Comment by keithwhor | Hacker News Reader

Comment by keithwhor | Hacker News Reader

Isn't an alternative explanation just that well-aligned jaws and spaced teeth are selected for via both (A) natural selection, esp. in predator species (well-aligned jaw = more efficient prey capture, chewing, defense) and (B) sexual selection as a holdover result of the first? As in, if ancestors relied heavily on their jaws and, behaviourally, a selective preference for well-aligned jaws and teeth helped favor offspring with the same, that selective behavior itself would be selected for -- and could perhaps persist even after "well-aligned jaws" were no longer a deciding factor in objective fitness?

So now there's significantly less actual natural selective pressure on well-aligned jaws / teeth in human populations, meaning phenotypic variability in human jaw / teeth is simply the result of genetic drift in a very large population. The holdover, of course, being there's a market for and industry around striving for perfect jaws and smiles in Hollywood because we have a behavioural (but no longer naturally selective) predisposition towards that signal of health and fitness.

... that's always sort of been my mental model of orthodontia as an industry, at least.

I think a stronger argument, along similar lines, would be to observe that the human skull underwent pretty drastic architectural changes in the last 200K years because of the rapid growth in size of the brain cavity. And, since the importance of teeth fell behind the importance of intelligence, not enough selective pressure existed to "fix" the teeth given the other radical skull changes.

Yep, this can mechanistically explain why the phenotypic variability exists, for sure.

In addition, let’s not forget the invention of cooked food. Soft cooked food no longer requires a massive jaw with jaw muscle attached to the top of the skull. Furthermore, the higher sugar content of the (post)agrarian diet creates a healthy environment for bacteria.

“Catching Fire: How Cooking Made us Human” has a lot to say on this subject.

The article addresses that with this: https://onlinelibrary.wiley.com/doi/abs/10.1002/ajpa.1330620...

Within one generation, the palate size decreased. So that can't be explained by natual selection. Well, unless small jaws killed a huge portion of the population before modern times.

I'd love to read it, but it's not Open Access and I'm not a member of any academic institution. Do you have a copy of the paper?

Edit: It's not that small jaws killed anybody, the hypothesis would be that in a pre-industrialized society sexual selective (behavioural) mechanisms favoured well-aligned jaws / teeth but with the rise of globalization stronger selective mechanisms superseded them. This would be mostly due to rapid real living standard + wealth accumulation gains more strongly correlated with other behaviours.

IIRC the orthotropics explanation for this is because we have much easier to chew foods, we chew much less and have poor tongue posture, leading to skinnier inter-molar width (IMW) and jawbones that are more down and in as opposed to up and out

There hasn't been nearly enough time for changes in selective pressure to develop through the population. Orthodontics as we know it was invented in the mid-18th century, and it wasn't affordable until the late 19th century. That's like, four or five generations to the present with access to medical tooth alignment.

I'm not sure what the point you're making is, can you clarify?

My point is orthodontia has a large market because, outside of actual pain and health symptoms associated with malocclusion, we have a predisposition towards well-aligned jaws and teeth.

I'm not making any assessment of the effects of orthodontia on long-term selective biases in human populations.

... but also, four or five generations is not insignificant. Evolution is just a change in allele frequency in a population over time, and five generations is more than enough time to objectively measure a change in frequency with such a large population sample. (I think the better argument would be that that the number of people that have had access to medical tooth alignment is a statistically insignificant fraction of the global population.)

One can also interpret it as that, once human learnt to use fire, cooked food made jaws and teeth less important, so people with bigger brain would reproduce more, no matter how their teeth look like.

There's also cultural difference in the aesthetics of teeth, for example, Japanese culture considers yaeba as cute.

Couple of evolutionary pressures that would make sense to me:

1) Human ancestors didn’t live as long, giving them less time for teeth to get misaligned in a terrible way.

2) Animals will tend to lose teeth for various reasons, so some extra teeth (including eg. wisdom teeth that appear during the prime of life) are probably useful spares more often than not. Not enough teeth was probably a more likely cause of death than too many.

Both of my parents had well-aligned jaws with teeth suited to them. Unfortunately for my elder brother and myself, we had our father's teeth in our mother's jaws. Crowding ensued, not helped by the fact that both of us have a couple of senility teeth (my neologism for our fifth molars).

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