Primatologist Explains the 1% Difference Between Humans & Apes | Richard Wrangham | EP 249

We only know two species on Earth in which males live in groups, often with their relatives, and go out on raids to kill members of neighboring communities, and those were chimpanzees and humans.

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Hello everyone. I'm very pleased to have today as a guest on my YouTube channel and podcast, Dr. Richard Wrangham of Harvard University. He's an anthropologist and primatologist, and not only an anthropologist and primatologist, but one of the top, certainly one of the top people in his field. I ran across Dr. Wrangham's work back in 1996. He wrote a book with Dale Peterson, "Demonic Males," very provocatively titled, a study of aggression in primates, including human beings, and an analysis as well of sex differences. I learned an awful lot from that book, and since then he's published two others: "Catching Fire: How Cooking Made Us Human," also not a title that you would expect because it's not as if people popularly think about cooking as something that made us human, so that was very interesting. It's a great book. And then more recently, "The Goodness Paradox: The Strange Relationship Between Virtue and Violence in Human Evolution," which was published in 2019.

Dr. Wrangham began his career with Jane Goodall studying chimpanzees in Gombe Stream National Park in Tanzania, and began an association there with Dian Fossey, another stellar primatologist who worked primarily with gorillas. He was then a professor at the University of Michigan and finally Ruth Moore Professor of Biological Anthropology at Harvard, where he is now. He's also a MacArthur fellow, which makes him a recipient of the prestigious prize popularly known as the Genius Grant.

So we're going to talk today about human evolution, primate evolution, aggression, the use of fire, and all those things. So welcome, Dr. Wrangham. Thank you very much for agreeing to talk with me today.

Dr. Wrangham: Oh, great to be here. Great to meet you!

Yeah, thank you. Thank you. So, I didn’t know—or perhaps I had forgotten till I looked into you again this week—that you more or less began your career with Jane Goodall, and that's pretty interesting. So do you want to walk us through your—how did your career develop? How did you develop an interest in primatology and anthropology? And maybe you could also define those fields for everyone.

Dr. Wrangham: I love being in the wild. I love being in nature. As a kid, I liked birdwatching, and that took me to more and more remote places. By the time I was 17, I spent nine months in Zambia in a national park which was called Kafui National Park. It had something like 20 people living in it, and together with its border areas, it was slightly larger than Switzerland. So it was a wonderful opportunity to really get a sense of what it's like to live in the wild. And I went to Oxford University to read zoology as an undergraduate, and after that, I wrote to Jane Goodall and said, "Is there any chance of being able to work with her?" The reason was that I was just fascinated by animals that could serve as some kind of entry into thinking about the way in which our ancestors had adapted their behavior to their own problems of life in the wild.

So I've got two questions that stem from that. So what in the world were you doing living in that park when you were 17, and how did you manage that? And how did that come about? And then the next question is, why were you convinced—or are you convinced now—that the study of non-human animals is a useful means of shedding light on human behavior?

Dr. Wrangham: Two very different questions. I told you I just loved the wild. I'd been on some expeditions to more remote parts of Britain in my adolescence, but I saw the opportunity to do more. I wrote I don’t know, a hundred letters to anyone whose address I could find saying, "Could I become a research assistant joining you in your field studies?" A guy called John Hanks, who ended up as the chief conservation scientist for the World Wildlife Fund in South Africa, was doing studies of elephants and other things in Zambia, and he took me on. I was paid a shilling a day, and I just had the most incredible time.

So what were your living conditions like in that particular time?

Dr. Wrangham: Well, they were very civilized. I mean, you know, I had a regular little cabin in a small camp that was ultimately destined to become a tourist venue—it wasn't really at that stage. It was a remote area; you had to drive a couple of hundred miles to get there. But of course, we did go camping, and so that was a major thrill. You know, going camping in an area that was full of really wild animals and no opportunity to call for help as it were—you know, you're really living on your own. So if you ran into wild animals, you had to know what to do. In my first few weeks there, I went for a walk in an inadvisable area and found myself catching up on a rhino, and was only stopped from bumping into it in thick vegetation by the arrival of, well, I was lost. People were shooting rifles to tell me where to go. So you know, this was just a little adventure for a 17-year-old learning about self-sufficiency in the wild.

You mentioned you were studying elephants at that point?

Dr. Wrangham: I was participating; I wasn't studying anything myself.

Right, right. And yes, I mean, this is very dramatic stuff because we were trying out a new drug which has since become a standard drug for immobilizing animals, mammals in the wild—immobilon it's now called. But at that time, the delivery systems were poor and the doses were unknown. So we would walk up to elephants, and John would fire a crossbow dart into the elephant and would then hope that the elephant would fall asleep so that we could take measurements, mark it, extract parasites, and so on. The amusing thing in retrospect about this is that when an elephant falls asleep, it stands absolutely solid; it doesn't necessarily fall over. You know, it's like a table—got four strong legs. And so then the question is: is it asleep? It seems like an important question.

Yeah, well, exactly! So instead of all of us swarming up to it, one person—and I'm very happy to say that it wasn't me—I was deputed to go up and pull its tail. And since you know, you didn't know whether or not this is asleep or awake, I think this is very brave of John to do this, who took that upon himself. And so then it turned out that he always got it right; it was asleep. And then what we had to do was to get the elephant onto its side, and this involved four of us getting onto one side of it and pushing as hard as we could. But this only affected a sway—so the elephant would totter over as it were to the right side if we were pushing on the left, and then it would sway back towards us, so we had to run backwards and then catch it again as it swayed further forwards and so on. Yeah, I mean, you can see what a sort of thrill all this was for a young aspiring naturalist.

It sounds like a study that would be very difficult to get through a research ethics committee.

Dr. Wrangham: Yes, and rightly actually, because in the previous years, the person in my position had been killed by elephants. So there were serious aspects to this, and we did have to take what precautions we could, certainly.

Right, yeah. Well, I wasn't necessarily saying that favorably. I mean, to gain knowledge requires a certain degree of risk, and I suspect that's particularly true when you're out in the wild observing wild animals. That's not something you can make 100% safe. I mean, obviously, you don't want to be foolhardy, but there is the thrill of the adventure that goes along with that, and that's a necessary part of it. I think.

Although, by the way, when a woman called Nancy Howell had her son killed while she was doing fieldwork in South Africa—or I think it might have even been Botswana—she did an analysis of deaths among anthropologists doing fieldwork and found that the major source of death was car accidents.

Right, right. You know, the roads are bad.

Right, yes! Well, car accidents are a major killer everywhere, so maybe that's a good rule of thumb for danger—if it's safer, if it's no more dangerous than driving a car, which is something almost everyone accepts, maybe that's safe enough.

So, okay, so you got accustomed to this, and you found that you liked it. You went back to Oxford and studied zoology, and then you wrote to Jane Goodall. I've got that timeline correct.

Dr. Wrangham: This was the late 1960s that I was studying zoology, and there were two writers whose work had sort of really impinged on the public imagination at that time in relationship to human evolution. There was Robert Ardrey, a playwright who wrote a book called "African Genesis," another called "Territorial Imperative" in which he produced some bold, sweeping ideas about how human competitiveness and conflict had arisen from our time when our ancestors were in Africa.

