According to Weston A. Price's seminal 1939 book based on decades of research travelling the world to examine people's dental health, the short answer is: diet.
Price was a dentist who examined the teeth, jaw and facial bone structures of both indigenous and "modernized" populations around the world. His findings were extremely consistent among diverse populations. Those indigenous people with diets rich in meat and animal fat had superior straight, white, healthy teeth and superior dental arches, jaws and facial development.
Populations of those same ethnic groups who had been at least somewhat integrated into modern civilization and exposed to what Price called a "Western diet" for a single generation saw massive physical degeneration, poor teeth, poor facial structure, and generally poorer health.
Across these populations, the three ingredients common to "Western diet" foods which Price linked to dental degeneration were flour, sugar, and vegetable oil.
Sugar was not readily available outside of the upper classes prior to advances in agricultural processes in the 17th and 18th centuries, and then mainly in wealthy, Western countries. Vegetable oil was developed much later in the US during the early 1900s.
By the time price conducted his studies, sugar and vegetable oils were readily available in shelf-stable processed foods consumed in modern cities around the world, but plenty of indigenous populations in nearby rural areas maintained mostly subsistence diets based on hunting and livestock agriculture.
This provided an ideal environment to compare the differences between otherwise very similar populations.
Summary of major findings: