Richard Wrangham, a primatologist and anthropologist, has spent four decades observing wild chimpanzees in Africa to see what their behavior might tell us about prehistoric humans. Dr. Wrangham, 60, was born in Britain and since 1989 has been at Harvard, where he is a professor of biological anthropology. His book, “Catching Fire: How Cooking Made Us Human,” will be published in late May. He was interviewed over a vegetarian lunch at last winter’s American Association for the Advancement of Science meeting in Chicago and again later by telephone. An edited version of the two conversations follows.

Q. In your new book, you suggest that cooking was what facilitated our evolution from ape to human. Until now scientists have theorized that tool making and meat eating set the conditions for the ascent of man. Why do you argue that cooking was the main factor?

A. All that you mention were drivers of the evolution of our species. However, our large brain and the shape of our bodies are the product of a rich diet that was only available to us after we began cooking our foods. It was cooking that provided our bodies with more energy than we’d previously obtained as foraging animals eating raw food.

I have followed wild chimpanzees and studied what, and how, they eat. Modern chimps are likely to take the same kinds of foods as our early ancestors. In the wild, they’ll be lucky to find a fruit as delicious as a raspberry. More often they locate a patch of fruits as dry and strong-tasting as rose hips, which they’ll masticate for a full hour. Chimps spend most of their day finding and chewing extremely fibrous foods. Their diet is very unsatisfying to humans. But once our ancestors began eating cooked foods  approximately 1.8 million years ago  their diet became softer, safer and far more nutritious.