Is retrognathism a progressive trait?

Answer by Tim Dalinian Jones:

Is what-now a progressive trait?

I confess that I had to look it up, so here is a working definition of “retrognathism”:

Retrognathia (or retrognathism) is a type of malocclusion (a misalignment of teeth or incorrect relation between the teeth of the two dental arches) which refers to an abnormal posterior positioning of the maxilla or mandible, particularly the mandible, relative to the facial skeleton and soft tissues. A retrognathic mandible is commonly referred to as an overbite, though this terminology is not used medically.”

~ Wikipedians, from…
» Retrognathism – Wikipedia Advanced  
» Malocclusion – Wikipedia Advanced

Reading between the lines, my hunch is that this is a question about the cultural emergence of (non-abnormal) mandibular retrognathism, ie: a pronounced overbite, within our recorded history, in relation to our widespread adoption of cutlery usage.


Three cousins – guillotine-like incisor alignments, meeting edge-to-edge, in three extant great ape species, (L to R, not to scale) chimpanzees, gorillas, and pre-cutlery humans.


In popular culture, the possibility that our ancestral adoption of food-morsel-handing cutlery (eg: chopsticks, fork, spoon, spork) has changed human skull morphology towards an overbite emerges with the (Oct 2012) publication ofConsider the Fork: A History of How We Cook and Eat’, by British food writer and historian Bee Wilson. [1]


Human incisors before cutlery – an early modern human skull demonstrates an ape-like incisor alignment, meeting edge-to-edge, the better to scissor through chunks of food held in the hand.

The Skhul 5 skull, from Es Skhul Cave on the slopes of Mount Carmel, Israel; early modern human, from c. 81,000-101,000 years ago.


Ms Wilson sheds light on the work of American anthropologist, C. Loring Brace, who has noted the geohistorical correlations between the widespread adoption of forks and chopsticks, and a change in the way our incisor teeth fit together, from a pre-cutlery cutting-edge-to-cutting-edge guillotine-like fit, to a post-cutlery top-in-front-&-bottom-behind overbite.


Human incisors after cutlery – sagittal X-ray imaging of contemporary human skulls demonstrates a modern cutlery-using incisor alignment, meeting in a top-in-front-&-bottom-behind overbite, the better to pluck morsels of food from hand-held cutlery, eg: a fork or chopsticks.


Here is Bee Wilson, in her own words, from a book-promotion media interview in 2013:

“Until around 250 years ago in the West, archaeological evidence suggests that most human beings had an edge-to-edge bite, similar to apes. In other words, our teeth were aligned liked a guillotine, with the top layer clashing against the bottom layer. Then, quite suddenly, this alignment of the jaw changed: We developed an overbite, which is still normal today. The top layer of teeth fits over the bottom layer like a lid on a box.

This change is far too recent for any evolutionary explanation. Rather, it seems to be a question of usage. An American anthropologist, C. Loring Brace, put forward the thesis that the overbite results from the way we use cutlery, from childhood onwards.

What changed 250 years ago was the adoption of the knife and fork, which meant that we were cutting chewy food into small morsels before eating it. Previously, when eating something chewy such as meat, crusty bread or hard cheese, it would have been clamped between the jaws, then sliced with a knife or ripped with a hand – a style of eating Professor Brace has called "stuff-and-cut."

The clincher is that the change is seen 900 years earlier in China, the reason being chopsticks.

As with any such thesis, we will probably never have definitive proof that the overbite results from the adoption of the fork, but it does seem the best fit with the evidence.

The first time I read Brace's work, I was truly astonished. So often, we assume that the tools we use for eating are more or less irrelevant – at most, a question of manners. I found it remarkable that they could have this graphic impact on the human body.”

~ Bee Wilson, in » 'How Forks Gave Us Overbites and Pots Saved the Toothless', by Scott Douglas in The Atlantic (17 Jan 2013)

If we can agree that forks and chopsticks are culinarily progressive, then it’s fair to say that the accompanying change in skull morphology – within our recorded history, towards mandibular retrognathism, ie: a pronounced overbite – is also a progressive trait.


Feline incisors after cutlery – whether feline ethologists will investigate a parallel (non-abnormal) mandibular retrognathism in chopstick-fed companion kittehs remains to be seen ;-D
 


[1] ‘Consider the Fork: A History of How We Cook and Eat’, by British food writer and historian Bee Wilson – see, eg:
» Consider the Fork: A History of How We Cook and Eat, by Bee Wilson, at  Amazon.com: Books

Is retrognathism a progressive trait?

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