Every gram of brain uses up more energy than every gram of body. And bigger brains, which have more neurons, consume more fuel. On their typical diets of raw foods, great apes can’t afford to fuel more neurons than they already have. To do so, they would need to spend an implausible amount of time on foraging and feeding. An ape can’t evolve a brain as big as a human, while still eating like an ape. Their energy budget simply wouldn’t balance.
Our ancestors overcame this constraint when they learned how to cook. Cooked food offers more calories than raw food, and is easier to chew and digest. These early chefs could gain more energy from the same amount of eating time. That, in turn, fuelled more neurons and larger brains.