And there was Desmond Morris, who wrote a book called "The Naked Ape," in which he emphasized in particular the sexual side of human evolution. What those books did for me was to say that there was a world out there waiting to be explored of really understanding, in a way that had not been attempted before, where humans come from in terms of our behavior.

Simultaneously, this was a time when the discovery of the social structure of all sorts of large mammals was taking giant leaps forward. You know, the main study that I was doing with John Hanks in 1967 in Zambia was a study of the behavioral ecology of an antelope called the waterbuck. It's a very widespread antelope. Behavioral ecology means understanding the social behavior in relationship to all the environmental pressures. This was the first such study done on waterbuck, no particular surprise because many of the first studies were being done at that time on any animals, on lions, and gorillas, and many different things.

So there was a sense of impending discovery. And what happened even in the late '60s, but particularly in the early '70s, was the discovery of a lot of differences in the different species. Even quite closely related species might have not just differences in group size and flexibility, but differences in their social relationships in what at first seemed totally mysterious ways. You know, one species would have a society in which females all lived in the same group as they were born in, would never leave, and males would come in from outside around adolescence. Other species, the opposite would happen. Some species would have females that readily mated with every different male in the group, others would have a society in which females would only mate with one male. All sorts of fascinating differences were emerging, and hints were coming through about how you could explain these.

So this was a very exciting moment because when you combine it with the opportunity to think about humans, then we could get a long way beyond the kind of very naive political science interpretations of human behavior; we could really embed it in the environment in which humans had evolved.

So that's a tricky issue, you know, because, well, one of the things your work highlights—and this has been the case with other primatologists, particularly more recently—is that, because you just mentioned that species that are even very closely related can have radical differences in their fundamental behaviors. And you highlight the differences, for example, between bonobos and chimpanzees, which are very closely related.

And so then you ask yourself, well, if bonobos and chimpanzees can be so biologically similar, structurally similar, let's say, but so behaviorally different, how is it that you decide what's reasonably generalized to the human case? And why should you believe that anything could be given? Right? Because there's similarities and differences between all of the apes. So how do you prioritize the similarities versus the differences? And how do you decide when you can draw conclusions that are more universal rather than local to that particular species?

Dr. Wrangham: Yeah, I mean, this is the kind of stuff that we grapple with all the time, of course. And I think it's quite a long story working out what we can say about humans in relationship to, call it, the other apes or the great apes. You know, we have a very strange position as humans. It used to be thought when I started, for instance, in the 1970s, it was fascinating to go and study chimpanzees, but they were just one species of great ape among the other main three—gorillas, bonobos, and orangutans—that all seemed so roughly equally important for understanding human evolution. There was the apes and there were humans. An enormous shock happened in 1984. In 1984, two ornithologists at Yale, Calhoun and Powell, published a paper in which they used techniques that they'd been applying to the study of the evolution of birds to the apes and humans.

And this technique was to assess the degree of similarity of DNA. So they'd worked out this DNA and kneeling system, and they applied it to apes and humans, and they claimed that chimpanzees and bonobos were the most closely related species, and then chimpanzees, bonobos, and humans were more closely related to each other than any of them were to gorillas. So that kind of put us in the chimp category.

Dr. Wrangham: Now let me ask you just there. I want to ask you a question about that DNA technique. Now, if I remember correctly, the way that was done to begin with was to take DNA from separate species and to heat it so that it split apart and then to have the DNA from the different species join together. And then to assess how tightly bonded the DNA was between species, and the hypothesis was: the more tightly bonded it was, so the harder it was to pull apart once it regrouped, the more closely related the species was. That was the first way of doing DNA similarity analysis. That's been superseded. But was that the 1984 work?

Dr. Wrangham: Yes, absolutely! And the technique was a little crude—um, unzip things and zip them together and see how tough they are to pull apart—and that crudeness meant that people could challenge the results, and it took maybe a decade for people to become, you know, the profession as a whole to become really convinced at these extraordinary results. And the reason they seemed so extraordinary was because if you take the difference between a chimpanzee and a gorilla, they look as if they're very closely related. They look so closely related because if you just imagine allowing a chimpanzee to just keep on growing, it basically turns into a gorilla. And many of the differences between gorillas and chimps can be understood just in terms of body size, gorillas being bigger than chimps.

So when the argument is made that—or actually, you know, that's the normal argument. I mean, just absolutely clear right now that chimpanzees and humans are more closely related to each other than either is to a gorilla. What that means is that the common ancestor of chimps and humans is very likely in the mold of the chimpanzee.

Gorilla, either they're the same—very similar kind of animal—which if it was very large would be like a gorilla; if it was very small it would be like a chimpanzee. Either that or there's been a fantastic degree of evolutionary convergence between some mysterious ancestor which would become more gorilla-like and become more chimp-like, but no one thinks that that is the case.

Right, so the idea is that—and this is—this is, how long ago did gorillas split from chimps hypothetically? Is that like 10 million years ago?

Dr. Wrangham: Ten million.

So the common ancestor of chimps and gorillas, and the chimps in this discussion include us, roughly speaking—so that's 10 million years. And then 7 million years ago we split from chimps, and the bonobos and the chimps split 2 million, something like that, or one or two million years ago?

Dr. Wrangham: Yes, right. So maybe more like one nowadays. We’re still getting an increasingly confident assessment of that, but anyway, much more recently—not long after we had left the chimpanzee-bonobo line, right.

And we might just also throw out finding—I suppose the hypothesis that Homo sapiens sapiens—that's us—that we’re about 300,000 years old in our current configuration, something like that.

Yes, and in between we got the genus Homo, so the genus to which we belong, and that emerged about 2 million years ago.

So you've got the common ancestor with chimps 6 or 7 million years ago, and then between then and 2 million years ago we had these animals that are rather like chimpanzees standing upright—Australopithecines—and they gave rise to our genus Homo about 2 million years ago.

And ever since then, our ancestors have basically looked like us. I mean basically in the sense that you could take the earliest Homo—Homo, true Homo, Homo erectus—and they could walk into a clothing store on Main Street and pick clothes off the peg, and they wouldn’t—they’d fit reasonably well. They’d be, you know, they need the big size—heavily muscled. But nevertheless, we can estimate the difference. The—that we can estimate when these different species split from one another by looking at the degree of genetic difference and inferring the split from such things as divergent mutations because we know that mutations occur at a fairly standard rate.

Is that—and then there’s obviously fossil dating, but what other techniques are used to estimate the divergent states?

Dr. Wrangham: The more recent times, obviously, we've got the fossils. So, you know, we have a very good fossil record back to more than 2 million years ago, and we can be very confident that our ancestors were indeed Homo erectus, which as I say, I mean, was the first thing that was fully bipedal in the human style, going back to 2 million years ago. And then we got a pretty good record, but obviously, the older you go, the more broken it is. But of the Australopithecines, and by the time we get to three, four million years ago, it's getting increasingly broken. But nevertheless, there’s half a dozen different species and more that have been recognized in the African habitats.

And then by the time you're asking the question about when it was that you have chimpanzees and humans as a common ancestor with a common ancestor, there you do have to rely on genetic data, as you say, the rate at which mutations accumulate, because we do not have any good fossil evidence for that animal. And the reason for that is partly just as getting old, and also probably because it would have lived in a forest, and forests don't preserve bones well, you know—they tend to be too acid—and the bones just decay very quickly. But I bet eventually someone will find a pretty good, you know, something close to the chimpanzee-human common ancestor.

