that we can say are exclusively human,” Duke University’s James Pampush tells Robert Siegel for NPR. “[T]hose animals all walked on two legs. The one thing that really sticks out is the chin.”

Over the last century, scientists have proposed many ideas to explain why humans evolved chins, from helping us chew food to speaking. Pampush argues that many of these theories don’t hold up under further scrutiny. He published this idea recently in the journal Evolutionary Anthropology.

“The chin is one of these rare phenomena in evolutionary biology that really exposes the deep philosophical differences between researchers in the field,” Pampush tells Yong.

One of the most popular ideas is that our ancestors evolved chins to strengthen our lower jaws to withstand the stresses of chewing. But according to Pampush, the chin is in the wrong place to reinforce the jaw. As for helping us speak, he doubts that the tongue generates enough force to make this necessary. A third idea is that the chin could help people choose mates, but sexually selective features like this typically only develop in one gender, Pampush tells Siegel.

When it comes down to it, the chin may have no real purpose. According to Pampush, it could just be something called a "spandrel," or an evolutionary byproduct left from another feature changing. In the chin’s case, it could be the result of the human face shrinking over time as our posture changed and our faces shortened, or a remnant from a period of longer jaws.

“It seems that the appearance of the chin itself is probably related to patterns of facial reduction in humans during the Pleistocene,” Nathan Holton, who studies facial evolution at the University of Iowa, tells Yong. “In this sense, understanding why faces became smaller is important to explaining why we have chins.”

The spandrel hypothesis is as good a theory as any, but it too has its problems. It’s hard to find evidence to test if something is an evolutionary byproduct, especially if it doesn’t serve an obvious function. But if researchers one day do manage to figure out where the chin came from, it could put together another piece of the puzzle of what makes us different from our primate and Neanderthal cousins, Yong writes.

“Perhaps it will tell us really what gave us that last little step into becoming anatomically modern,” Pampush tells Siegel.