288 - Richard Wrangham: How Humans Caught Fire

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1:06Last week, I interviewed your colleague, Dan Lieberman, and that was really fun for me because he's somebody whose work I've been aware of for many, many years. and incidentally I encountered your work around the same time and I when I read Catching Fire which I also loved and totally kind of changed the way I look at humans so I've really been looking forward to this conversation and I'm looking forward to talking about more of your work beyond just catching fire. So before we get started, I mean, you've, well, I mean, this is us starting, but you have studied chimpanzees, bonobos, early humans, cooking, catching fire, violence, social tolerance.

2:01What first got you interested in these topics and what led you to think they belonged together? Well, you say I've studied those things, and I've thought about a number of them, but really the only species that I've studied personally in any depth is chimpanzees. And chimpanzees provide a kind of perfect entryway into thinking about human evolution, because even though we don't know exactly what the ancestor of humans looked like around six or seven million years ago. I think there's a very persuasive case for saying that they would have been so similar to a chimpanzee that we would have wanted to put them in the same genus and conceivably even the same species as living chimpanzees.

2:58And so if we look at chimpanzees as a sort of time machine of our ancestry, just before our ancestors set off into the outside the forest habitat, at, the savannah areas in which the Australopithecines and then later Homo evolved, then it's astonishing that we have in chimpanzees a species that we can actually look at and compare with ourselves. And I think most people find it really intriguing to look at similarities between chimps and humans. And I agree with that. That is totally fascinating. But I've come to the sort of complementary conclusion that the differences are super revealing as well, because the way in which we differ starts to on thinking about why we differ from chimps in the ways that we do.

4:16Well, I would really like to come back to these differences and similarities, but another context-setting question I was wondering about is, obviously, we're interested in the evolution of humans because we are humans. But beyond that, as an evolutionary biologist, Do humans present special puzzles that other animals don't, given how different we are from other animals? Well, I mean, every animal has its own special puzzles, but some of them are more unusual than others. And I think that it's fair to say that the human case is particularly unusual. Now, I must admit that I've always had a sort of slight suspicion or a slight feeling that one of the puzzles that anthropologists are totally fascinated by, which is bipedalism, erect bipedalism, is sort of, to me, slightly less interesting than some other problems because it's the kind of thing that has happened rather repeatedly in animal evolution.

5:34that you've got animals that started out on four legs and ended up on two. And so that's a puzzle that is, you know, in many ways, it's rather a sort of choice puzzle because it means that you have the more greater opportunity to test ideas than you do with features that are more unique to humans. but obviously you know the things that are totally unique to humans like language like culture that is inherited in a ratcheted way just building on culture like aspects of our cognition are particularly fascinating and and they are also very frustrating because the fact that they're unique means that it's difficult to test them against other species.

6:38So, you know, there's always a danger of getting into storytelling. And the trick, I think, is to try and find elements in these issues that can be tested by comparable kinds of features in other species. talking about the the dangers of storytelling i mean we are as you mentioned unusual from unusual in the animal kingdom for many ways we're very intelligent i mean i don't want to talk about myself i don't want to say that humans in general are intelligent we have culture i guess bipedalism isn't that unique. But do you see these different traits as all emerging from one single evolutionary story?

7:35Or are they kind of separate mysteries that we can pick out from different times in human evolution? Oh, I absolutely think that different things happen at different times. and it was not some lightning bolt from the blue that created all of the human distinctive features that separate us from the great apes. No, I think that's one of the exciting things. And I mean, the sort of simplest way that I have to think about this is that we are members of the genus Homo and the species Sapiens. And the reason that we call ourselves Homo and Sapiens is because Sapiens is a distinctive species from other species in the genus Homo, and Homo is a distinctive genus from the other genera that gave rise to our lineage, namely the Australopithecines, and specifically Australopithecus.

8:40well those two names i think are signposts to major changes and the major changes that happened that made us sapiens as opposed to other species of homo or homo as opposed to species of Australopithecus were separate things. And I've worked on topics related to both of those changes. And to a large extent, they are, I think, separate topics. Specifically, I think that fire and cooking, the control of fire and the ability to cook were massively important for contributing to the evolution of the genus Homo from the pre-Homo apes, the Australopithecines. Whereas with sapiens, I don't think that cooking had anything to do with the emergence of sapiens from other species of Homo.

9:48I think that a quite different process of self-domestication and alliance formation happened to create sapiens out of the other homo. And I'm sure there are many other examples that differentiate different features of humanity. Do you think, so now I think we're at a little bit of a branching point because there are these two main topics that I want to discuss, which are cooking and then violence and warfare, and I guess the flip side of that, tolerance. And you mentioned that cooking is very important for the evolution of Homo, but has nothing to do with the emergence of sapiens from Homo. I'm wondering if in this evolutionary time span, you think it makes more sense to begin with cooking or violence?

