The famed Australian orthodontist “Tick” Begg recognized this mismatch back in the 1920s. He found that Aboriginal peoples living traditional lifestyles wore their teeth down more than his dental patients of European ancestry did. They also had perfect dental arches—their front teeth were straight, and their wisdom teeth were fully erupted and functioning. Begg reasoned that nature expects wear between adjacent teeth to reduce space requirements in the mouth.
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Begg was right about the mismatch between teeth and jaws, but he got the details wrong. According to anthropologist Rob Corruccini of Southern Illinois University, the key change was not to the abrasive environment but to the stress environment, meaning the mechanical stresses jaws experience during eating. And the teeth were not too big—the jaw was too small.
Remarkably, Charles Darwin made the connection between stress and jaw size in his 1871 book The Descent of Man. But Corruccini was among the first to offer definitive evidence. He had just started teaching at Southern Illinois when a student from nearby rural Kentucky told him that in his community seniors were raised on hard-to-chew foods, whereas their children and grandchildren had more refined, processed diets. Follow-up study showed that older residents had better bites, despite almost no professional dental care, than younger ones did. Corruccini explained the difference in terms of dietary consistency. Thus, the dental differences were not genetic but environmental. Corruccini went on to find many other examples, including the Pima of Arizona before and after they had access to store-bought foods and rural peoples near Chandigarh, India, who had diets of coarse millet and tough vegetables as compared with urban dwellers, who ate soft bread and mashed lentils.
Corruccini reasoned that tooth size is preprogrammed to fit a jaw subjected during growth to levels of mechanical stress in line with a natural childhood diet. Subsequently, when the jaw does not get the needed stimulation during development, the teeth become crowded at the front end and impacted in the rear. He confirmed this hypothesis with experimental work on monkeys evincing that those fed softer diets had smaller jaws and impacted teeth.