Have you ever wondered why you have a chin? Science has the answer: it's because the human skull has shrunk with the evolution of our species.

Previously, the leading theory was that the chin had been shaped over time to assist humans with chewing, which involves the jaw exerting force via the molars. The theory explained that as physical exercise builds muscle mass, the chewing process increases bone mass — thereby developing the human chin. 

What challenges this theory is that it doesn't take into account why the Neanderthals and other primates lack outstanding chins. They have a stronger bite than humans — and yet, a prominent chin is a defining feature differentiating humans from other primates, including our closest relative, the Neanderthal. 

Nathan Holton, a postdoctoral anthropologist specializing in craniofacial structure at the University of Iowa, formed a research team to complete the Iowa Facial Growth Study. They selected 37 subjects – 19 male and 18 female – and examined X-ray images of their skull development, monitoring their facial measurements from age three to young adulthood.

The researchers found no evidence that chins possess mechanical function — in fact, they found that some chins are worse at resisting mechanical forces as the body matures.

"Overall," Holton said, "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." 

Holton and his associate, University of Iowa anthropologist Robert Franciscus, theorized that the reduction of the human face through time has made the chin seem relatively large. The chin didn't grow — it was the rest of the face that shrank. The reason for that shrinkage can be linked to changes in the human species' prehistoric violent behavior.

The team explained that early humans began to undergo accelerated change as they migrated from Africa around 20,000 years ago, settling down into societies and forming communities. This marked an evolutionary period wherein humanity developed more social traits, such as communication and even the creation of art. There was less violent competition between groups of humans and more exchange of goods and ideas.

Scientists believe this triggered a decrease in levels of hormones such as testosterone, which resulted over time in the shrinking of the craniofacial structure — or the softening of the face.

This study was published in the Journal of Anatomy.

Photo: James St. John | Flickr