So, 10 million years—okay, so let's go back to the biography now. You came out of Africa, you studied zoology, you wrote to Jane Goodall, and you told her about your experiences in Africa, so she knew that you could probably handle it. Why? And by that time, you had an undergraduate degree?

Dr. Wrangham: I did, that's right. So you started working with her in—where?

In Tanzania?

Dr. Wrangham: In Tanzania.

Tanzania, yeah. In 1970. So what was that like, and what were you—what were you doing during that period of time?

For my first year, I was a research assistant to her; I was just learning the job. And I would say that within about 20 minutes of seeing my first chimpanzee in the wild, I recognized that there was a mind in that animal that was different from that of a waterbuck. You know, this is not just an antelope, and it’s difficult to say exactly what's going on, you know? But there’s something about the use of eyes and something about, you know, the way that they're evidently concocting strategies that very rapidly tells you that you’re dealing with an animal that is, like other animals, but also is cognitively, you know, quite sophisticated. And of course, that makes it incredibly interesting.

Now this is 1970, I said, long before I had any kind of concept that we were looking at a good model of the human ancestor. So this is just, you know, another ape, but here’s what was so amazing—you know, I happened to arrive there at a time when some very fundamental discoveries were being made. Jane Goodall had already discovered that chimpanzees had very strong relationships among males—very, very brotherly, very fraternal relationships somewhat recalling the kind of thing that you can see in, I don't know, fraternities in contemporary humans.

She discovered that, amazingly, they would go hunting and would kill antelope, pigs, baboons, other monkeys, and eat them. She discovered that they would shed—they valued meat; they valued meat to eat. But they would also share it a little bit. And by the time you're racking up these male bondings, the meat eating, the meat sharing, this is looking extraordinarily human-like. No other animal does these things.

Dr. Wrangham: Did they share for sexual favors? Did she discover that the males would give meat to the females? I can't remember if I'm remembering that correctly.

Well, you may indeed remember what you read. Some colleagues and I have published a paper saying we don't believe that for a minute—that it does not look to us as though any of the evidence that has been brought forward in favor of the idea that chimpanzees—that male chimpanzees will give meat to females in order to get them to mate—none of that evidence is good.

Right, that's pretty sophisticated behavior; it would imply some knowledge of trading with future gain in mind, it would seem to me.

Well, I don't think the problem of sophistication is difficult. I think chimpanzees could easily handle that based on other kinds of interactions they have. The thing is that among chimps, the sexual system is very different from others, and females really, really want to mate with other males as much as possible. I have seen female chimpanzees go into a tree containing 10 males, climb to the highest-ranking male present, hoping he will mate her, and he turns his nose up. Matter and she then goes ranked by rank down the ranks to the different males, and finally, she'll get some juvenile male to mate with her.

That's just symbolic of the fact that she is desperate to get mated as much as possible. And she—she's also with the chimps—do the females, do they have an estrous period that’s not hidden, correct?

Dr. Wrangham: It's very visually striking.

That's right. The labia increase under the influence of estrogen so that you have quite a large swelling and very pink—very obvious from hundreds of yards away. So males know when she has the swelling, but there is a little bit of subtlety to this. So in the first—I mean, she has a swelling for maybe 10 days at a time once a month during a period that she's trying to get pregnant, and during the first week of the swelling, the males are not particularly interested in her. They appear to know, as it were, that she is unlikely actually to have ovulated at that time. They get much more interested towards the end of the period of swelling, and that's when the big males come in—the high-ranking males—and exert their dominance to be able to compete for the female. But she is interested in mating throughout this period, and she can mate sometimes 50 times a day.

And the reason is very clear: the reason is that any male who has not mated her is dangerous to her subsequent infant because a male who has failed to mate, the logic is that he cannot be the father. And if he's not, then the infant is worthy of being killed—because—

How does he track that?

Dr. Wrangham: Let's not know. It’s almost certainly memory, but we don’t know for sure, you know. And there are animals in which it happens with mechanisms other than memory. So do you know the story about how it happens in mice?

No, no.

Well, there's a wonderful study of mice in which they assess the infanticidal tendency of males by seeing how desperate they are to get at infants. And what they were able to show is that the tendency to be infanticidal happens 21 days—which is the length of gestation of the mouse—after a mating. And the way they're able to show that it’s an account of the number of days is they manipulated the length of the day-night cycle, so that if you had day-night cycles eight hours long, and eight hours—eight hours a day and eight hours of night, then they would come in and try and commit infanticide after 21 of those. If you extended them to 16 hours or longer of a day and 16 hours of night, then they would commit infanticide; they would try to commit infanticide 21 of those cycles later. So some animals can have just an internal clockwork regardless of the actual time.

So, if they hadn't mated 21 days earlier, they would assume that those infants aren't theirs.

That's right! That's right.

So they do get the inhibition; I hope I said that right. It’s in the—after 21 dark-light cycles.

That's remarkable! Such a sort of mechanical system applies to chimpanzees. It's much more likely that they actually have a memory of when and how often and under what circumstances they mated a particular female. So we don't know that exactly, but at any rate, it certainly fits with all that we know from other primates, that where there is some direct experimental evidence about this, that females risk the lives of their infants if they do not mate with all of the males in the group on a regular basis.

And is that also characteristic of the bonobos as well?

Dr. Wrangham: Yes. In bonobos, the females famously have even more sex than in chimpanzees, and no infanticide has been recorded. And so all one can say is that if there is an infanticide threat, the females have overcome it by successfully persuading every male every time that he is a potential father.

Right, right. So you can’t see any exceptions to that. So, well, so that's a marked—as you pointed out to begin with, that’s a marked difference with human behavior—all of that.

No, including the—in human females, famously have concealed ovulation as well, which is also a massive difference. So human males cannot tell with any degree of certainty when a female is most likely to be impregnated, and it’s—that’s an interesting evolutionary divergence. And that's something—have you thought about—Is that something that you've developed a particular theory about or thought about in it to any great degree? Is that something we could pursue? Because it's a fascinating—

Dr. Wrangham: It is a fascinating difference, yeah. I mean, I would say that no, I haven’t thought about it in detail, and I would say nobody has a really convincing story. But the basic story is that with humans we have shifted away from females mating all males in the group; females mate with one male at a time, essentially. There’s a little bit of gene shopping, but basically, females are bonded to a particular male. And I do see the bonding as quite strongly associated with cooking.

Okay, that’s not an obvious connection, so I would love to hear about that.

Dr. Wrangham: Well, why do you believe that? If you look now at people living in open societies, then what you see is a sexual division of labor in which females are cooking food for themselves and their children and for a male, and their big job in life is to produce food for the male. From the point of view of their relationship with the male, he absolutely needs to rely on having a meal when he comes back in the evening from whatever he’s been doing, whether it's been hunting or politicking with people in a neighboring group, or searching for enemies, or whatever. And the reason he needs it is because he needs cooked food just like every other human, and he doesn't have time to cook the food himself. So if his wife doesn't cook the food for him, then she’s in trouble.

