Soft foods cause crooked teeth. Ancestral humans ate raw tubers and other very chewy things that exercised their jaws, and the growing human body needs this resistance/exercise. If you look at modern hunter-gatherer tribes with traditional diets, they have a far lower rate of crooked teeth we'd solve with braces than the developed world.

Sterile environments/too-low pathogenic load causes allergies. The immune system expects a base-rate of infection; if it doesn't see that, it upregulates. Allergies are a first-world disease; they're far less prevalent in less developed countries, and you even see the effect in immigrants- their allergy rates go up after a few years of living in a developed country.

> Or do you seriously believe all prehistoric humans had 20/20 eyesight?

The prehistoric realities were harsh and nearsighted individuals had little chances of survival and procreation (natural selection). In the modern society perfect eye-sight is not as important for survival as in the prehistoric society, thus more tolerance to its greater variability.

Crooked teeth really didn't exist in the farther out past. If you look at the jawbones of Neolithic people, they all have perfectly aligned teeth.

The theory is that we're not chewing hard enough food, which leads to insufficient jawbone growth as a kid, leading to not enough space for your teeth.

If they killed that many people, how did they escape natural selection?

Only Americans are afraid of wisdom teeth. 90 percent of the rest of the world keep them with no problems whatsoever.

(Something something medical cartel...)