A couple of million years ago or so, our hominid ancestors began exchanging their lowbrow looks for forehead prominence. The trigger for the large, calorie-hungry brains of ours is cooking, argues Richard W. Wrangham, the Ruth B. Moore Professor of Biological Anthropology at Harvard University's Peabody Museum of Archaeology and Ethnology. He hit on his theory after decades of study of our closest cousin, the chimpanzee. For the Insights story "Cooking Up Bigger Brains," appearing in the January 2008 Scientific American, Rachael Moeller Gorman talked with Wrangham about chimps, food, fire, human evolution and the evidence for his controversial theory. Here is an expanded interview.

You have been the director of the Kibale Chimpanzee Project in western Uganda since 1987. Have chimpanzees always been a big interest of yours?

I have always been interested in nature. I began as a bird-watcher and then wanted to go to wild places. I had a gap year between high school and college in Zambia, and that set off an interest in behavioral ecology—I was an assistant to a biologist there working for the game department. It was an amazing place, miles and miles of bush with all sorts of animals.

Did you study chimpanzees there?

No, I wasn't focused on primates at this point. But then I came to college at the University of Oxford, and on my first day I went to the expeditions club to see if there were any opportunities to work in Africa again. By the time I left college, I'd already had quite a bit of experience of Africa. I'd become really interested in thinking about animals as a way to get at the evolution of human social systems: If there are similarities between humans and animals, then let's find out where they come from. I wrote to Jane Goodall in July of 1970 [to ask to work with her], and in November I was in Gombe.

What was it about Africa that excited you—the adventure? What kept drawing you back to look at these animals?

I think the natural history is tremendously exciting and rich. And I think that, even then, I had a sense time is running out, things are changing and, when it's possible, it's necessary to explore all of these fascinating animals and ecosystems. But the sense of freedom and adventure was palpable, of course, as well.

When you first went to work with Goodall, what kind of research did she have you conduct?

She gave me the opportunity to spend a year following four pairs of chimp siblings. It was a wonderful time because I was free to develop my own thoughts and interests about chimps. It's embarrassing in some ways, but my subsequent career has all been playing out the thoughts that I developed then. It was looking at the ways in which ecological pressures affect chimpanzee society. It's very obvious with chimps, because in different seasons you get different distributions of food, and the chimps respond in very marked ways. That is a window into the larger question of the relationship between ecological pressures and social systems and how they vary between species.

It must be exciting to be among the first to really understand the social behavior of chimps.

Yes, that's right—it's fabulous! It's interesting with any animal, because every species has its strange twist, but it's particularly dramatic with chimps. The reason that it's dramatic, actually, only became apparent later with the genetic data that says how closely related we are to chimps. In the 1970s, when we were discovering that chimpanzees have all these amazing similarities to humans, what Jane discovered was that they really like to eat meat, they use tools, they make tools, they have relationships between mothers and offspring that in many ways recall what's going on in humans. There are all sorts of things—cultural transmission of a wide range of behaviors. It gives a general sense that there was more biology to human behavior than we had formerly given credit for.

Your theory that cooking spurred the evolution of modern humans occurred to you while you were sitting in front of your own fireplace?

Yes, about 10 years ago, right after the start of the academic term, I was thinking about what stimulated human evolution. The fire just started drawing me in to the comparison with chimps, because I tend to think about human evolution through the lens of chimps: What would it take to convert a chimpanzeelike ancestor into a human? And as I thought about how long we have had fire, I realized what a ridiculously large difference cooking would make. It's a very simple thought; anyone who had ever taken an anthropology course should have had it long before this.

Just how would cooking make a difference? What's wrong with raw food that chimps eat?

I know chimpanzee foods fairly intimately, I've tasted the great majority of the things I've seen them eat, and I know what a huge difference there is between a chimpanzee diet and the human diet, because we cook. And that set me off thinking about whether or not humans really could ever survive on a raw diet. And my instant assumption was no, because of my experience with chimpanzee diets, which said to me we couldn't possibly do this—so that raises all these fascinating evolutionary questions. I'd had the experience of seeing a close relative eating all those foods and seeing how unpleasant they are and how difficult it would be for humans to survive on a diet like that. Maybe people assume that the kinds of places in which humans live would have apples and bananas dripping off the trees, but it's not like that.

What are the foods like, then?

The typical fruit is very unpleasant, very fibrous, quite bitter; the net effect is that you would not want to eat more than two or three of them before running for a big glass of water and saying, "That was not a pleasant experiment, I hope I don't get sick." They're not nice to eat. Not a tremendous amount of sugar in them. So there were very few fruits that I've tasted that I can actually imagine getting a stomach for because most of them are unpleasant to eat. Some make your stomach heave.

But maybe if we—or ancient humans—were accustomed to them, we would be able to eat them.

I recognize that I've got a palate softened by ease, and it may well be that if you're hungry in the bush you might be prepared to eat a lot of these nasty tasting things, but I've worked with Pygmies in eastern Congo in a forest where I knew a fair number of fruits were being eaten by chimps, and I would ask them, and they'd say there's no way I would ever eat this stuff. Chimpanzees eat, on average, 60 percent of their food as fruit. Humans couldn't do that. So one of the fascinating things for me as I ventured into this was really learning about what hunters and gatherers eat—and it turns out that there are no records of people having a large amount of their food come from raw food. Everywhere, everyone expects a cooked meal every evening.