What the female is bonded to the male in the sense that the male needs her to provide food for him. If she is sexually promiscuous, then for obvious reasons the male is upset, and so he will punish her—he will beat her, he will maybe dispose of her and get another female if he can. And she needs him to do the things that he’s out doing, having been fed—like providing high-quality protein, for example.

The standard story is that she needs him to produce meat and other good foods. I think there’s something else, though, that is critical. Once you get to cooking, and the reason that she needs him once you have cooking is that cooking exposes the cook to theft.

So when you cook, you need a fire. Fire produces smoke; everybody knows you’re cooking from a long way away. People can detect the fact that you are cooking; they can smell the smoke; they can see the smoke. Just it’s a very practical thing that means that for people who are lazy, people who don’t want to cook their own food, people who don’t even want to find their own food, people who maybe are sick, a person who is cooking food is vulnerable to having their food stolen.

And a woman cooking food is particularly vulnerable to men—such as a man who is not yet got his own wife. So bachelors are a problem for a woman, and so might be the elder children of other females. So might be another woman. You have all sorts of sources of risk, and this is where a woman’s bond to a man is really important once cooking emerges because he can be relied on as somebody who can protect her—not necessarily from actually being there at the time that a bachelor appears and tries to pinch some of the food on her fire, but because she can go to the husband and say, "That lousy rat has taken some of my food." And, you know, we’re getting ahead of ourselves in terms of thinking about the nature of human society because the thing I have to bring into this stage is that in all human societies, you have a collection of men who form an alliance among themselves. So that the husband of the cook is not just a man who can stand up for his wife and threaten any bachelor who approaches her with theft in mind; he’s more than that because he can go to the rest of the men and say, "You know, guys, we’ve got a trouble. We got some guy who is not respecting the norms in this society." And that’s a much more potent threat than one guy standing up for a one-on-one fight because all of a sudden you’ve got the whole society now led by those men ready to enforce some quite severe punishment.

Yeah, you're starting to explore that idea, and we’ll go back to that pretty deeply in your last book on what you described as the self-domestication of human beings—that enforcement of moral norms and the control of, well, hyper-aggressive behavior but also behavior that’s breaking rules. Yes, so we’ll come to that.

But to come back to the question of what you asked about do male chimpanzees share food, share meat with estrous females, right? Females who are available to mate—one of the reasons, other than just the shortage of direct observations, that you don’t expect this is because a male does not need to produce meat to persuade a female to mate with him. In humans, that is necessary—or, you know, under the appropriate circumstances. But not in chimps. More likely that a female would have to pay a male to mate her. I mean, not that really needs to happen, but anyway, this is an example to me of a behavior that was written up quite early in the study of chimpanzees in the 1970s on, I think, too optimistic an assumption that something that you saw that was reminiscent of human behavior might easily be interpreted as being equivalent.

So, you know, occasionally a male might allow an estrous female to take some meat, and everyone leaps on it and says, "Hey, look, just like humans!" As it turns out, females who are not estrous are more likely to get meat than a female who is estrous. As it turns out, when you have females with estrous sexual swellings, then there is less hunting going on than when those females are absent. There’s all sorts of evidence now that this is not a confident relationship at all. So I think it’s an object lesson in the care you need for thinking about similarities between humans and primates and chimpanzees.

Right. Well, one of the ways you kind of check yourself against such things as a scientist is that before you generate any theory, you know, of spectacular originality, you might want to ensure that multiple lines of evidence suggest it and that those lines of evidence have been drawn from divergent and non-overlapping sources.

Yeah, that’s right, exactly.

It checks you against your own projections.

Yeah, that’s right. And that kind of caution applied particularly to one of the additional features that was emerging as I was joining the chimpanzee study in the 1970s, which actually really happened while I was there. And that is the discovery of something even more shocking than hunting, meat sharing, and tool use—the famous tool use. You know, all these things that were very similar between chimpanzees and humans. Well, then it turned out that chimpanzees were holding territories against other chimpanzees and would sometimes go to the territorial boundaries, look for opportunities to stalk and hunt and kill members of neighboring groups, and those members of neighboring groups would be almost entirely adult males.

So now we had for the first time something that looked like a primitive kind of war.

Yes, and that the importance of that can hardly be overstated. I mean, when I first read that, it just shocked me to the core because I thought, well, you know, if you’re an optimist in some sense, you might assume that the human proclivity for war is a consequence of, let’s say, maladaptive socialization or something like that that could hypothetically be easily controlled. But to see an analog of that so striking in chimpanzees was, well, it was an indication of just exactly how deep that proclivity is—that proclivity to dehumanize, let’s say, so to speak, outgroup members and to treat them as prey.

Yeah, it’s an indication, but, you know, as my great advisor, Robert Hinde, the animal behaviorist, endlessly emphasized to me, you know, you’ve got to be really, really cautious about thinking how to understand what this chimpanzee behavior means for humans, and I think, you know, we have been as a profession pretty cautious about it. But, um, yeah, so one of the first things that happened was people said, “Well, it’s very likely that this pattern of chimpanzees killing members of neighboring groups, males in neighboring groups, is something to do with disturbance to that particular population.” And it won’t turn—it’ll turn out not to happen in other populations, and it took, gosh, 30 years to develop enough confidence in what was happening in all of the other populations to be able to say, “You know what, this is a characteristic feature of chimpanzees.”

And by the time we had those data coming in, we could also say what it was associated with, and the answer is it was associated with high population density and large numbers of males in the aggressive community. So you get lots of males together, and they will look for opportunities to kill members of neighboring groups.

And what is it not associated with?

Dr. Wrangham: Uh, it’s not associated with measures of human disturbance—whether or not the forest had been subject to a bit of logging, or how many people live nearby, that sort of thing. The chimpanzees don’t care about that sort of stuff; they’re living their own lives, and their social dynamics are concerned with what’s going on in their own species’ lives.

So why is a preponderance of males in the attacking? Is that just a matter of outnumbering the enemy, or are there other factors at play?

Dr. Wrangham: Numbering looks the important thing, yes.

Right, because they generally don’t attack—chimps generally don’t attack outside their boundaries unless they clearly outnumber those that they are targeting, correct?

Dr. Wrangham: And that explanation for that—they have a rudimentary sense of number, or amount, something like that? Is it number or amount?

Dr. Wrangham: They have a rudimentary sense of that at least, yeah. I mean, yeah, they’re very smart. And it turns out that the average ratio of the number of males in an attacking group to the number of males in the victim group is 8 to 1. And what this comes down to is: you each have four males grab one limb, and then you have a helpless victim, and the remaining four can do what they like to that victim, and they do. And you know, they can terrorize, pull his testes off and twist their arms until the bones break, and blood is coming out from everywhere, and so on. It’s a really nasty business. And, you know, chimpanzees are, depends on exactly what measure you like, but you know, they might be three or four times stronger than humans. They are immensely strong. And so a chimpanzee fighting for its life could in theory impose immense damage on its attackers, but the chimps that attack are so smart and figured out exactly how to do this that there is not a single case out of some 50 attacks that have been reasonably well documented of any of the aggressors being damaged beyond a scratch.

