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