All I want is my two front teeth:
From CBC Radio's "Quirks and Quarks" podcast:
Dr. Joy Richmond, Professor of Pediatric Dentistry at UBC, just mentioned that many of the present dental problems are actually caused by our highly processed food.
In the past (and as recently as the Dark Ages), our food was quite coarse, containing a lot of grit and abrasive material that ground our teeth down. This wear allowed our teeth to fit better in our jaws. That wear is no longer a factor, and our teeth retain their points much longer than is natural, which causes much more force to torque the teeth each time we chew (or grind our teeth), and it is this that causes much of the crookedness and crowding in modern teeth.
This is visible when we look at the statistics of dental problems in ancient skulls - in general, older skulls have fewer dental problems than modern ones (although of course, if you did have problems, they were probably pretty agonizing, since there was little that could be done about it).