So they know what they’re doing. And this is the, you know, the imbalance of power hypothesis that what has happened in chimpanzee social evolution is that because you have variation in the number of companions that individuals have within their communities as they walk about looking for opportunities to eat as well as possible and find females and all that sort of stuff, you have variation which exposes occasional victims to occasional large groups. And just the fact of having a lot of males in your group means that you have safety when it’s attacking.

Imbalance of power is enough to induce attack. So, you know, for me, this is classic Lord Acton: power corrupts, and absolute power corrupts absolutely.

And so what’s in it for the attacking chimps? What is it that they gain? They don’t gain immediate food; they don’t gain immediate females. You know, they don’t walk back to their communities with a kidnapped female or anything like that. They don’t show in the next few days increased acquisition of fruit from a tree on the boundary—nothing like that. But what they do gain is increased confidence in moving in an area that was previously more evenly used between the two communities.

Oh, so they decrease the probability that they’re going to be someone who’s one against eight.

It’s partly the safety problem like that, but also it gets them access to more resources in that area. In other words, during the months and years to come, they’re able to spend more time in such and such a valley where they have killed. And we now have nice data from two different studies—one in Tanzania, one in Uganda—showing that you get an occupation of the area where the killings have occurred, and in one case beautifully showing all of the expected consequences for the quality of the diet: so a better food, increased body mass, even known of the individuals that are able to occupy a larger area, reduction in the time between births for those females. So the females are feeling better, the males are feeling better, and even increased survival of the offspring in that community as a whole.

And how much killing does it take to produce such effects? I mean, is it—is it the elimination of one or two individuals from a neighboring troop sufficient to do that? Like, what scale does this have to occur at before those consequences emerged?

Dr. Wrangham: Anyone know? In those two best-known cases, in one case, all the males in the neighboring community were killed.

I see. So it was a complete obliteration of the competitors.

Dr. Wrangham: It was initially a small community; there were only seven males in it, and they were all killed. I shouldn’t say they were all killed; I should say they all died. And in four, maybe five cases, they were known to be attacked and killed.

So, is that typical? Is the typical behavior to move in those situations towards complete elimination?

Dr. Wrangham: It’s typical, and it may well not be. I mean, you know, if you've got a very large community neighboring, then I could well imagine that they kill several and it just means they are able to dominate an area that was previously more evenly shared.

So in the case of the Ugandan story at Ngogo in Kibale National Park, you had something over 20 kills that were made in various different parts of the boundary of the killer community. But not enough was known about the neighbors to be sure what proportion that meant or that was of the neighbors. It’s unlikely that many neighboring communities were completely eliminated, though those are immense communities. You know, that killer community at that time had 25 fully adult and over, I think, 35 adult and adolescent males—this is an immense power—and they were living in an area with just tremendous food productivity and a number of communities that had a large number of males.

But they were probably the biggest boss on the block.

So you were working with Goodall when that initial raiding behavior was documented?

Dr. Wrangham: Yes, I mean, I saw some of the first raids.

And it’s no wonder you wrote your first book, that were inspired to do the work to write your first book, which was "Demonic Males." I mean, that must have really—I mean, that’s quite a bombshell, that discovery. Like, Goodall was—if I remember correctly and informed him correctly—Goodall was quite hesitant about sharing those results for some period of time, I believe. Is that—have I got that right, or was she just being cautious, or what’s the story there?

Dr. Wrangham: Well, I don't think that’s right.

Oh, okay.

You know, it’s funny—William Boyd wrote a book called "Browserville Beach," which was obviously inspired by what happened in Gombe, and in that case, the protagonist, the Jane Goodall figure, was very reluctant to share the results. And I think that that may have affected the, you know, the popular understanding.

But no, Jane, in 1979, she published a paper that was describing some of these results, and I think she’s always had a very faithful and honest approach to presenting this. She didn’t hide it.

Yeah, well, I wasn’t—I certainly wasn't suggesting that. I mean, I can imagine, though, that a scientist in her position would want what would you call to be damn sure of the proposition before releasing it on the world.

Well, fair enough, yes. And I think she’s been appropriately cautious. You know, she described what was seen. I think I probably did go a bit beyond her in inferring the more general tendencies.

And so, but you know, by 20 years later, in '96, when I published "Demonic Males" with Dale Peterson, I think it was pretty clear what was going on.

And then, as I said, you know, another 30 years later, the data were really coming in very, very solidly.

And I mean, in some ways, I wasn't too keen on the title "Demonic Males" because it’s a little bit in your face.

I wanted to make—is that it is very extraordinary because we only know two species on Earth—or at that stage we only knew two species on Earth—in which males live in groups, often with their relatives, and go out on raids to kill members of neighboring communities. And those were chimpanzees and humans.

Well, when I picked up the book—I mean, I was very interested in the scientific study of aggression at the time—and when I picked up the book, I thought the same thing about the title. I thought, "But the book itself is a very scholarly examination of this proclivity."

And so for people who are listening who are interested, it’s a very, very solid book, and it isn’t—it isn’t, let’s say, accessible.

Yes, well, it’s accessible, but it’s also careful—it’s not sensationalistic at all.

Yeah, and that’s the case with all of your work. It’s very serious work.

And so, so walk us through "Demonic Males." You’ve done some of that, so we’re going to jump ahead a bit because you’ve done a lot of things, and I want to cover a lot of it before we close. Anything else you want to say about your first—your work with Jane Goodall and that setting you up for the long-term study of chimpanzees?

Dr. Wrangham: Sure! Well, I mean, I actually started feeding behavior when I was working with chimpanzees. And I think it’s a great thing to study because it emphasizes, you know, the most important aspect of an ordinary animal’s life.

The just this daily search for food. You know, people, when they come to film chimpanzees, they often are expecting to see tool use or dramatic sexual behavior or something exotic in their first day or two, and they say, "This is so boring. They’re spending six hours a day just sitting in trees eating."

Yeah, well, I think it was in your—in the book "Catching Fire" maybe? Is it in that book that gorillas—the documentation of gorillas spending up to eight hours a day doing nothing but chewing leaves?

Dr. Wrangham: That’s right. They have to do that because their diet is not particularly rich, and so they don’t have time for much else.

And with chimps as well, like, it takes a lot of calories to keep a chimp going, and so there’s—and that’s also perhaps offers some insight into why it took human culture so long to explode in the way it does. It’s very difficult to get beyond hand-to-mouth living.

Yeah, I just think it’s a really helpful sort of embedding of the reality of animals’ lives, and it’s not something that necessarily comes across when you read about their social behavior, but they are spending most of their time strategizing about how to get as much food as possible. And I found that immensely helpful. But on the other hand, it took me a long time before the penny dropped, you know, I actually tried to live like a chimpanzee a little bit in the sense of eating what chimpanzees eat. You know, I ate everything chimpanzees ate, and I discovered very rapidly that I got extremely hungry if I did that.

The penny finally dropped in the '90s when I realized that there was a huge difference between chimpanzee and human diets. And that is that humans cook our food. And that, you know, developed a whole new story.

So, but I mentioned the importance of seeing chimpanzees feeding and spending time thinking about their attitude to using their environment to maximize the amount of food they can get because it’s a big problem for humans too. And, you know, this is nature in the raw: How on earth do you get enough to satisfy those endless pangs of hunger?

I think you’ve got to think of these animals as being hungry all the time, and it’s difficult for us to get into that mentality because we are never hungry. We can satisfy our food needs all the time.