10:49Well, I think it makes much more sense to begin with cooking because the dramatic human-specific features of our pattern of aggression happen much later than cooking. So cooking gets us started at the appropriate time, which is 1.9 million years ago. And what is the basic cooking hypothesis? The idea is that we became biologically adapted to needing to have access to a fire and cooked food at roughly two million years ago. And that biological adaptation ushered in the genus Homo. It's what made us a human rather than a pre-human ape. what i'm about to ask next i mean most of our listeners are not evolutionary biologists and i think one of the cool things about speaking with one is getting to learn how you work and how you think so this divergence happened maybe two million years ago what is the evidence and the thought process that led to the development of this hypothesis in the first place?

12:19Well, the background to really appreciate is the evidence from the fossils that show when the genus Homo arose, and I'm going to say, call it two million years ago, in a way that foreshadows, as it were, the current anatomy of our species. So what I'm trying to say is that if you go back for the last two million years, then pretty much throughout that entire time, well, in fact, yes, not just pretty much, but specifically throughout that entire time, you have a species that could walk down Main Street into a clothing store and put on a suit in a way that a gorilla or a chimpanzee cannot. Because they're basically the same shape and size as us.

13:27I mean, sometimes there are small, sometimes there are larger individuals, but they all look like us in that sense. So that is the identification, the simple way to think about the genus, the full genus Homo.

13:48sophisticated listeners here might know that there is a species called often called homo habilis that precedes uh two million years ago so the around two million years ago roughly 1.9 million years ago you get the emergence of homo erectus homo erectus the erect human the one that is standing up bipedally, is no longer a very good climber, but is a very good strider. Prior to that, you have a species that, as I say, is often called Homo habilis, but it is also sometimes called Australopithecus habilis. And that species is called both of those names because it is somewhat intermediate between the full erect human and the much more chimpanzee-like australopithecines that preceded it.

14:54So there's a difficulty about Homo habilis too, which is that the amount of material, fossil material that has been found, is still too little to be really confident about what it was like.

15:08So it's at, say, 2 million years that you get the emergence of the species Homo erectus that looks like modern humans, except that it was much more heavily built and the brain was a lot smaller. but the change to the modern kind of anatomy is so striking that that's why it has been called homo with no uncertainty at all and that therefore identifies a critical time for biological adaptation? What was it that made us, at that point, made our ancestors successful in that kind of anatomical confirmation as opposed to the previous ones? Well, to come to the cooking question,

16:12the special feature of the human body that we can identify very closely by studying it nowadays, because we have all the soft tissues available, which of course we don't when we look back in the past, is that we have a very reduced digestive system compared to our cousin apes. We have small guts, we have small teeth, we have small mouths. Our guts are something like two-thirds of the size that they would be if we were a chimpanzee or a gorilla or an orangutan. and I have made the argument that the reason that we are now very strongly associated with cooked food in our diets is because the adaptation to small guts, small teeth and small mouths has meant that we're actually unable to live and survive on a raw diet in the wild.

17:24And I feel very confident about that argument, based on particularly what we see with people who adopt raw diets living in cities, where they can have the best possible raw food diet, come from all over the world and aided in its preparation by electrical machines and all sorts of other things. And yet people steadily lose weight on a raw diet. And the argument is that the anatomy, the hard anatomy, the bony anatomy and the teeth anatomy of all the members of the genus Homo, back to Homo erectus, is indicative of a reduction in the size of the gut and the performance of the digestive apparatus in a way that means that at that point, that was when we first see the evidence of adaptation to cooked food, inability to eat raw food.

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19:15Print postage for certified mail, registered mail, and packages in seconds. Then schedule a pickup right from your home or office. For a limited time, go to stamps.com and use code podcast for a free welcome gift. Taxes and fees apply. That was such a good answer. I can tell you've been thinking about this for a while. I was going to ask, but you already answered how this reflected in our changing digestive anatomy. How did this transition two million years ago result in changes in our heads, our brain, other than our teeth, our jaws? I mean, particularly our brain size. And what is the connection between that and cooking?

20:01Well, this is certainly more speculative, but I think there's a pretty strong evidence that there is a relationship between cooking and the brain based on the fact that the brain is an extremely expensive organ. So in a fairly typical human, the brain occupies about 2.5 % of body weight. And the amount of energy it consumes is about 10 times that, 20 to 25 % of our body weight. So you can think of every fifth hamburger you eat as being just directed to going to service the brain. and the question that arises for any animal that might be as it were needing to be more intelligent is how do you get the brain to be able to take more than its fair share of the metabolic energy that is being produced by the animal.

