A primate the size of an early human would be expected to spend about half of its day chewing, as chimpanzees do. Modern humans spend less than an hour a day, whether you're American or living in various subsistence societies around the world. So you've got four or five hours a day freed by the fact that you're eating relatively soft food. In hunter-gatherer life, men tend to spend this time hunting.
That observation raises the question of how much hunting was possible until our ancestors were able to reduce the amount of time they chewed. Chimpanzees like to eat meat, but their average hunt is just 20 minutes, after which they go back to eating fruit. Hunting is risky. If you fail, then you need to be able to eat your ordinary food. If you hunt too long without success, you won't have enough time to process your usual, lower-quality fare. It seems to me that it was only after cooking enabled individuals to save time on chewing that they could increase the amount of time spent on an activity that, for all its potential benefits, might not yield any food.
You have also suggested that cooking allowed the brain to expand. How would cooking do that?
With regard to the brain, fossils indicate a fairly steady increase in cranial capacity, starting shortly before two million years ago. There are lots of ideas about why selection favored larger brains, but the question of how our ancestors could afford them has been a puzzle. The problem is that brains use a disproportionate amount of energy and can never be turned off.
I have extended the idea put forward by Leslie C. Aiello, now at the Wenner-Gren Foundation in New York City, and Peter Wheeler of Liverpool John Moores University in England that after cooking became obligatory, the increase in food quality contributed to reduced gut size. Their newly small guts were energetically cheaper, allowing calories to be diverted to the brain.
In 2012 Karina Fonseca-Azevedo and Suzana Herculano-Houzel of the Federal University of Rio de Janeiro added a new wrinkle. Their calculations showed that on a raw diet, the number of calories needed to support a human-sized brain would require too many hours eating every day. They argued that cooking allowed our ancestors the extra energy needed to support more neurons, allowing the increase in brain size.
Cooking is not the only way to make food easier to digest. How does it compare with other methods?
Simply reducing the size of food particles and the structural integrity of food—through pounding, for example—makes it easier to digest. Carmody did a study that looked at tubers and meat as representative types of food that hunter-gatherers eat and asked how well mice fared when eating each of these foods, either raw versus cooked or whole versus pounded. She very carefully controlled the amount of food that the mice received, along with the amount of energy they expended moving around, and assessed their net energetic gain through looking at body-mass changes. She found that pounding had relatively little effect, whereas cooking led to significant increases in body weight whether the food was tubers or meat.
This is incredibly exciting because, amazingly, this is the first study that has ever been done to show that animals get more net energy out of their food when it is cooked than when it's raw. Second, it showed that even if pounding has some positive effects on energy gain, cooking has much bigger effects. [Editors' note: Wrangham was a co-author on the study, published in 2011.]
Is there any genetic evidence to support the cooking hypothesis?
There is essentially nothing published yet. But we're very aware that a really interesting question is going to be whether or not we can detect, in the human genome, evidence of selection for genes related to utilizing cooked food. They might be concerned with metabolism. They might be concerned with the immune system. They might be concerned somehow with responses to Maillard compounds, which are somewhat dangerous compounds produced by cooking. This is going to be a very exciting area in the future.