Peter Ungar: It’s a uniquely modern problem because we don’t grow our jaws long enough to accommodate our teeth. It turns out that nature has selected our jaw length on the basis of what it expects us to be doing during the period of time the jaw is growing. The more frequently you put force on the jaw, the longer the jaw grows. Nature has to guesstimate how long your jaw should be for teeth of a given size. Today we don't achieve that because we’re eating mush as kids.

Zhang: Does that mean we’re going to see Paleo diets where you chew on tough grass, or Paleo jaw workouts?

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Ungar: You know it’s funny you say that because there are orthodontists that are starting to use treatments that actually lengthen the jaw. The traditional treatment is to shave down or take out teeth to fit everything in properly. And now there are orthodontists that are starting to put spacers to lengthen your jaw. They’re taking a lesson from what you might call evolutionary dentistry.

Zhang: What about cavities? You sometimes hear that the invention of agriculture is tied to a big increased in cavities.

Ungar: The conventional wisdom in bioarchaeology is that the onset of agriculture and the increase in carbohydrate consumption led to more cavities. These carbohydrates—and especially later on when we hit the Industrial Revolution and the wide availability of refined sugars—provide a bed to which the bacteria that cause cavities can stick. They provide food for the bacteria as well. These microorganisms they eat, then they reproduce, and they poop. And it’s that poop basically that is acidic and erodes the teeth and causes cavities.

There’s certainly some evidence in a lot of the world where that’s the case, particularly the New World where people started to consume a lot of maize, a lot of corn. The cavities rate went way up with corn consumption. But we really don’t see it that much in, say, the Middle East where people started to eat wheat and barley. And even less in Far East where people started to consume rice. That leaves a complicating factor.

Zhang: You suggest that teeth have evolved to wear down over time and could even become more efficient at chewing as they’re worn down. I think my dentist would tell me wearing down my teeth is bad. So how does that work?

Ungar: I’m not necessarily sure human teeth per se are evolved to wear down. But teeth are evolved to wear in a way that keeps or makes them functionally efficient. Because if you look at prehistoric people or people today still living a traditional foraging lifestyle, like the Hadza, the group I study in Tanzania, their teeth are worn. They tend to be much more worn than our teeth. Evolution isn’t going to say oh well, we’re just going to choose shape are unworn when we’re trying to make a traditional biting machine. Nature is going to select for teeth that wear in a way that keeps them efficient.