Chuck Bednar for redOrbit.com – @BednarChuck
Modern humans developed chins not for chewing or other mechanical functions, but as the result of an evolutionary adaptation involving face size and shape that may be linked to changes in our hormone levels due to domestication, according to a new study.
In a paper published this week in the Journal of Anatomy, a team of experts from the University of Iowa also concluded that the development of a human chin is a secondary consequence of our lifestyle changes from isolated hunter-gatherer groups to more cooperative social networks.
Chewing alone would not necessitate jaws
Nathan Holton, an assistant professor and anthropologist in the UI College of Dentistry, and his colleagues recruited almost 40 people of various ages, and conducted a series of different facial and cranial biomechanical analyses. They discovered that chewing and other mechanical forces alone would not have been enough to spur on the development of jaws.
“Part of what we’re trying to do is gain a better understating about the role of jaw function in facial development and evolution,” Holton told redOrbit via email. “Many aspects of facial form in archaic and modern humans have been suggested to be tied, in one way or another, to jaw function. A projecting chin in modern humans is a good example of a part of the jaw that is thought by some to be a response to mechanical stresses during chewing.”
“What we wanted to do was examine how whether the development of a prominent chin from around three years of age into adulthood increased the ability for the mandible to resist certain types of stresses,” he added. “Based solely on the geometry of the mandible, we didn’t find any evidence that a chin was better a resisting stresses in the anterior region of the mandible. In fact, we found that as the chin develops, the mandible is less capable of resisting certain stresses.”
Instead, they report that the formation of new bone in the lower mandible was actually the result of simple geometry: as our faces became smaller during our evolution into modern humans, the chin became a bony prominence at the end of our face that develops as our head size increases. The finding could settle a long-standing, on-again/off-again debate into the reasons why people have chins and how the structures formed in the first place.
Hormone changes, facial growth involved in chin development
Furthermore, Holton’s colleague Robert Franciscus and a team of UI anthropologists concluded that the human chin was a side-effect of lifestyle changes that started roughly 80,000 years and saw human evolve from isolated hunter-gatherer groups to increasingly-connected societies that would cooperate and were more likely to express themselves in art and other mediums.
In particular, males would have become less aggressive over this period, opting to fight less and negotiate more (as evidenced by an increasing exchange of goods and ideas) that would benefit all members of the group, the study authors added. This shift in attitude is correlates to reduced levels of testosterone and other hormones that would have also resulted in reduced face size and other physiological changes in men, the Iowa research team explained.
“What we’re arguing,” Franciscus, corresponding author on the study, said in a statement, “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.”
The new study supports the notion that the chin did not develop from mechanical exertion such as chewing, and is based on an examination of its reaction to the mechanical forces that cause the chin area to become stressed. In theory, the processes being analyzed created new bones on the microscopic levels, and that these processes resulted in the development of the chin.
In looking at subjects from three to 20 years old, however, the authors found no evidence of this. In fact, they found the opposite what true: those individuals with the most mechanical resistance were most similar to three- or four-year-olds, meaning that they had very little chin. Rather, they found that chin growth had more to do with how each part of faces adapts as head size increases, fitting together like a series of pieces in a shape-changing 3D puzzle.
“These results provide evidence that the development of a prominent chin is unlikely to be the result of mechanical function,” Holton told redOrbit. “Instead, the development of the chin probably has more to do differential growth of different regions of the face that affect the spatial relationship between different parts of the jaw.”
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