Right, so we underestimate how relevant that is by a massive degree. I mean, your book "Catching Fire" was striking to me, I suppose, in almost the same way that my encounter with the data showing that chimpanzees went on raids. I mean, you made the claim, for example, that, well, you think human beings have been using fire for about 2 million years, which is an awful long time.

Well, in some sense, it's longer than many suppositions.

And you also make a very strong case that our proclivity to use fire and cook has radically altered, well, our whole morphology, our whole physiology, our intestinal system, our digestive system, and that that’s provided us with the additional calories necessary to expend some resources on brain power—more resources on brain power. Have I got that right? I hope I'm not doing your book an injustice.

Dr. Wrangham: No, that’s all right, and the other big thing that it did is to hugely increase the amount of time we can spend on things other than choice.

Right, right, exactly—to free us up to...because to do nothing long enough to think about something other than, like, immediate food acquisition.

Cooking softens food, and it softens it so much that we can spend just a fraction of time chewing compared to any of our ape relatives. You know, so if we were eating our food raw, we would be spending probably more than six hours a day chewing our food.

And as it is, we spend less than an hour a day chewing our food. So that saves us five hours a day, and gosh, you know, that is important.

It has different importance for females and for males in terms of how females and males actually spend their time. Now and the irony is that females do a lot of food preparation. I mean, this is worldwide. The only exceptions to it are in modern urban society, but much of the saving of chewing time is translated into females gathering food, preparing it, and cooking it for males.

There’s much more freedom given by that saving of time to go off and do high-risk, high-gain activities like hunting, but also politicking, visiting people in the neighboring camp, chasing women in the neighboring camp, and so on.

And the contribution to cultural production, artistic production, like aesthetic production, bead making— all of that.

That’s all that, exactly. That’s right!

I mean, you know, all of a sudden you have a totally different attitude to time from other people. You have some of it exactly right!

And so, two million years ago, so what were our ancestors like two million years ago when they discovered fire, and how has that come about? Well, one of the things that’s really interesting about fire, as far as I’m concerned, is that it’s archetypally interesting. You know, if I spend time at my parents’ cottage and I used to bring my little kids up there, and you know, adults—human adults will sit in a circle and look at two things. They’ll look at infants and little kids like intently, watch them non-stop as if they’re in eternally interesting; they don’t habituate to that.

But we also don’t seem to habituate to fire. And I’m wondering if, you know, two million years ago, there was a mutation that occurred that made someone, some ancestor absolutely unable to stay away from fire because he was no longer able to habituate to it. So it was endlessly fascinating.

So I don’t know what you think about that idea, but no, though there were probably various kinds of adaptations, psychological and physiological adaptations to being near fire, to being drawn to fire, and so on.

But I think they would follow the discovery of being able to control it and just how useful it was. Exactly how that happened, I don’t know. I mean, you know, my personal fantasy goes along these lines that you’ve got Australopithecines. So that’s, you know, like a chimpanzee standing upright. It’s just got a big jaw and eating raw foods—big teeth. It’s by the size of a chimpanzee.

And it’s clear that there was increasing evidence in the fossil record of meat eating and how that comes about—who knows? But if they’re eating more meat, then meat is not that easy to eat if it’s raw, but it’s a lot easier to eat if you pound it—if you just, you know, do a steak tartare on it.

And so my fantasy is that they were eating more meat, they were using rocks to pound the meat sometimes. Sometimes they’d use wood, but when they use rocks, sparks would come out, and sparks would sometimes start little fires. And so they repeatedly would be exposed to fires in relationship to their activity, and this would happen often enough that out of this would learn—they’d learn the opportunity to leave meat near a fire and taste the value.

How often are animals, edible animals, trapped in grass fires in Africa? You know people often refer to that idea, but I don’t think it happens very often at all. I’m not—I don’t believe there are any studies in which people have actually, you know, documented a number.

Most animals are able to escape by either borrowing underground or running away from the fire. You know, it’s conceivable that that is the way it happened, that it happened often enough that people, you know, figured it out, that Australopithecines figured out what was going on.

You know, it’s all speculation as to how fire was first controlled, but what is clear is that once it was first controlled, it would have had huge effects. You know, we know that all animals like their food cooked compared to like eating it raw or all that have been tested.

So not just our domestic animals, but wild animals too—all the apes, for instance, prefer it cooked. They just haven’t figured out how to cook it.

And how do you account for that? It seems very strange that how is it that could possibly be the case? Like, why do animals prefer cooked food?

Dr. Wrangham: It’s because what cooking does is essentially pre-digest food. And animals like their food as much digested as possible because the more of digestion you can have done for you, then the less you have to do it yourself, and there’s a major cost, right?

Well, I can see—I can see that it makes sense. I just can’t see how they would have possibly developed the taste for it or the odor preference or any of that.

I mean, do you—you don’t come to come, I mean, is it—is it in some sense like partially decomposed food? You know, maybe with that a little bit?

Dr. Wrangham: So, you know, softness. I mean, in general, animals like their food soft because harder food is tougher, and you’re going to do more chewing, and so there’s a texture issue; there’s a texture with you, and probably the taste. You know, as you say, a little bit of decomposition.

So if proteins have been partly denatured, then you have more exposure to the amino acids that have a bit of sweetness to them, have a bit of umami flavor, and all of those are indicators to even to an ape that the food is relatively going to be relatively easy to digest.

Right, it’s bioavailable and ready for use—right, right, right. But actually, no one has—the spontaneous interest in the odor of cooked food, and that’s something would be really fun to do. But certainly, a striking character.

Right, well, it’s certainly a striking characteristic of human response to especially caramelized meat, which has a particularly distinctive and delicious odor.

Yeah, right, right. So, you know, this is still early science. We don't know that much about it. But what we do know is that animals, including the great apes, prefer their food cooked even when they haven't had any exposure to it. You know, when they're naive.

And that once you are eating cooked food, you are getting more calories; you're saving yourself the cost of digestion, and you’re actually increasing the amount of the food that you are able to digest as well. So the net result—in experiments that my colleagues have done—I mean, you get indications of 30, 50 percent increase in the number of calories. I mean, this is huge, you know? For five percent more milk out of their cows, you know, they’re a millionaire. And now we're talking, you know, some tens of percent—right, right! Essential a doubling of available calories, and—and you also pointed out, not only the caloric improvement, but the radical decrease in the amount of time it takes to do the processing, like the chewing.

Right, so yes, so this would be really big. And I think what it, I mean, the reason the argument I make about why this happened surely at two million years ago is kind of twofold. One is that everything fits at that point. So that’s the point in which you see smaller mouths, smaller teeth, evidence of smaller guts to judge from the shape of the ribs and the pelvis.

And by the way, for the first time, a commitment to sleeping on the ground. Because for the first time you have a construction of the organism, you know, the human.

In a way that is not easy to climb trees.

Ah, so that made it possible for us to not have to climb trees. That’s the idea at least, yeah.

And the Australopithecines could climb trees, so they surely would have slept in trees, but humans nowadays—you can’t imagine them being able to regularly climb up into trees and always be able to make a nest in the way that a chimpanzee or an Australopithecine could.

So they’d have to be nuts to sleep on the ground if you don’t have fire to protect you.

