Video: The Secret of Our Success: How Culture Is Driving Human Evolution, Domesticating Our Species, and Making Us Smarter

The ability of human groups to socially interconnect and learn from one another has allowed us to create ingenious technologies, sophisticated languages, and complex institutions that have enabled successful expansion into myriad environments. Drawing insights from lost European explorers, clever chimpanzees, mobile hunter-gatherers, neuroscience, ancient bones, and the human genome, Joseph Henrich, author of The Secret of Our Success, will discuss how our collective intelligence has propelled our species’ evolution.

"Mind-stretching…Henrich’s book will take you on a prodigious journey through human nature and society." —Alun Anderson, New Scientist

Joseph Henrich, Professor, Department of Human Evolutionary Biology, Harvard University and Co-director of the Centre for Human Evolution, Cognition, and Culture, University of British Columbia

Co-sponsored by Harvard Museum of Natural History

Recorded February 24, 2016


[00:00:05.14] And it's a real pleasure to be here to introduce my colleague Joe Henrich. Joe studied at Notre Dame and UCLA where he earned degrees in both anthropology and aerospace engineering-- just an interesting combination. And he taught at Emory and the University of British Columbia. And he's the only person on the planet, I believe, who-- I could be wrong, but I'm pretty sure that this is a safe statement-- he's the only person on the planet has been granted tenure in the following four disciplines-- anthropology, psychology, economics, and human evolutionary biology. But despite this array of accomplishments, we managed to recently woo him to Harvard, and I'm really pleased that he's now a professor in the Department of Human Evolutionary Biology.

[00:00:48.71] And we really couldn't be more thrilled to have him here because, if alien biologists ever landed on this planet, and they were asked to describe various creatures, including us, you can be sure that the feature they'd probably single out as most distinctive about human beings is our capacity and proclivity to use culture in almost every context imaginable. And maybe our second most distinctive characteristic is our proclivity to cooperate as we're all doing here making seats for each other in the room. It therefore follows that if you ever-- if we ever really want to understand why humans are the way we are, and we need to understand, therefore, how culture evolves, how and when we cooperate, and how cultural evolution interacts with biological evolution, which made all this possible. And that's what Joe studies, and that's why we're here tonight, to hear him. Because he's probably the foremost scholar in the world who studies the evolution of cooperation and the capacity for culture in humans.

[00:01:48.57] He does this using several major approaches. First, he conducts really ingenious, and creative, and innovative cognitive and behavioral experiments in many different cultures across the globe. Second, he's a very sophisticated theoretician who has developed a lot of important new mathematical techniques for analyzing cultural evolution. And finally, he goes out into the field and collects empirical data from small scale societies in many places, including-- most of them seem really warm and pleasant, like the Amazon, rural Chile in the South Pacific-- South Pacific sounds particularly nice at the moment-- and he uses this using a variety of ingenious techniques from psychology, and cultural anthropology, and economics.

[00:02:31.66] He's published hundreds of papers, but the most cited, by far, is his very famous Weird Paper, in which he points out that most research subjects that are used in studies of behavior are highly biased. They're Western-educated, industrialized-rich, and democratic-- weird. And although these adjectives characterize all of us in this room, I hope you'll agree that we are definitely not very representative of our species as a whole.

[00:02:57.91] And so by now you know that he recently published a book on these topics, The Secret Of Our Success: How Culture Is Driving Human Evolution, Domesticating Our Species, And Making Us Smarter. The latter part sounds very important. And by the way, having read it, I can promise you it's a really entertaining, delightful, and thought-provoking book. I particularly recommend chapter 5. So, I hope you enjoy the lecture, and without further ado, it's a real pleasure to welcome Joe Henrich. Thanks.

[00:03:25.40] Thanks Dan. And hi everyone. Thanks for making it out on this wet evening.

[00:03:29.84] So I want to begin by talking about the ecological success of our species. And of course, in the modern world, it's obvious that we're highly ecologically successful. So 98% of the terrestrial vertebrate biomass consists of humans and our domesticated animals.

[00:03:45.47] But the puzzle of our species' immense ecological success extends well back into our species evolutionary history. So long before the origins of agriculture, or the first cities, or industrial technology, humans are expanding out of Africa. So our species emerges in Africa and expands out-- so 100,000 years ago or a little bit. More recently, we expand out of Africa, 70,000 years ago into the southern part of Asia. And then, sometime after 60,000 years ago, we arrived in Australia, after 50,000 years ago into Europe, and then into the Arctic some time after 40,000 years ago, and then eventually into the New World, all the way down to the tip of Tierra Del Fuego, before the origins of agriculture.

