"I tend to think of the advent of cooking as having a huge impact on the quality of the diet. In fact, I can't think of any increase in the quality of diet in the history of life that is bigger. And repeatedly we have evidence in biology of increases in dietary quality affecting bodies. The food was softer, easier to eat, with a higher density of calories—so this led to smaller guts, and, since the food was providing more energy, we see more evidence of energy use by the body. There's only one time it could have happened on that basis; that is, with the evolution of Homo erectus somewhere between 1.6 [million] and 1.8 million years ago... Homo erectus is the species that has the biggest drop in tooth size in human evolution, from the previous species, which in that case was Homo habilis. There wasn't any drop in tooth size as large as that at any later point in human evolution. We don't know exactly about the gut, but the normal argument is that if you reconstruct the ribs, you have reduced flaring of the ribs. Up until this point you have ribs that went out to apparently hold a big belly, which is what chimps and gorillas are like, and then at this point [when Homo erectus arose] the ribs go flat, meaning you've got now a flatter belly and, therefore, smaller guts. And then you have more energy being used; people interpret the locomotor skeleton as meaning that the distances traveled every day are much farther. And the brain has one of its larger rises in size." (Scientific American: A Q&A with Richard Wrangham)