So I think that all points to why it makes sense that fire was first controlled around 2 million years ago. Literally about 1.9 million years ago.

And then the other reason is that subsequently to that point, no dramatic events happen in human evolution which could be consistent with the acquisition of something so important as the control of fire.

So, you know, there is no subsequent time that makes sense in terms of, “Oh yeah, they must have got fire and therefore what?” Nothing happens. You know, you get a steady increase in brain size; you get a variation in tooth size where generally it’s declining, but no big things.

Okay, so we’ve touched on autobiographical issues; we talked about your work with Google; we touched on "Demonic Males," and the raiding. And now we’ve touched on cooking and fire use.

So maybe—because I want to get to your third book before we run out of time too—you got really, really interested in aggression, particularly aggression among males, and the particular forms it takes in human beings and our close relatives. But then in your last book, "The Goodness Paradox," you really turned your attention to how that proclivity for extreme aggression came under some degree of social control. And that’s a fascinating issue in and of itself.

So maybe we could talk about that. You distinguish first between two kinds of aggression: proactive and reactive—a classic distinction.

So do you want to start there and then elaborate out the thesis? Is that a reasonable way of approaching it?

Dr. Wrangham: Yeah, it is, so yes, for early—so "The Goodness Paradox" is the title of this book, and it refers, as a paradox, to the fact that humans are so extreme with regard to aggression and non-aggression.

So we’re extremely aggressive in the sense that, like chimpanzees, we have these demonic tendencies to go off and kill members of neighboring groups using our overwhelming power when we can get a big group attacking a small group and so on, in a way that no other species—not the mammals do! And in a way that horrifies us in retrospect often and makes us drop our jaws at our own behavioral possibilities.

Yeah, I mean, you know, they were still living in the shadow of World War II and the Holocaust. And so, you know, for so many—I mean, almost everybody who writes about human behavior is affected by that and still thinking about how did that come to be and how do we avoid it in the future?

So, you know, there’s that angle on humans, and the other angle which makes human behavior so paradoxical is that we are the kindest and most tolerant and most gentle of animals. And people since the ancient Greeks have said, "Well, like a domesticated animal."

You know, we meet strangers; we’re so nice to each other. We don’t have automatic aggression. We’re nothing like wild animals. That we share food and make that the basis of many of our social interactions.

Yeah, and so, you know, for decades—well, for centuries—people have tended to solve this paradox by saying either that we are naturally aggressive and we learn culturally to be nice to each other, or vice versa.

And so, you know, the famous debate between Hobbs and Rousseau—as people put it now—Hobbs takes the naturally aggressive perspective, and Rousseau takes the naturally kind perspective.

And this goes on. I mean, there’s a book recently published by a Dutch historian called Ronald Bregman who’s called "Human Kindness," saying, "Well, actually, humans are spontaneously naturally kind."

So, you know, it’s absurd in retrospect to think that people are trying to arbitrate between these two views, that one is correct or the other is correct, because they're both right.

You know, I think it’s very, very clear that humans have got tendencies for appalling violence, which will be elicited under the appropriate circumstances regardless of what ethnicity, or culture, or religion you come from.

And equally, it is very clear that people grow up to be spontaneously thoroughly moral and kind and tolerant with each other. We have these two tendencies.

And you mentioned this division between two types of aggression—a proactive and reactive. It’s a division that, as you, as a psychologist said, it’s a familiar distinction, that’s fine.

But I will take credit for bringing it into biological anthropology because people thinking about the evolution of human behavior for some reason did not apply it.

Well, the short story is this: The way to think about human aggression and non-aggression is that we are relatively elevated for the propensity for proactive aggression because all of the war—and the Holocaust and your stuff you’re talking about—that’s all proactive, right?

That’s planned—that’s multi-party—that’s organized—it’s social. It’s—it’s one group against another, yeah.

And it follows exactly the chimpanzee principle of imbalance of power. You only do it if you can get away with it and feel very comfortable that you’re not going to get hurt in the process, so, you know, the killers of the Jews and the gypsies and the homosexuals, and the Holocaust—very, very rarely got any pushback. They were butchers.

And that’s the nature of proactive aggression. So humans are elevated for that in the sense that we have a very high propensity to do it if we—the circumstances are right— which is right.

And part of that circumstance is that we’re defining the—the entities upon which that aggression is afflicted as outside what constitutes human because, within the human definition, all the standard rules of morality apply, and that’s sort of equivalent.

Correct me if you think I'm making an error here. That’s sort of equivalent to the chimp distinction between the chimps that are in their own group and the chimps that are from another group altogether.

Dr. Wrangham: You're right. And by the way, in a shocking addition to that, if you take the ethnographies of people living in small-scale societies—including hunters and gatherers and small-scale farmers—you find the same thing. You find that within the ethno-linguistic society, in other words, the people who speak the same language, we are humans, and the other people are not humans.

Right, right.

And that’s a very common—well, and it’s, I guess it’s also partly because you can rely on those who are like you. It’s almost the definition of them being like you that they accept your definitions of right and wrong so that you can predict their behavior. You can enter into a social contract with them with implicit understanding, whereas with an outsider, you don’t know what rules apply, and so their behavior isn’t predictable, and they don’t obviously fall within the overarching definition of moral.

And maybe that’s what underlies the definition of human in some sense for us.

Yeah, I mean, that seems like a reasonable explanation. But the net result is that even a total stranger in a completely weak state, the first thing that you do—you’re living in a small-scale society—when you encounter such a person is not to say, "Here, have a cup of tea and let’s find out your morals." It’s a killing.

Right, right!

So, you know, I say this because there’s a romantic view that among small-scale people particularly, hunters and gatherers, there is this extension of generosity of spirit to people of different languages.

And you can argue very strongly against that.

So let me ask you a question that just popped into my mind about that. I mean, it’s a compelling idea; I know that in the— in I think it’s in Genesis, in the Abrahamic stories, there’s tremendous emphasis on the hospitality that has to be shown to a stranger.

And so that seems to be an ex—that seems to be an exception to that general principle. And so I think it’s a definition of the stranger that’s the issue.

You know, to me, what you’re talking about would typically be a stranger who you do not know personally but who speaks the same language as you—who’s part of, you know, the larger series of Judaic tribes.

Right, right.

Yeah, right!

So still in—still encapsulated within the idea of what constitutes the central people, right?

Yeah, okay, okay.

Okay, well, that could well be. And that, I mean, and obviously that’s going to be the case from a historical perspective because the idea of everyone who’s morphologically similar being human—that idea obviously must have moved out from tribe to slightly larger group to larger groups still and so forth as our groups got bigger and bigger.

Yeah, well, and maybe we’ve got some more—one world!

Right, right, right!

Well, at least we do that to some degree.

Automatic enemy is horrendous, right? He says you really have to think yourself back into a very different past to understand this.

So that’s all proactive aggression—the proactive aggression—the use of power to damage anyone outside your group.

And then the reactive aggression that you describe is also characteristic of other—of many, many other animals who engage in male-to-male conflict.

So that’s not unique to human beings. And that involves emotional reactivity. It’s impulsive, it’s immediate. All this defensive.

That’s right; it’s all those things! So it’s what we ordinarily think of as aggression.

