A strange scientific debate that has gone on for the better part of the century, right alongside why men have nipples, may have been solved: why are humans the only species with a chin?
Researchers at the University of Iowa delved into the question of why humans have chins, comparing modern human jaws with the jaws of early humans and Neanderthals. One theory has our chins developing due to mechanical forces, such as chewing. But lead researcher and assistant professor in the Department of Orthodontics, Nathan Holton explained in a paper published in the Journal of Anatomy that mechanical forces actually discouraged chin growth. Analyzing 40 people from toddlers to adults and using facial and biomechanical analyses, they found that chewing simply did not give enough stress to encourage evolving a more powerful bone to compensate. Something else was the reason why humans have chins.
That something else was our faces getting smaller. As our species evolved and our faces receded, the lower mandible simply stayed, pointing out from what was once a deeper sea of flesh. The thing to wonder next in order to answer the chin question is: why did our faces get smaller?
University of Iowa, anthropologist and contributing author to the paper Robert Franciscus explained that the human chin is likely an epiphenomenon of our changing lifestyle about 80,000 years ago. As we became more gregarious, talking more with other groups and trading items, ideas, and protection, there was more of a selection toward cooperation than attacking and defending. This selective process led to a decrease in the hormone tied to aggression, testosterone. One effect of testosterone is to physically make the face larger (one reason why men on average have larger faces than women). As humans became more social, individuals with heavy testosterone were selected out, leaving the smaller faced individuals to propagate in the groups. And as our faces retracted, our chin stayed put.
The researchers did notice how chins grow and vary within modern humans after studying the 40 participants, finding that chin growth has more to do with complex spatial dynamics as we age. Children have very small chins, and as we grow, all of the features of our face and head bumble about until they find a way to fit together. They also found an inverse effect with chewing. The more resistance your jaw has when you chew, the less it will actually grow, resembling that of a child.
In the theme of marveling at humans, watch this short about a marvel of human courage:
https://youtu.be/nQ-s1OL_1OM
