Chinless wonders and too much testosterone

A new paper by Nathan Holton and colleagues at the University of Iowa suggests that our chins (which Neanderthals and other primates did not evolve) resulted from hormonal changes which themselves resulted from increasing social interactions among humans and not from evolution due to mechanical forces such as chewing.

Using advanced facial and cranial biomechanical analyses with nearly 40 people whose measurements were plotted from toddlers to adults, the UI team concludes mechanical forces, including chewing, appear incapable of producing the resistance needed for new bone to be created in the lower mandible, or jaw area. Rather, they write in a paper published online in the Journal of Anatomy, it appears the chin’s emergence in modern humans arose from simple geometry: As our faces became smaller in our evolution from archaic humans to today—in fact, our faces are roughly 15 percent shorter than Neanderthals’—the chin became a bony prominence, the adapted, pointy emblem at the bottom of our face.

They sperculatively  suggest that

 ….. the human chin is a secondary consequence of our lifestyle change, starting about 80,000 years ago and picking up great steam with modern humans’ migration from Africa about 20,000 years later. What happened was this: Modern humans evolved from hunter-gatherer groups that were rather isolated from each other to increasingly cooperative groups that formed social networks across the landscape. These more connected groups appear to have enhanced the degree to which they expressed themselves in art and other symbolic mediums. Males in particular became more tranquil during this period, less likely to fight over territory and belongings, and more willing to make alliances, evidenced by exchanging goods and ideas, that benefited each and all.

…… the change in attitude was tied to reduced hormone levels, namely testosterone, resulting in noticeable changes to the male craniofacial region: One big shift was the face became smaller—retrenching in effect—a physiological departure that created a natural opportunity for the human chin to emerge”.

This last is more than a little speculative since the research is really just evidence that mechanical forces are probably not the cause of chin evolution and development. I am not at all convinced that the evolution of the chin could actually be due to hormonal levels resulting from social behaviour, but it could well be that it is merely a feature not explicitly selected for, but which is a geometrical consequence of other facial changes which came about as AMH evolved. It cannot either be due to just the increase in brain size since Neanderthals had somewhat larger brains. I wonder if it may be a consequence of changes due to development of speech?

N. E. Holton, L. L. Bonner, J. E. Scott, S. D. Marshall, R. G. Franciscus, T. E. Southard. The ontogeny of the chin: an analysis of allometric and biomechanical scaling. Journal of Anatomy, 2015; DOI: 10.1111/joa.12307

Abstract

The presence of a prominent chin in modern humans has been viewed by some researchers as an architectural adaptation to buttress the anterior corpus from bending stresses during mastication. In contrast, ontogenetic studies of mandibular symphyseal form suggest that a prominent chin results from the complex spatial interaction between the symphysis and surrounding soft tissue and skeletal anatomy during development. While variation in chin prominence is clearly influenced by differential growth and spatial constraints, it is unclear to what degree these developmental dynamics influence the mechanical properties of the symphysis. That is, do ontogenetic changes in symphyseal shape result in increased symphyseal bending resistance? We examined ontogenetic changes in the mechanical properties and shape of the symphysis using subjects from a longitudinal cephalometric growth study with ages ranging from 3 to 20+ years. We first examined whether ontogenetic changes in symphyseal shape were correlated with symphyseal vertical bending and wishboning resistance using multivariate regression. Secondly, we examined ontogenetic scaling of bending resistance relative to bending moment arm lengths. An ontogenetic increase in chin prominence was associated with decreased vertical bending resistance, while wishboning resistance was uncorrelated with ontogenetic development of the chin. Relative to bending moment arm lengths, vertical bending resistance scaled with significant negative allometry whereas wishboning resistance scaled isometrically. These results suggest a complex interaction between symphyseal ontogeny and bending resistance, and indicate that ontogenetic increases in chin projection do not provide greater bending resistance to the mandibular symphysis.

xray showing chin comparison between toddler and adult

University of Iowa researchers find that we develop chins as our head size increases, from childhood to adulthood. At about 4 years of age (left), we have little indication of a chin, but by our 20s, we have a prominent point at the bottom of our faces.

And that leads us to the “chinless wonders” of the Royal House of Windsor

Members of the upper classes, by repute, often have minor genetic abnormalities like receding chins. This disparaging term is often used to describe members of the British upper classes and in particular the royal family. This is probably an implied reference to the effects of the supposed inbreeding of the upper classes and, again, particularly the House of Windsor. This is mostly just name calling, but is supported by the fact that Queen Elizabeth II and her husband have the same great great grandmother – Queen Victoria, and that she had a rather receding chin, as have several of her descendants.

Too much testosterone?

HEIR AND A SPARE Prince Andrew and Prince Charles arrive in a carriage at the Royal Ascot horse race, 2006. By David Hartley/Rex USA. via Vanity Fair

 

 

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