It’s Friday, and that means that today I make all the students in my eco-devo class do all the work, while I sit back and observe. It’s too bad I can’t do this every day of the week, but I guess I have to do something now and then to earn my gigantic paycheck. Anyway, on Fridays I pick a paper relevant to the subject of the course, throw it to two student volunteers, and tell them to lead a discussion.
This week the paper is The Jaw Epidemic: Recognition, Origins, Cures, and Prevention by Sandra Kahn, Paul Ehrlich, Marcus Feldman, Robert Sapolsky, and Simon Wong. It’s about the fact that our jaws have been shrinking rapidly in some cultures, and speculating about why. The answer the paper gives is that it’s an epigenetic response to environmental factors, specifically diet but also respiratory phenomena. You might be able to see why this is of interest in an eco-devo class.
Some scholars, although they accept all or some of our narrative, still assert that part of the problem must be genetic or hereditary. They apparently do not realize that, because every attribute of all living organisms must to some degree be traceable to their DNA (or RNA), the statement is nonsensical. Nonetheless, some scientists continue to push partial blame for the epidemic toward genetic evolution while ignoring the etiology of jaw shrinkage and distortion. The success of some clinical techniques to normalize jaw growth in young children and abundant evidence that jaw shrinkage is a factor in both obstructive sleep apnea and the advancement of maxilla and mandible are key treatments, in addition to other surgical techniques. This further makes clear the largely environmental cause of the epidemic.
This confusion over etiology is a possible result of the genetic determinism that is characteristic of much of popular science. For instance, recent genome-wide association studies (GWAS) studies aimed at orofacial issues have been focused on possible genetic factors involved in the variation in the eruption of third molars (wisdom teeth). But they in no way suggest that selection and widespread genomic evolution explain the rarity of impacted third molars in hunter-gatherers compared with their common occurrence in settled or industrialized human populations). Similar problems occur when “racial” differences in the occurrence of jaw-related disease are discussed. For instance, Weinstock and colleagues (2014) found that African-American children were about 20% more susceptible to pediatric obstructive sleep apnea than children of other ethnic groups. But, unhappily, possible key environmental variables such as allergen concentrations at home or the length of nursing were ignored, as were different head shapes in different human groups that could make some more susceptible to the impacts of environmental change. In short, despite the great attention paid to a possible genetic evolutionary cause of the jaw epidemic, precious little evidence of genomic change being a significant factor has been uncovered.
I hope this sparks some good conversation. It’s a bit over-the-top to call it an “epidemic” of jaw shrinkage, but the hyperbole might trigger some arguments.
P.S. Their instructor is no gigachad. I grew up with a horribly crowded mouth with crooked teeth every which way that was treated crudely, by just yanking out a half dozen teeth to make room — we couldn’t afford braces or any finesse. Also, I had painfully impacted wisdom teeth that required an oral surgeon to take a hammer and chisel to my face.
James Nestor’s book Breathe is a good popsci introduction to this, and also has a lot of interesting health tips.
If it’s any help, even with nice braces, yanking teeth is still often required. My poor kid had good to honest shark teeth, one growing behind the other.
The percent of children in the USA that get orthodontic treatment is high.
The estimates run around that 75% of all US children will see an orthodontist for functional or cosmetic reasons.
Isn’t the most common dental procure in Minnesota, Hockey. I lost 3 teeth in my teens that way and it contributed to my getting dentures in my 50’s.
I also needed my lower molars chiseled out. Great fun!
While I agree with the authors that there are plausible hypotheses outside classical evolution that could explain changing jaw sizes, they seem a bit desperate to exclude evolution as a possibility, calling any evolutionary explanation ‘nonsensical’ because ‘every attribute of all living organisms must to some degree be traceable to their DNA (or RNA)’, which seems to me a very strong argument against confidently excluding evolution as a possible contributor to observed phenotypic changes over time.
Also, they insist on the canard that ‘a few centuries’ is far too short for evolutionary changes in humans. Evolution can proceed very rapidly under certain circumstances. Manchester’s famous peppered moths are a great example — the dark moth variation went from rare in 1811 to 98% of the population by 1895. Moths of course have a faster generation cycle than humans, and the selection pressure from camouflage was probably much higher than that for jaw size in humans, but still I am wary of arguments that evolution can completely excluded as a component cause of human changes on the scale of centuries. Just for one example, there is evidence of malaria resistance evolving in the people of Cabo Verde over a span of ~560 years.