What about the way our bodies are set up to digest food—besides not liking the taste, can we digest the foods chimps eat?

I think we can probably digest them—this is guesswork because we don't really know—but the point is they're very full of indigestible fiber. So the average human diet has, even in the more fibrous hunter-gatherer types, 5 to 10 percent, say, indigestible fiber. With our chimp studies, they eat 32 percent indigestible fiber. So that is something that the human body is not designed to handle. And the reason we can say that is that we have small colons and small stomachs which are adapted to food that has high caloric density. And food the chimps eat has low caloric density.

When you looked at the archaeological evidence, what were the clues that indicated to you that fire spurred the development of Homo erectus?

The archaeology of fire is historically a confusing area because people have derived stronger claims than, I think, they should have done. What they did was say that we see quite a lot of evidence of fire back to a certain time, and then we see much less evidence; so let's assume that fire started at that break point. And I think the way they should have read it is we know that fire has been used some time in the past, but we don't see any clear cutoff, so we can't draw any conclusions.

Is it possible to narrow down the time when humans first used fire?

Some people say that fire began 40,000 years ago, some people say 200,000, some people say 300,000, some say 400,000, some say 500,000—it's all over the map. There are quite a few sites back to 1.6 million years ago where the people who excavated the sites say, "Well, I've got some evidence of fire back here." And other people say, "Well, you might have it but it's not good enough to convince me." So to me, the way to look at the archaeology of fire evidence is simply to say the archaeology doesn't tell you anything. All it tells you is it is possible that fire was controlled back to 1.6 million years ago.

And you believe cooking with that fire spurred the development of modern humans.

Here's the way I tend to ask the question: I tend to think of the advent of cooking as having a huge impact on the quality of the diet. In fact, I can't think of any increase in the quality of diet in the history of life that is bigger. And repeatedly we have evidence in biology of increases in dietary quality affecting bodies. The food was softer, easier to eat, with a higher density of calories—so this led to smaller guts, and, since the food was providing more energy, we see more evidence of energy use by the body. There's only one time it could have happened on that basis; that is, with the evolution of Homo erectus somewhere between 1.6 [million] and 1.8 million years ago.

What exactly is it about Homo erectus that fits these criteria so much better than earlier or later human ancestors?

Homo erectus is the species that has the biggest drop in tooth size in human evolution, from the previous species, which in that case was Homo habilis. There wasn't any drop in tooth size as large as that at any later point in human evolution. We don't know exactly about the gut, but the normal argument is that if you reconstruct the ribs, you have reduced flaring of the ribs. Up until this point you have ribs that went out to apparently hold a big belly, which is what chimps and gorillas are like, and then at this point [when Homo erectus arose] the ribs go flat, meaning you've got now a flatter belly and, therefore, smaller guts. And then you have more energy being used; people interpret the locomotor skeleton as meaning that the distances traveled every day are much farther. And the brain has one of its larger rises in size.

Smaller guts and bigger brains resulted from extra calories, then. So it is possible that our ancestors simply found richer foods?

There's this lovely theory by Leslie Aiello [president of the Wenner-Gren Foundation for Anthropological Research] and Peter Wheeler [at Liverpool John Moores University in England] saying that larger brains are made possible in primates by smaller guts. And they previously argued that guts were getting smaller at that time, but they said it was because of meat eating. I'm suggesting that this was instead because of cooking, partly because there's no other time that satisfies the expectations that we would have for changes in the body that would be accompanied by cooking.

There are people who believe that just a switch to eating meat caused these changes, even though a million years had lapsed between the adoption of meat-eating and the evolution of Homo erectus?

Yes, one or two people have written articles saying this doesn't make sense! There is some diversity of opinion, and I find it helpful that there are people who say the old story is too simple.

Do most people adhere to the meat theory, or are there other, more popular theories?

There's an amazing lack of theories, actually. I mean, this is human origins, and there's so much willingness to go with a rather well established, yet not very deeply thought-out, idea. One of the things that amazed me was the difficulty of eating raw meat. Raw meat is not that attractive, particularly the kind of meat you cut off an animal that has been living under stressful conditions in the African savanna: tough, antelope mostly, hippos and rhinos. And I've tried chewing raw meat. It probably wouldn't take them long, though, to realize you could pound the meat. To pound the meat they would have gotten more energy out of it.

Is cooking meat better than pounding it to increase digestibility?

I've turned up some studies in the literature that have not been interpreted the way I've been interpreting them, which show that digestibility of animal protein increases when it is cooked. And that's because it's denatured—the protein is unfolded. It's normally packed solid, with the hydrophobic groups on the inside, the hydrophilic groups on the outside. Denaturation is the process of it opening out. And once it opens out, the proteolytic enzymes can now go in and start snipping. Heat predictably causes denaturation, so I think one of the major effects of cooking is to denature proteins, opening them up to the point where proteolytic enzymes have easier access.

What additional studies would lend support to your theory?

It would be very interesting to compare the human and Homo erectus genetics data to see when certain characteristics arose, such as, when did humans evolve defenses against Maillard reaction products?