Not proof, but logic: Without machinery to do the work of processing food, it has to done by hand or not at all, which leads to a lot more jaw work. Especially if you're eating a lot of animals where you'll be eating off of bones. But even with plants too.

I think the theory is that modern jaws are too narrow to contain all the teeth, causing them to overlap and grow in "unintended" directions. The reason for the narrowness is lack of exercise in childhood due to the soft nature of prepared foods. I'm also a bit sceptical.