In Catching Fire: How Cooking Made Us Human, [Richard Wrangham] proposes that the big breakthrough of almost 2 million years ago that generated another 1,000 ideas and changed who we are forever was this: Drop food in fire, eat it. We are because we cook.
. . . The assumption is that once we became modern, we worked out how to cook. Wrangham, by contrast, thinks we were cooking 1.8 million years ago—and that the activity was not an outcome of being human but that being human was an outcome of cooking. Cooking physically transformed a creature that was more ape into the earliest version of us, Homo erectus (perhaps more Conan the Barbarian than Jamie Oliver but still fundamentally human).
. . . Apparently, the idea that cooking was the crucial difference between their diet and ours came to Wrangham as he stared into the fire at home. Though there's no archeological evidence of controlled fire before 800,000 years ago, he realized that a cluster of changes in the human face, brain, and gut 1.8 million years ago could be explained by only one thing—regular cooked meals.
His argument begins with the odd spend-money-to-make-money aspect of digestion: You must burn calories in order to release calories from food (a fact deeply cherished by celery-chewing teenage girls). Because raw food is harder to digest, it takes more calories to get the calories out of it, and you get fewer calories from it anyway.
So I guess human beings looked pretty gaunt way back when.