With our supersized brains and shrunken teeth and guts, we humans are bizarre primates. Richard Wrangham of Harvard University has long argued that these and other peculiar traits of our kind arose as humans turned to cooking to improve food quality—making it softer and easier to digest and thus a richer source of energy. Humans, unlike any other animal, cannot survive on raw food in the wild, he observes. “We need to have our food cooked.”
Based on the anatomy of our fossil forebears, Wrangham thinks that Homo erectus had mastered cooking with fire by 1.8 million years ago. Critics have countered that he lacks evidence to support the claim that cooking enhances digestibility and that the oldest known traces of fire are nowhere near as old as his hypothesis predicts. New findings, Wrangham says, lend support to his ideas.
Scientific American: How did you come up with the cooking hypothesis?
Wrangham: I think of two strands. One is that I was trying to figure out what was responsible for the evolution of the human body form, and I was sensitive to the fact that humans everywhere use fire. I started thinking about how long ago you would have to go back before humans did not use fire. And that suggested to me the hypothesis that they always used it because they would not have survived without it. Humans as a genus [Homo] are committed to sleeping on the ground. I do not want to sleep on the ground in Africa without fire to keep the wild animals at bay.
The other strand is that I've studied chimpanzees and their feeding behavior for many years. I've eaten everything that I can get ahold of that chimpanzees eat. And I have been very much aware of the deeply unsatisfying nature of those foods because they are often quite fibrous, relatively dry, and contain little sugar, and they are often strong-tasting—in other words, really nasty. So here we are, two very closely related species with completely different dietary habits. It was an obvious hypothesis that cooking does something special for the food we find in nature. But I was astonished to discover that there was no systematic evidence showing what cooking does to the net energetic gain that we get from our foods.
For the past 14 years I've been focused on that question because to make a satisfactory claim about humans being adapted to cooked food, we have to produce some real evidence about what cooking does to food. Experiments conducted by Rachel N. Carmody of Harvard University have now given us the evidence: if we cook, we get more energy from our food.
Other researchers hold that increased access to meat allowed the teeth and gut to shrink. Why do you think cooking better explains these changes?
It's quite clear that humans began eating meat from large animals by 2.5 million years ago and have left a steady record of cut marks on bones since then. The cooking hypothesis does not deny the importance of meat eating. But there is a core difficulty with attributing changes in digestive anatomy to this shift.
Selection pressure on digestive anatomy is strongest when food is scarce. Under such conditions, animals have very little fat on them, and fat-poor meat is a very poor food because if you have more than about 30 percent protein in your diet, then your ability to get rid of ammonia fast enough is overwhelmed. Nowadays in surveys of hunters and gatherers, what you find is that during periods of food scarcity, there is always a substantial inclusion of plants. Very often it's tubers. To eat those raw, you would have to have the digestive apparatus to handle tough, fibrous, low-carbohydrate plant foods—that is, large teeth and a big gut.
So your idea is that by cooking those plant foods, our ancestors could evolve a smaller gut and teeth—and avoid overdosing on lean meat. Let's turn now to what happened when food was not so scarce and animals were good to eat. You have argued that cooking may have helped early humans eat more meat by freeing them up to hunt. What is your logic?