[00:04:26.90] And as we spread into those environments, we ended-- or we entered an immense diversity of environments. So from the arid deserts of Australia to the malarial swamps of New Guinea, up into the Arctic tundra in Siberia, and eventually Canada, mountains in Tibet, and then an immense diversity of environments in the New World. And what's interesting about our species is we go into all these environments, but we have very few genetic adaptations to these in diverse environments.

[00:04:58.00] And this important in thinking about our success because when we look at other species that have been very ecological-- ecologically successful-- so if we look at ants, we find that ants, they control a lot of biomass, they go into a lot of different environments, but they have an immense number of environment-specific genetic adaptations. In fact, they've speciated into over 14,000 different species. But we're a relatively genetically homogeneous species, yet we inhabit this immense diversity of environments.

[00:05:24.29] So the question is, how did we do it? Now, for probably for many of you this wasn't-- you hadn't thought much about this being a problem because there's an obvious answer. The common sense answer is we're intelligent. We have big brains, lots of mental firepower, and we can just figure out how to solve problems. And it's this ability to build causal models on the fly that allows us to enter all these different environments.

[00:05:47.26] And I want to start this talk by trying to challenge this idea that it's just our intelligence, our raw brain power that gives us our ability to enter diverse environments. So I want to begin by dipping into the lost European explorer files. Now this is a set of files that Rob Boyd and I have been putting together. And there are cases where lost European explorers, or sometimes Americans, end up stranded while exploring in some environment that's inhabited by hunter gatherers.

[00:06:15.91] And so the explorers then need to survive. They need to find food and water. They need to know how to travel and make tools. And I like cases where-- well, there always has to be a group of hunter gatherers that's been surviving there, preferably for centuries or millennia is usually the case.

[00:06:33.25] And they need-- I like them to have lots of food and security at the beginning. So I don't want them in an emergency situation. They need lots of time to warm up to the environment. And then what be find in case after case is they flounder. They can survive as hunter gatherers.

[00:06:46.43] So this is the story of Burke and Wills. So it's 1860, and they're endeavoring to be the first Europeans to head across the center part of Australia. It's a very interesting story. I'm going to cut it kind of short, get to the punchline.

[00:06:59.83] They make it up to the Gulf of Carpentaria, and they're heading back, and things are starting to go wrong, and they decide to make a last ditch run for a police and ranch that's prophetically called Mount Hopeless. So they begin heading along Cooper's Creek, and their last camel-- they'd imported camels from India for the expedition. The last camel dies. It gets stuck in the mud and dies.

[00:07:20.90] And this means they can't get the water they need because they needed the camel to carry the water to get across the last stretch of desert, to get to the ranch and police outpost. So they're essentially marooned along Cooper's Creek, and they're hoping a rescue party is going to come from Melbourne. So they try fishing and hunting, and they can't get enough calories from fishing and hunting to stay alive. They occasionally get stuff, they have guns but they're running out of bullets and things are going wrong.

[00:07:45.03] But things start to look up when they meet the Yantruwanta So those are the local hunter gatherers who have lived in this environment for a long time, and they're able to get gifts of fish from the locals. And they see the locals making cakes and gruel from a small sporocarp.

[00:07:59.89] And they manage to track down the sporocarp after quite a bit of wandering around. They initially thought it would be in a tree, and it turned out to be on the ground. And they know how to bake bread, so they grind up the sporocarp. And it looks like they're going to get enough calories because they're getting the occasional gifts of fish, and they're able to make lots of gruel and cakes from these things.

[00:08:19.69] The problem is when they were in the aboriginal camps, they didn't notice that this is processed by the women by two possible methods. One is to grind it, leach it, heat it, and then use it with a mussel shell to eat it. Or grind it, leach it, and then bake it in ash.

[00:08:35.60] And either of these methods has an important effect on the nardoo because nardoo is toxic and indigestible unless properly processed. You've got to remove this enzyme called Thiaminase, otherwise it depletes the thiamine in your body, the B1, and you get a horrible disease called Beri Beri, and eventually you die. And this is what happened to Burke and Wills.

[00:08:55.49] Now there was a third member of their party still alive at this point named King. King, he's having the same kinds of symptoms, and we know this in part because William Wills is-- he's a good Victorian right? So he's writing in his diary as he's dying describing the experience and everything. You can read it online it's actually interesting reading. So King wanders out into the desert, and he's eventually picked up by the Yantruwantu and eventually rescued by a search party that came from Melbourne. So we know the story from King's account and also from William Wills' diary.

