"The origins of the genus Homo are murky, but by H. erectus, bigger brains and bodies had evolved that, along with larger foraging ranges, would have increased the daily energetic requirements of hominins1, 2. Yet H. erectus differs from earlier hominins in having relatively smaller teeth, reduced chewing muscles, weaker maximum bite force capabilities, and a relatively smaller gut3, 4, 5. This paradoxical combination of increased energy demands along with decreased masticatory and digestive capacities is hypothesized to have been made possible by adding meat to the diet6, 7, 8, by mechanically processing food using stone tools7, 9, 10, or by cooking11, 12. Cooking, however, was apparently uncommon until 500,000 years ago13, 14, and the effects of carnivory and Palaeolithic processing techniques on mastication are unknown. Here we report experiments that tested how Lower Palaeolithic processing technologies affect chewing force production and efficacy in humans consuming meat and underground storage organs (USOs)....
"...Although cooking has important benefits, it appears that selection for smaller masticatory features in Homo would have been initially made possible by the combination of using stone tools and eating meat."
Lieberman's previously published research showing that soft food causes deformations of the jaw that's similar to what we see in modern humans who require teeth to be pulled and braces.
Here's some color from the NYT:
"...Other patrons got three courses of meat (goat, in this case). Dr. Zink grilled the meat in the first course, but offered it raw and sliced in the second. In the third course, her volunteers received an uncooked lump of goat flesh.
"In some of the trials, the volunteers chewed the food until it was ready to swallow and then spat it out. Dr. Zink painstakingly picked apart those food bits and measured their size.
"“If that was all my dissertation was, I would have quit graduate school,” Dr. Zink said. “It was as lovely as it sounds.”...
"...As long as 3.5 million years ago, scientists have found, hominins were making stone tools. Cut marks on mammal bones suggest that the tools were used to carve meat from carcasses."
P.S. Here's a short (60-Second Science!) interview with Prof. Lieberman.