Because so many people think of proactive aggression as something that is sort of just cultural and just human-taught and that sort of thing, and even though there are very important cultural elements to it, it’s part of our biology.

But reactive aggression is what people, if you look up aggression in a textbook, animal behavior, it’s almost all about reactive aggression—often exclusively about it. So reactive aggression is testosterone-fueled; it’s losing your temper, as you say. It’s impulsive; it’s motivated by anger—not only by anger but anger, frustration, shame associated with emotions.

That’s right.

And what’s striking about humans is that we are very downregulated for reactive aggression compared to our close relatives.

So—and the way that you can see this manifest is that the rate at which you get actual physical aggression happening in a small-scale society in humans compared to in a group of wild chimpanzees is two to three orders of magnitude—difference. That is to say, hundreds to thousands of times less frequent in humans than in chimpanzees or in bonobos too. You know, the famously peaceful bonobos. But nevertheless, bonobos are not nearly as peaceful as humans. So we have—we’re way down the scale of reactive aggression, and at the same time, we’re way up the scale on proactive aggression.

And "The Goodness Paradox" is a story of how did we get this astonishing mixture, and I think—you know, we actually have a really good story for it now—a really good understanding.

And it, uh—well, some of it in the book, some of it you outlined—intense attempts by people within human social groups everywhere to socialize children into controlling their reactive aggression.

Yeah, and I know there’s literature. I interviewed Richard Tremblay on this YouTube channel.

I saw the podcast!

Oh, okay!

So, you know, we talked about the studies showing that a small percentage of two-year-olds are spontaneously aggressive if you put them in groups of other two-year-olds. It’s only about five percent, and virtually all of them are male. And virtually all of them are socialized out of that by the time they’re four, but a small proportion aren’t—and they tend to be lifetime aggressors, and those are the ones that—

Well, I mean, I think that’s the residue of a population that would have been 100% like that if you go back 300,000 years ago.

You know, all our babies would have been highly aggressive and would have retained that aggressiveness throughout life. And the reason we can say that is, you know, there are two main points. First of all, our anatomy compared to our earlier ancestors looks like the anatomy of a domesticated animal compared to a wild animal.

And what I’m saying, our earlier ancestors, I’m thinking very specifically of what happened around 300,000 years ago. So this is when we first get the first glimmerings of our species moving into Sapiensization—Sapiensization, the process of becoming Homo sapiens.

People now say it started about 300,000 years ago; that’s thanks to fossil discoveries in Morocco. And that’s when you first start getting smaller teeth and smaller mouths indicative of a trend that will get increasingly strong.

By the time you have Homo sapiens as a recognizable species, you have several of the characteristics that archaeologists use to mark a domesticated animal compared to its wild ancestors. The four characteristics they use are smaller teeth and jaws, reduced differences between males and females, reduced sexual dimorphism, and a reduction in body size. And those three things all happen fairly early in Homo sapiens.

And then the fourth thing is a reduced brain size. Astonishingly, there is evidence for reduced brain size in Homo sapiens compared to our earlier phases.

Right, but not necessarily any loss of cognitive power—just like in domesticated animals, there’s no evidence of a loss of cognitive power in domesticated animals compared to their wild ancestors, even though they have smaller brains.

And you, I think, you posited that that was a consequence of decreased size of the brain areas associated with reactive emotions.

That’s part of it anyways. Yes, you may be extending slightly beyond what I said, but I actually do think that that’s right; it seems to me that there’s quite a bit of evidence that part of the contribution to brain size is associated with reactive aggression.

And so, you know, bonobos are less reactively aggressive than chimpanzees; they have smaller brains than chimpanzees. Females are less reactively aggressive than males; females in a whole bunch of species have smaller brains. Domesticated animals all have smaller brains than their wild ancestors.

And there’s even some very provocative evidence that if you give testosterone to humans, then it increases the size of the brain—even to adult humans.

So you mentioned in "The Goodness Paradox" some early hypotheses about how human beings might have become domesticated because obviously we domesticated animals. And there were wild hypotheses, like some sort of super race that had domesticated us all and then disappeared.

And these were very early speculations, but you have a hypothesis about how this might have come about that doesn’t involve such, what would you call, extreme speculation.

Yeah, and it comes originally from Christopher Boehm, who was an anthropologist who went to look at chimpanzees, and he was really struck by the huge difference between humans and chimpanzees in the existence of an alpha male bully.

In chimpanzees, you have an alpha male bully who gets what he wants by using his personal physical power. In humans, you don’t have that. And we often talk informally about humans having alpha males, but it’s not an alpha male in the primate sense because the human supposed alpha does not get that status or achieve what he wants by using his personal physical power; it’s all through coalitions.

But in chimps and bonobos and baboons and every other primate, it’s not by coalitions; it’s by his personal physical ability to defeat everybody else. Now, you sometimes get in humans—in small-scale societies, you sometimes get a man who tries to do that—who tries to, you know, kick sand in the face of every other male and take their wives and take their resources and make them feel small.

And thoroughly mean to them. Just by being the bully on the block. And when that happens, there is a consistent solution because you have to think about societies in which there’s no police, there’s no one to help you, you’re just on your own in your society.

And here is this guy who is being incredibly objectionable, and may actually kill other people, but even if he doesn’t kill them, he’s trying to—he’s taking their wives, he’s pushing them around. So there’s various kinds of social responses.

People can try pleading with him to behave better; they can laugh in his face; they can ostracize him; they can ignore him; they can try and move away and exile him just by going away. But none of those mechanisms will work in the face of a really determined desperate.

So in the end, he gets killed.

Now that’s what happens nowadays, and what Christian’s weapons—weapons would have contributed to that too, I suspect, because it’s easier to be a bully when you’re huge and everyone else is small, and they don’t have weapons.

But once weapons emerge, the advantage of physical strength is decreased substantially, at least in principle. Is that—

Well, yeah. I mean, weapons play a funny role, and it’s almost certain that the kinds of weapons that would be needed for a bully to use them existed long before 300,000 years ago.

You know, there are very nicely preserved spears from 400,000.

Right, yeah, I’m thinking more about weapons used against the bully.

Dr. Wrangham: Well, yes, but, you know, animals without weapons can kill others quite safely. Even nowadays, you have descriptions of humans killing without weapons. So how much weapons you really mattered is unclear.

You can kind of argue it both ways, but either way, the—you see in the present is the argument that is taken back into the past, and I like this argument a lot because I think it is a really tidy explanation for the fact that beginning around 300,000 years ago, we had this reduction in reactive aggression, and there is no other explanation other than a communal effort at executing that can account for the removal of this would-be desperate—this bully who uses his physical strength.

That’s why no other primate has escaped having an alpha male bully. Only humans have converted an alpha male bully into an alliance of males among whom there is a sort of formal level of equality.

And if anybody in that alliance tries to throw their weight around, they know what will happen—they will get taken out as the bully originally did.

Right, so you paused in "The Goodness Paradox" that there’s been enough of that in human societies over the last very long period of time that have markedly decreased the propensity for reactive aggression to such a degree that it’s also transformed our morphology and our and our psychology.

Absolutely, at the biological level.

Right, right. And, you know, it probably accelerated over time—the loss of reactive aggression—the move towards this domestication-like species that we are now.

People sometimes want to suggest that other forces,