[00:09:27.51] So what this and many, many tales like it tell us is that Burke and Wills, they had three with three months of food with them initially. They couldn't survive as hunter gathers in a place where humans have been surviving for over 60,000 years. So no modules fired up that allowed them to find edible plants and make fire. No instincts kicked in. No general intelligence bailed them out. So they couldn't do basic things like find water or identify edible plants, but they were able to find deadly plants.

[00:09:55.03] Now you might think that that's kind of a lot to ask to be able to do these things, but we have a little bit of a controlled experiment because I mentioned those camels. Well some of those camels had escaped from Burke and Wills earlier in their adventure, and there's now thousands and thousands of feral camels all over central Australia. So the camels survived the lost European explorer.

[00:10:14.01] Camels can smell water from a mile away, and they have taste cues which allows him to find certain kinds of plants. They're really good a detoxifying plants when they eat it. And they actually have taste cues that help them find plants that are high in protein.

[00:10:27.42] So they've got lots of great adaptations that Burke and Wills and other humans lack. So the camels beat the test. And their brain is less than half the size of the Homo Sapiens.

[00:10:38.95] So these guys, they couldn't hunt effectively or make any of the tools. And one example of the knowledge they lacked was this plant called spiniflex. If properly processed, spiniflex makes a powerful cement-like adhesive. But you have to know that you have to harvest these grains from this kind of-- doesn't look like a great plant to me-- and put them in fire and actually mix kangaroo dung in with them to get them to really work like a good cement, very non-intuitive aspects of the process. So they can't do that.

[00:11:07.64] But any local adolescent could have done these things. So you can ask yourself, well what were Burke, and Wills, and King missing that these local adolescents would have? Well the answer is obvious, that they were missing this large body of inherited know-how that teaches you about processing nardoo and teachers you about spiniflex and helps you find water in the desert, where otherwise this would be very difficult to find. So this body of cultural knowledge seems to be key to understanding humans in a way that it's not for understanding camels.

[00:11:36.41] OK. Now I want to come at this from another direction, which is to look at some work done by Mike Tomasello and Esther Hermann and their colleagues at the Max Planck Institute for Evolutionary Anthropology in Leipzig. So what these researchers did is they created a contest between three different apes, so orangutans, chimpanzees, and then humans. The humans in this case are 2 and 1/2 year olds, so I substituted a picture of my son Josh who happens to be 2 and 1/2 years old.

[00:12:05.75] And they gave me a battery of cognitive tests, 18 different cognitive tests in different areas. So you can break the test down into tests about space, quantities, causality, and social learning. So there's a bunch of subtests in each of these.

[00:12:18.89] And this is the percent correct. And each of these species is very interested in snacks. So if you get the test right, you get a snack. So there's incentives for performance.

[00:12:29.10] And so you can see the humans and the chimps, the 2 and 1/2 year olds and the chimps, do about the same in the space test, the orang's a little worse. Chimps slightly beating them-- although within the margin of error for sure. In the quantities test, causality test, slight victory to the chimps, margin of error though, and the orangutans seem to lose by a little bit each time.

[00:12:47.94] But the place where the kids clean up is in the social learning test. So, in this case, the kids are up in the 80s and the apes-- or the non-human apes-- are all down here below 10%. So this is a big difference. In fact, this is actually deceptive because, when you talk to Esther Herman, the lead author on the study, it took her a while to find a test where she could get the apes off the floor from 0% and get the kids away from the ceiling, get them down from 100%, because kids are such good social learners.

[00:13:16.42] OK. Now you might think, well this is kind of unfair to the humans because you're using 2 and 1/2 year olds and these apes are all different ages. So there are some infant apes-- so less than five years old-- and then older apes. And that might seem unfair. But what's interesting is the apes don't get better with age on these cognitive tests. In fact, in some of the cognitive measures, the infants-- the five-year-olds, four-year-olds-- actually do better than the older apes.

[00:13:40.23] But in the case of Josh and his cohort, they're going to continue to get better and better at these tests into the third decade of life at least. And by the time they get to be in their 20s, they would ceiling on all these tests. They get 100% on all the things, and so they would crush the apes. So something is going on over these few decades. And I'm going to be telling you about that in a second, or at least part of what I think that is.

[00:14:02.53] I wanted to mention, though, in the book, I also discuss interesting research which compares undergraduates in terms of their working memory abilities, which is often thought to be a center piece of IQ and intelligence, the working memory, in which the chimps do quite well in comparison to the humans, the undergraduates, in working memory. And in measures of strategic thinking recently done by some Cal Tech economists, in collaboration with some Japanese primatologists, the apes outperform the humans. They're able to zoom in on the Nash equilibrium, the optimal behavior, in a way that the humans systematically miss it. So they're better Machiavellians than humans.

