We have the smallest guts of all primates, and the biggest brains. Blame cooking.

Question: How does cooking play a part in evolution? 

Richard Wrangham: Well, cooking is a huge influence on the availability of energy. And ultimately, cooking does much more than provide energy, but the first great thing it does, which has not amazingly been appreciated until very recently, is that cooking gives us more energy than eating our food raw. And when our ancestors first learned to cook, then the great huge initial impact would have been that they got so much more energy that they would have had more babies than they had before. And their babies were to survive better, and the adults were to survive better simply because they were able to eat more and have more regular menstrual cycles and put more energy in the immune system. So, the first big thing about cooking is, it gives us a huge increase, but we don't know yet how much. But a big increase in how much energy we get out of our food. 

When I was studying chimpanzees, I would normally take sandwiches with me, but there were days that, for one reason or another, I didn't, and because I was studying the feeding behavior of chimpanzees, I would regularly eat what they eat. If I eat everything that they eat that I could find. And on the days when I didn't have any prepared food with me, I would try to rely on what they eat. And you can eat it in the sense that you can chew it up and try to swallow it. Some of it is very strong tasting, which is kind of code word for really unpleasant, and some of it is okay. But it was not possible to find anything that I could fill my stomach with. 

Well, the short story is that I realized after a bit that I simply could not get enough energy out of a chimpanzee diet, and then I started thinking, well what would be the best place that you would get a raw diet in the wild? And there may be better places that the chimpanzee forest, I don't think there are many. And this sent me off looking for the difference between humans and other animals in terms of their ability to survive on raw food. And I very rapidly discovered that although there are some myths to go in the opposite direction, humans are different from other species because we have adapted biologically such that we cannot survive on raw food in the same way that other animals can. 

In some ways I shouldn't say we cannot survive, but we can't survive in the same way other animals can in this sense that whatever environment we are in, raw food is an unsatisfactory source. And the most dramatic example of this is that in the best study of the people who choose to live on raw food in modern urban environments, which is a great way to lose weight and can be very healthy and takes a lot of will, but is nevertheless has got many admirable aspects, but in the best study of people who do that, then first of all, a high proportion of people suffer energy shortage. They just are not getting enough energy to be able to maintain their bodies well. And the most dramatic point is that half of the women who eat all of their food raw are amenorrheic. That means that their ovarian system is closed down completely. Now, this is despite the fact that they are under ideal conditions. They are eating the best possible kind of foods being domesticated. There are no seasonal food shortages because they're eating from the global food resource; when it's not available in Germany, you can get it from Israel. They're eating food that is processed by blending and grinding and many raw foodists are even drying up to 114 degrees Fahrenheit and they're taking relatively little exercise compared to if you're gathering in the hot sun. 

So, despite all of these advantages, still 50 percent of women living on an ideal raw food diet, aren't able to make a baby. In fact, it's more than 50 percent, because those are the 50 percent that are completely amenorrheic, whereas, the chimpanzee on that diet would be pumping out babies. So, humans, there is clearly something different than others. And we actually know what it is. Compared to other primates, we have tiny intestinal volumes, compared to other primates, we have these very small guts, and we have very small teeth. So, these are signals of an adaptation that occurred in our evolutionary past to loosing the ability to eat raw food. Why should we do that? Well, because we were eating cooked food and the cooked food was great for us, we didn't need raw food anymore, so get rid of the ability to do it. And that's what we did. 

So we've been committed to eating cooked food for a period of time that is still under dispute, but I think it's 1.9 million years ago, all the way to the beginning of our genus. 

Question: What happened to the size of the brain and the intestine as a result of eating cooked food? 

Richard Wrangham: Well, there's this fascinating set of possibilities that relate the size of the brain to what has happened to the reduction to the size of our guts. And the background for this is that, in order to understand how brains get big over evolutionary time, you have to think both about the advantages of being smart because that's why you have big brains, of course. And the costs of fueling the brain because though our brains only represent only, what is it, 2.5% of the weight of our body, they represent about 20% of our basil metabolic rate. So they are disproportionately hungry in terms of the amount of calories they consume. And that means to have a big brain; you have to supply calories to it at a high rate. So, how do we do that? Well, is it by having a high basil metabolic rate? Not at all. We have exactly the basil metabolic rate expected of any other primate. Is it by taking some of the energy we use to feed some other organ and supply it to the brain? Yes. It has to be.  So which organ is it? 

Well in the primates, the only way that they can find energy to give to their brains, as it were, is -- I shouldn't say the only way, but a major way, is through reduction of the size of the gut. Those primates that happen to have small guts because they happen to be evolved to eat a high quality diet are able to have some spare energy that they would have used for the gut, they no longer have, and they divert that to the brain. 

In other words, primates with small guts have big brains. Well, we have the smallest guts of all; we have the biggest brains of all. So, it looks as though there is a connection there and since the reason to for our small guts is the fact that we cook, that suggests that it's cooking that really facilitated this. And by the way, the time when our brain really takes off in size was about two million years ago. 

The reason it looks as though we started cooking about two million years ago is that it's around that time, 1.9 that you first see evidence of our ancestors having two signals that are associated with a small gut. And that is a narrower rib cage, and a narrower pelvis. In addition, you have, for the first time -- or no, not for the first time, but at that point, you have the biggest drop in the history of human evolution in the size of the teeth, the chewing teeth. Well that's associated with another effect of cooking, which is it makes your food softer. And because it makes your food softer, you don't need big teeth. Small teeth seems to be an advantage because maybe because they are less easily damaged than big teeth. So that happens then. 

And there's a third thing that happens around the same time which is this is the point in our evolution when we stopped being ape-like in the sense that we abandoned the morphology of the shoulder and the upper arms that allowed us to climb. Now for the first time, we look like us. And that means we are not very good at climbing in trees. Well, that means that we slept on the ground, and how are you going to sleep on the ground? In the middle of Africa with elephants and rhinoceroses and lions and leopards around? The only way that you would be willing to sleep like that nowadays is with a fire, if you are out in the open. And so that suggests that that was a time when our ancestors first controlled fire, enabled them to sleep on the ground, lose their climbing adaptations. Soften the food; get the teeth smaller. The food became much more digestible and they could have a smaller gut. 

So, those are all the things pointing to cooking emerging immensely older than people used to think. And people used to think, maybe 200,000 years ago. But I would say ten times as far as that.