January 4, 2012
When you think about what makes modern humans unique, the chin is probably not the first, second or even third thing that comes to mind. Yet this bony protrusion at the end of the lower jaw is not seen in any other hominid species. So what’s a chin good for? Over the years, researchers have thought up a variety of explanations for why we have chins.
Perhaps the most common explanation is that our chin helps buttress the jaw against certain mechanical stresses. Ionut Ichim, a Ph.D. student at the University of Otago in New Zealand, and colleagues suggested in the journal Medical Hypotheses in 2007 that the chin evolved in response to our unique form of speech, perhaps protecting the jaw against stresses produced by the contraction of certain tongue muscles. Others think the chin evolved to safeguard the jaw against forces generated by chewing food. Last year, Flora Gröning, a biological anthropologist at the University of York in England, and colleagues tested the idea by modeling how modern human and Neanderthal jaws withstand structural loads. Their results, which they reported in the American Journal of Physical Anthropology, indicated the chin does help support the jaw during chewing . They suggested the chin may have evolved to maintain the jaw’s resistance to loads as our ancestors’ teeth, jaws and chewing muscles got smaller early on in our species’ history.
A completely different line of reasoning points to sexual selection as the driver of the evolution of the chin. Under sexual selection, certain traits evolve because they are attractive to the opposite sex. Psychological research suggests chin shape may be a physical signal of the quality of a mate. For example, women may prefer men with broad chins because it’s sign that a man has good genes; likewise, a woman’s narrow chin may correlate with high levels of estrogen. Zaneta Thayer, a graduate student at Northwestern University, and Seth Dobson, a biological anthropologist at Dartmouth, examined the sexual selection hypothesis by measuring the chin shape of nearly 200 skulls in a museum collection, representing people from all over the world. The pair discovered that there is a small but distinct difference in chin shape between the sexes, with men having a taller, more pronounced chin. They argued in the American Journal of Physical Anthropology in 2010 that this difference is evidence against explanations that the chin evolved to resist mechanical stress. If chins evolved in response to eating or talking, then there should be no difference in chin shape between the sexes because, presumably, men and women eat and talk the same way.
Maybe the evolution of the chin is more complicated than any one scenario. For example, is it possible the chin initially evolved to handle particular mechanical stresses and was later shaped further by sexual selection? I’d like to see someone test that idea.
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