Here we see an impressive, but chinless, Neandertal skull and a puny, but bechinned, human skull. Why do we have weird, low, skull ridge? We don't know, but we know one of the reasons why we shouldn't have it.
A study done at the University of Iowa took a look at our chinny-chin-chins and discovered that when we first heard the story associated with "chinny-chin-chins" (The Three Little Pigs) we didn't have them. Small children, like the four-year-old pictured in the x-ray, have just a tiny bump where a chin bone should be. Without the chin bone, we don't have a modern adult human's chin. The skull grows and changes as we do, and so by our early twenties we, like the second skull pictured in the x-ray, have a definite ridge jutting out in front of our jaw.
The researchers conducting the study looked the stresses that we put on our bodies as we grow — and they did it over a very long period of time. Participants were studied from the age of three to the age of twenty. Throughout that time, the participants were subtly breaking their bones.
Bones don't only break in two. Tiny stresses and jolts break down bone tissue, which often grows back thicker and stronger. This is why exercise and gravity keep your skeleton nice and dense. It is not, researchers concluded, why you grow a chin. In the study, the people who put the most stress on their jaw had the least chin to show for it.
Looking at the bigger picture, that makes sense. Neandertals had a tougher life than we do, and they had no chin to speak of, despite having skulls much larger than ours. In fact, the scientists doing this study believe it's possible that the chin emerged as other forces caused the skull to shrink. As the bones in our skulls wasted away, our chins emerged like an old city lost under sand.
Top Image: Tim Schoon, University of Iowa, Second Image: Nathan Holton lab, University of Iowa
[Source: The Ontogeny of the Chin]