21:12Well, the gut is a very expensive organ. And for years it has been shown that the size of the gut is associated with the amount of energy that is directed towards the gut. and in humans we have been able to reduce the size of our gut and that is because cooked food is very much easier to digest than raw food. So the important point about the reduction in the size of the gut that we have compared to the great apes is that for our standard metabolic rate we have reduced the commitment that we need to make to fueling our guts. And it turns out, if you do the arithmetic, that the increased size of the brain is just about exactly what is available in terms of the energy needed to fuel that extra size of the brain.

22:28freed up by the reduction in the size of the gut. So there's the logic that we eat cooked food. The cooked food allows us to have smaller guts. We can use the energy saved by not needing the big guts to fuel the brain. and the sort of awkward aspect of that argument is that the brain takes a long time to actually increase in size. So we see the increase in size slightly before Homo erectus and increasing steadily at least until half a million years ago. so let's say it took a million and a half years and in fact even longer sapiens the history of the brain size in humans means that it was increasing really i think you can identify up to more like 100 000 years ago so i think of cooking is coming in as a as a very important feature pretty dramatically leading to this commitment that our species had something like two million years ago, and the brain responding over the next million and a half and more years.

23:56So the timing isn't ideal for the hypothesis, but the logic is. well this might be a bit more speculative even but i would love to hear what you think or what others think might have actually happened on the ground around two million years ago was there just a group of our lca's last common ancestors with chimpanzees who just became somehow isolated from all the other chimpanzees and then one of them discovered how to control fire accidentally? Or how do we think that the LCA branch of us that became us actually branched off from the LCAs? You're running a number of things together there. So just to be clear, the LCA of humans and chimpanzees, that would have been a species living six or seven million years ago, and the one that I think was likely to be very chimp-like.

25:04Thank you for correcting me there. And then you have some chimps getting isolated in a savannah habitat and adapting to a new lineage, the and these were chimp-like, chimp-sized animals that were bipedal when they were walking on the ground, but were still very good climbers. So these were relatively small, sort of 50, 60, up to 80 pounds, that sort of thing. Males quite a bit bigger than females. and that was the species that gave rise to homo so so when you're asking well how did it happen that the control of fire emerged um it's the australopithecines we're thinking about how did they figure that out and um i think that the way to think about it is not as a a single brilliant individual that sparked an idea that spread, it seems much more likely that it was the result of playing with fire for probably quite a long time.

26:28I mean, it could be several hundred thousand years. And part of the reason I say this is because this mysterious species, Homo habilis or Australopithecus habilis, whichever way you want to call it, is showing signs of hominization, including increased brain size, that I think are intriguingly interpretable as a species that is getting more energy out of its food than it had been previously. Could have been because it was starting to get fire on an irregular basis. and that eventually what happened was that our species were able to make sure that they could get fire so regularly that they could rely on cooked food and that's the point at which you get the anatomical commitment because until you can absolutely rely on being able to control fire and have cooking cooked food on a completely regular basis I mean, either every night or several times a week, then there would not be selection in favor of an evolutionary loss of gut size.

27:44Because until then, you'd absolutely have to be able to rely on raw food sometimes. And for relying on raw food, you need the big guts. So I think that you've got a slow period of learning about the control of fire, which would be in the half million years before Homo erectus, so roughly two and a half million years ago, up to two million years ago. And then you've got cooking taking over. and of course it is highly speculative

28:23people sometimes think that what you are going to need is an ability to make fire that doesn't seem to me to be right what you need is an ability to be able to get fire and that doesn't mean you have to make it because once there is a population in which many different subpopulations groups, whatever you want to call them, are making fire. And even when the heavy rainstorms come through, you're going to be able to go to some group and find fire and take it from them. So you imagine they would keep a fire going? Yeah, yes. And it gave or something 24 steps. It would take time to learn. Yeah. But just like hunters and gatherers nowadays, or hunters and gatherers, you know, living as hunters and gatherers in the very recent past, they would bank their fires at night and be very cross with anybody whose fire went out.

29:26Each family would bank their own fire and make sure it was still alive in the morning. And then when they move, they carry a log with them. and the action of carrying a log that has been flaming keeps the heat there. And as soon as they settle down, they put the log on the ground and give it some fuel and blow it up and say the gods need to be propitiated and that's what we're doing when we do this. So they're very, very conscious of the importance of keeping fire alive. Well, you see stories of occasional people that for one reason or another lost their fire and get into serious trouble and sometimes die as a result of having to live on raw food for a bit.

30:17so fascinating when you say that you said something to the effect of and they were probably playing with fire for a few hundred thousand years what sort of activities do you have in mind when you use that phrase playing with fire how did first coming into contact perhaps taming fire in some way end up with one of our ancestors some population of our ancestors using it really predictably for cooking? Well, I mean, you know, it's anyone's guess. And I don't have any special claim to have any better idea than anybody else. I still trust yours better than mine. Well, I mean, in Catching Fire, I think I suggested that literally playing with fire could have been important.

