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.
The strengthening of the jaw during childhood leads to a larger jaw, and more room for teeth. Our jaws are too small. "Straightening" (like I said earlier) isn't exactly correct.
