Chins of anatomically modern humans don't come from mechanical forces such as chewing, but instead result from an evolutionary adaptation involving face size and shape, according to a team of anthropologists at the University of Iowa.
Scientists say that primates, Neanderthals, and archaic humans don't possess a chin. This image shows a Neanderthal man. Image credit: Mauro Cutrona.
Using facial and cranial biomechanical analyses with nearly 40 people whose measurements were plotted from toddlers to adults, the Iowa University team concludes mechanical forces, including chewing, appear incapable of producing the resistance needed for new bone to be created in the lower mandible.
Rather, it appears the chin's emergence in modern humans arose from simple geometry. As human faces became smaller in our evolution from archaic humans to today – in fact, our faces are roughly 15 % shorter than Neanderthals' – the chin became a bony prominence, the adapted, pointy emblem at the bottom of our face.
"In short, we do not find any evidence that chins are tied to mechanical function and in some cases we find that chins are worse at resisting mechanical forces as we grow. Overall, this suggests that chins are unlikely related to the need to dissipate stresses and strains and that other explanations are more likely to be correct," explained Dr Nathan Holton, lead author on the study published online in the Journal of Anatomy.
"The human chin is a secondary consequence of our lifestyle change, starting about 80,000 years ago and picking up great steam with modern humans' migration from Africa about 20,000 years later."
The scientists think that anatomically 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.
These more connected groups appear to have enhanced the degree to which they expressed themselves in art and other symbolic mediums.
Males in particular became more tranquil during this period, less likely to fight over territory and belongings, and more willing to make alliances, evidenced by exchanging goods and ideas.
The change in attitude was tied to reduced testosterone levels, resulting in changes to the male craniofacial region.
"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. 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," said co-author Dr Robert Franciscus.
"Our study suggests that chin prominence is unrelated to function, and probably has more to do with spatial dynamics during development," Dr Holton said.
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N.E. Holton et al. The ontogeny of the chin: an analysis of allometric and biomechanical scaling. Journal of Anatomy, published online April 11, 2015; doi: 10.1111/joa.12307
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