11111Look at a primate or a Neanderthal skull and compare it with a modern human's. Notice anything missing? We have one feature that primates, Neanderthals, archaic humans — any species, for that matter — don't possess: a chin. “In some way, it seems trivial, but a reason why chins are so interesting is we are the only ones who have them,” says Nathan Holton, who studies craniofacial features and mechanics at the University of Iowa. “It’s unique to us.”

New research led by Holton and colleagues at the UI posits that our chins don’t come from mechanical forces such as chewing, but instead results from an evolutionary adaptation involving face size and shape—possibly linked to changes in hormone levels as we became more socially domesticated.

In a research conducted with nearly 40 people, the UI team concludes mechanical forces, including chewing, appear incapable of producing the resistance needed for new bone to be created in the lower mandible, or jaw area. Rather it appears the chin's emergence in modern humans arose from simple geometry: As our faces became smaller in our evolution from archaic humans to today, the chin became a bony prominence, the adapted, pointy emblem at the bottom of our face.

More intriguing, UI anthropologists led by Robert Franciscus think the human chin is a secondary consequence of our lifestyle change. What happened was this: Modern humans evolved from hunter-gatherer groups that were rather isolated from each other to increasingly cooperative groups that formed social networks across the landscape. The change in attitude was tied to reduced hormone levels, namely testosterone, resulting in noticeable changes to the male craniofacial region: One big shift was the face became smaller — retrenching in effect — a physiological departure that created a natural opportunity for the human chin to emerge.

Something else that the researchers did notice is that chin “growth” has more to do with how each feature in our face adapts as our head increases, much like you'd fit individual pieces together in an expanding, shape-shifting, three-dimensional puzzle. Children, for example, have flat, nearly imperceptible chins, much like what's seen in Neanderthals. That bony prominence only becomes visible as our heads and faces grow into adulthood. At the end Holton concludes that their study suggests the chin prominence is unrelated to function, and probably has more to do with evolution and spatial dynamics during development.






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