fire made eating animal protein much more efficient. imagine the patience of eating raw deer off the bone - hours of chewing. With fire, it converted a monotonous task to something efficient
"In essence, cooking—including not only heat but also mechanical processes such as chopping and grinding—outsources some of the body’s work of digestion so that more energy is extracted from food and less expended in processing it. Cooking breaks down collagen, the connective tissue in meat, and softens the cell walls of plants to release their stores of starch and fat. The calories to fuel the bigger brains of successive species of hominids came at the expense of the energy-intensive tissue in the gut, which was shrinking at the same time—you can actually see how the barrel-shaped trunk of the apes morphed into the comparatively narrow-waisted Homo sapiens. Cooking freed up time, as well; the great apes spend four to seven hours a day just chewing, not an activity that prioritizes the intellect."
Cooking made a big difference for vegetables because heat quickly breaks down some hard to digest fiber.