When we think of what makes us human, it's common to think of something like language or tool-making. Something that likely doesn't pop into mind is the chin - but humans are the only species to have one! The bony prominence is missing from the skulls of Neanderthals, archaic humans, primates, and indeed all other animals. (In the photo, the skull on the left is human, and the one on the right is Neanderthal).
Scientists have puzzled for more than a century over why chins developed, and the dominant theory has been that they resulted from mechanical forces like chewing. Bones under pressure sustain tiny tears that then enable new bone to grow, much like weight lifting does to muscles. But a new study conducted by University of Iowa researchers suggests that mechanical forces have nothing to do with it: It's more likely that chins resulted from shifting social dynamics.
The study, published in the Journal of Anatomy, capitalized on the fact that children don't have chins either - the bone underneath their lower lip is smooth, and the prominence develops with age. The study examined nearly 40 people ranging from 3-20 years old, correlating their chin development with various forces exerted by their cranio-facial anatomy (during chewing, for example), and concluded that mechanical forces don't play a role in chin development. In fact, those with the most mechanical force had the smallest chins.
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.
Instead, the researchers think that the chin results from the facial structure being rearranged as faces got smaller - human faces are 15 percent smaller than those of Neanderthals. This reduction resulted from a decrease in testosterone levels, which happened as males of the species benefitted more from interacting socially with other groups rather than fighting other males.
Robert Franciscus, PhD, professor of anthropology at UI and a contributing author on the study, also comments:
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.