31:12That is to say, juveniles finding the remains of a bushfire somewhere, finding something still glowing, picking it up and threatening another juvenile with it because, you know, they love to sort of play fight. and it's the kind of thing that just as juvenile chimps play with sticks and throw them at each other and might even hit each other with one, so they might have done this with fire.

31:47And I don't think it would take long before they discovered that if they rested a bit of food against the remains of a fire, that they would like the taste. You know, my group has tested great apes on their preference for raw food versus cooked food, and the short story is they prefer their food cooked. And I think this is going to be true for the great majority of animals. They will spontaneously like their food cooked because what fire does to food is it kind of pre-digests it. And the sensory apparatus of all animals is adapted to finding the most digestible food. So it doesn't matter that cooking is unnatural in a sense.

32:51It just happens to be a great way of denaturing proteins. It's a way of gelatinizing the starches. It's a way of making the sugars and the amino acids just a little bit more available. And our taste apparatus can detect that. So just as we can give naive great apes cooked food and they immediately like it. so I think that would have been true for australopithecines playing with fire finding a place where some by accident food has been rested against some ashes and hey this tastes nice and slowly over time they would have learned to put the food by the fire repeatedly and then very slowly get to the point where they can cultivate and control the fire in a way that means that they have a really predictable access to it.

33:53You mentioned that many animals prefer cooked food to raw food. What role, if any, do you believe that fire played in the domestication of dogs? Well, it is certainly possible that the story that people have of wolves coming around a camp and getting closer and closer because the food was desirable for them, a human camp. it is certainly possible that a major feature of that is that the food that was being tossed out as scraps unneeded by the hunters and gatherers was of special merit to the wolves because it was cooked. And there is now some biological evidence that the dogs may be one of the rare species, in fact, maybe the only species other than humans that have got some adaptations for cooked food.

35:11That's fascinating. Yeah. Yeah, both of my, and you've probably seen both of them at some point during this conversation. My dog and cat definitely prefer my cooked food to the raw food before I've cooked it, though that will not stop them, of course, from trying to get that too. Well, yes, and by the way, you know, a lot of people have problems with their cats and their dogs being too well fed, being obese. And the easiest way to solve that problem is to feed them on raw food. Because just as cooked food makes it easy for humans to be obese, it makes it easy for cats and dogs to be obese. Hmm.

36:01Interesting. It hadn't occurred to me that, I mean, it probably should have, that a raw diet would be an interesting sort of weight loss solution. It's never one that I've considered, but you did mention earlier that even people in cities who eat raw foods always tend to lose weight. they're almost always hungry all the time and and they absolutely on average uh lose quite a lot of weight yeah uh it's interesting comparison to uh meat eating you know where uh whether or not you're a vegetarian or a meat eater does not have the same effects at all the dramatic effects come just from cooking. The perfect lunch combo.

36:53That first bite of your favorite sub followed by ice cold Pepsi. Add a couple of friends and now that's next level. Suddenly, the laughs get louder, the stories keep flowing, and the food tastes better because Pepsi brings out more flavor, more fun, and more of the moment. Food deserves Pepsi. Grab a Pepsi Zero Sugar today. Hey, I'm running out to drop off a stack of letters and packages. Need anything shipped while I'm gone? Save the gas. Stamps.com lets you print postage and schedule pickups right from your desk. Sync your contacts into the address book, get detailed accounting insights, and get notified the moment your certified mail is delivered.

37:32All from my desk? All from your desk. Join the 4 million businesses using Stamps.com right now with code podcast and get your free welcome gift. Stamps.com code podcast. Taxes and fees apply. I'm struck by this idea of our ancestors carrying fire and banking fire. And I guess this might presage our discussion of warfare and tolerance. But I'm wondering how cooking might not have just changed our bodies, but also come to shape our social lives. Well, yes, I mean, the, you know, for me, the dramatic thing about cooking is what it does to us beyond our digestion. and um you know one of the first things it does by the way uh is it enormously reduces the amount of time we spend chewing so if uh you know i studied chimpanzee feeding behavior for my phd and i the way i did it was to go up in the morning uh with chimpanzees and be with them before dawn and watched them get out of their nests.

38:57And I stayed with them for about 12 hours, always following one individual, regardless of who they were with, until they went to bed at night. And so you can see how much time they spend eating. And the answer is, depending on the time of the year, around six hours a day, chewing, moving their jaws up and down. and humans might well spend an hour or two at a table with friends enjoying a meal but if you actually look at how much time they spend moving their jaws up and down how much time they really are chewing and the answer is about an hour a day regardless of whether or not they are hunters and gatherers or pastoralists or New York grandees.