(An interesting aside about Cabo Verde — it was completely uninhabited until Portugese settled there in 1462. What appears to have helped drive the evolution of malaria resistance was the fact that they brought Senegalese slaves with them who, of course, became genetically admixed with the Portugese settlers. So much for the genetic inferiority theories of ‘scientific’ racists.)
raven@3–
There is huge variation in orthodontic treatments around the world. Some of this is due to differences in health care systems.
chrislawson@5-
That must be it. Now, where and how) is this massive, selective culling of people with large jaws and straight teeth happening?
chrislawson: Immunity to diseases can be brought out in ONE generation. The Peloponnesian War tells of a plague, at the start of the big Athens-Sparta war, that killed off sizeable numbers of people at first, but then in a few years becoming — with no actual cure discovered, mind you — less deadly and less prevalent.
I just got out of class, and I’m happy to report that this paper caused some lively discussion. Some students were so bold as to call it “bullshit”! They noted that there was a distinct lack of good quantitative data presented, and part of it read like an advertisement for orthodontia.
Ah, good ol’ skepticism.
Raging Bee@8
But that would almost certainly involve rapid evolution of the bacterium or virus responsible for the plague. Not human evolution. It isn’t very successful if it rapidly kills its host. It is much more successful if the host lives a long life to infect as many other hosts as possible.
cates@7–
To be clear, I wasn’t disputing that change in jaw size is probably not driven by classic genetic evolution by adaptation (and if it contributes, it is probably a small factor not the driving force). I was criticising two of the arguments used in the paper. Also, there doesn’t have to be massive culling for rapid evolutionary change (although it helps). If there is a genetic/evolutionary component to smaller jaw sizes and fewer molars, it is quite likely to be driven, like the malaria example, by admixing in populations so that even a mild selection pressure can cause rapid change because it doesn’t rely on small numbers of beneficial de novo mutations in individuals which then spread through the population by descent.
Interestingly, on looking into this subject I found paper after paper asserting that jaw size is shrinking and that third molar agenesis is becoming more common…but the only references I could find to original papers on this were to studies in very ancient skulls such as Cro-Magnons. Many of the references were to differences between extant ethnic groups and populations with no time element, but for some reason these were quoted as part of the evidence of recent historical change. Has anyone found any papers directly testing historical rates of jaw size and/or molar agenesis over the last few centuries?
Raging Bee@8–
That’s a really interesting example. Thanks for bringing it to my attention. However, it’s hard to use as evidence for or against rapid evolution in humans because (i) as mallefowl says, it might have been evolution in the pathogen instead, (ii) immunity is more complex than most phenotypic traits, for example breastfed infants are helped by antibodies in the mother’s milk, so the generation after an epidemic gets some protection without any genetic change, (iii) we don’t know what really happened in ~400 BCE, quite possibly it was two separate epidemics with the first one being more severe. (As an example, the pathogen responsible for the far more recent Plague of Justinian was only recently identified as Yersinia pestis, and even so a lot of evidence doesn’t quite add up.)
Sounds like fun. Too bad I entered the IT field rather than biology. However, I doubt I could handle the labs. Oh, well.
Decades ago, a dentist told me that when large-jawed Nordics had offspring with lighter-boned Mediterraneans, said sprats could inherit teeth from one side and jawbones from the other, causing inevitable mismatch and gainful employment for dentists. I don’t recall how he reconciled that insight with my Irish-all-the-way ancestry and “wisdom” (late-adolescence) teeth woes.
Another dentist, along with various readings, taught me better, but the story seems apropos here.
I don’t recall much mention of dental woes in anything I’ve read about our primate cousins; if our branch of the order has exceptional difficulties in this area, the Intelligent Designer has some ‘splaining to do!
Reptiles and birds have a clever fix, let a muscular sack above the stomach grind the foo; that way you don’t need complex dentition.
If you want to stick to traditional morphology, do a “jaws” as in the Bond films. Great for melee fighting.
If we go to an all-insect diet, a long tongue could be an alternative to having many teeth…
Teeth. If we were intelligentkly designed wouldn’t they grow back when needed – and dittoknee cartiledge? FFS. Grr.. F this human body..
@ 17. birgerjohansson : Diitto a nectivorous diet too. Sweet!