Every animal on earth is constrained by its energy budget; the calories obtained from food will stretch only so far. And for most human beings, most of the time, these calories are burned not at the gym, but invisibly, in powering the heart, the digestive system and especially the brain, in the silent work of moving molecules around within and among its 100 billion cells. A human body at rest devotes roughly one-fifth of its energy to the brain, regardless of whether it is thinking anything useful, or even thinking at all. Thus, the unprecedented increase in brain size that hominids embarked on around 1.8 million years ago had to be paid for with added calories either taken in or diverted from some other function in the body. Many anthropologists think the key breakthrough was adding meat to the diet. But Wrangham and his Harvard colleague Rachel Carmody think that’s only a part of what was going on in evolution at the time. What matters, they say, is not just how many calories you can put into your mouth, but what happens to the food once it gets there. How much useful energy does it provide, after subtracting the calories spent in chewing, swallowing and digesting? The real breakthrough, they argue, was cooking.

Wrangham, who is in his mid-60s, with an unlined face and a modest demeanor, has a fine pedigree as a primatologist, having studied chimpanzees with Jane Goodall at Gombe Stream National Park. In pursuing his research on primate nutrition he has sampled what wild monkeys and chimpanzees eat, and he finds it, by and large, repellent. The fruit of the Warburgia tree has a “hot taste” that “renders even a single fruit impossibly unpleasant for humans to ingest,” he writes from bitter experience. “But chimpanzees can eat a pile of these fruits and look eagerly for more.” Although he avoids red meat ordinarily, he ate raw goat to prove a theory that chimps combine meat with tree leaves in their mouths to facilitate chewing and swallowing. The leaves, he found, provide traction for the teeth on the slippery, rubbery surface of raw muscle.