People living in "subsistence farming" communities around the world have shorter, wider jaws than those in hunting and gathering societies. This leaves less room for teeth, which have changed little in size or abundance over human history.
Changes in jawbone size arise due to "phenotypic plasticity", wherein environmental differences lead to anatomical changes. Longer jaws are advantageous for more intense chewing, whereas shorter jaws are more efficient for consuming agricultural fare like wheat, corn, or rice, which is relatively soft and consistent compared to hunter-gatherer grub.
Note: Children who chew tougher foods tend to have straighter teeth when they grow up.