from   http://www.beyondveg.com/billings-t/comp-anat/comp-anat-6b.shtml

  ... dentition of our ancestors decreased from Australopithecus to
  Homo erectus, coincident with the development of stone tools and
  increasing consumption of meat (hence decreasing consumption of
  coarse vegetable foods)...

  ...The cooking of food (meat) in earth ovens can be
  dated back over 200,000 years ago...

  Meat cooked in such a fashion can become quite tender indeed,
  and in such condition it requires less chewing to render it
  swallowable than would be the case if it remained uncooked.
  In turn, this should represent the relaxation of selection for
  maintaining teeth at the size level that can be seen throughout
  the Middle Pleistocene. The appearance of the earth oven in the
  archaeological record, then, should mark the time at which the
  dental reduction manifest in the Late Pleistocene had its beginning.

Philip Thrift
http://www.paleofitness.com