The answer in brief: our modern diet. Quoting from the article:
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.
Also on the list, to some extent:
- flat feet
- back problems
- forward head
- digestive problems due to posture
And surely many more.