[00:14:38.21] OK. Now why are we so smart? That's an obvious question. We know we're smarter than these apes. We're better at solving problems. Why? Well part of the reason is that we culturally inherit lots of pre-built solutions that have culturally evolved over long periods of time.

[00:14:50.24] So, some examples is if you think about simple tools like things like screws, springs, levers, and pulleys, these things turn out to be hard to invent, and I'll give you some examples in a minute. But if you grew up in a world where these tools already exist, you get to see them in operation, you get to learn their affordances, and you can redeploy them to solve new problems. But hard to figure out if you never get never get to see them before.

[00:15:10.69] My favorite example of these is the wheel. So if you learn your technological histories from the far side, you might think that the wheel goes well back into the human paleolithic, and that paleolithic peoples were using the wheel. But wheel's actually relatively late in human history, so about 6,000 years ago. You get a potter's wheel. You get wheels on carts like this, wheels for milling grain.

[00:15:36.94] But that only appears in Eurasia. So in the New World, no wheels except on Mayan toys-- no wheels in Oceania, no wheels in New Guinea, no wheels in Australia, no wheels in Africa. So something as simple as a wheel doesn't appear in these other places. Then once you have the wheel, you can do all this different stuff with it.

[00:15:54.60] Sometimes there is that-- I put this picture of the dog up there. This is in Belgium. There's this idea that there is no domesticatable animals to use the wheel, but everybody had dogs, and the Belgians hooked their wheels up to dogs. Also elastically-stored energy. So in Australia, you don't get any tools with elastically-stored energy or compressed air. So that's a whole continent which doesn't get these concepts.

[00:16:17.73] Number systems. So all of you, I think I can say with great confidence, have inherited a number system that allows you to count without bound, neatly packaged into groups of 10. You can perform all kinds of cool operations with it. But anthropologically-known societies, like the group I worked with in Peru, the Machiguenga, just count one, two, three, many. So they don't have a way to digitally distinguish between 16 and 17, or 34 and 35.

[00:16:43.23] And then when you study comparative number systems studied by anthropologists, you find not every combination, but almost any combination, some groups count to 11, some groups count to 12. It's often a kind of body part counting system. So you can see this group counts to 27, but then there's no way to do 31 and 32 with this system. And so the gradual evolution of numbering systems over time, we get this free of charge.

[00:17:09.56] Another interesting one is spatial cognition. So, as a consequence of speaking English, you all have access to three different spatial coordinate systems. So the first is absolute, so North, South, East, and West.

[00:17:20.74] The second is an object center. So you have a front, and a back, and a side. Your house has a front, and a back, and a side. You can say, I'll meet you in back of the house. I'll meet you in front of the house. Those things are readily distinguishable.

[00:17:31.25] And then there's a relative coordinate system. I could say, it's the guy to the right of the camera, and that draws a line between me and the camera and goes to the right if it or to the left. It's a relative coordinate system.

[00:17:42.31] But other groups studied by anthropologists just have one. They just have North, South, East, and West. So you'd always have to say it's the guy to the North of the camera or the South of the camera. There's no relative coordinate system to use.

[00:17:53.70] And you can imagine something like driving on the left or driving on the right, it has a certain kind of usage that allows you to solve new kinds of problems, where otherwise you'd have to say for every street you have to memorize North versus South or something like that. So it'd be awkward in some cases. So that's something that's evolved over cultural evolutionary time. OK. And one of the things in the book is you can read about lots and lots of these examples.

[00:18:19.86] So the case that I make is that it's not our intelligence, meaning or raw, individual ability to solve problems that leads to our species immense success, but our cultural capacities, that accumulated body of know-how the Burke and Wills didn't have. And the fact that we can learn from each other and we can pass this down over generations, in just a particular way that I'll talk about, gives rise to these cultural adaptations. So these are pre-built solutions to problems that we face in the environment.

[00:18:48.67] So a couple of keys to this, one is that in order to get this going-- well it works best if your high-fidelity cultural learner, so you can observe other members, your social group, and infer underlying goals and mental states and action patterns. You can get it going a little bit if you're less good at that, but the better cop you are, the better that goes. And also you have to be social. So the more interconnected you are, the better the system can run.

[00:19:14.78] Now you might-- some people try to say, well, this is actually a measure of intelligence. But when we-- anytime we try to, sort of in a common sense way, measure intelligence, one of the things is we say is you can't copy off the other guy's paper, which seems to be the kind of gut reaction. You don't have to do that with capuchins, for example, if you want to test their intelligence. You don't have a copying issue with them.

[00:19:34.52] And so, if this is the case, then it gives rise to the importance of collective brains. That actually, the ability of a group to generate knowledge and know-how and accumulate it over time, and come up with more sophisticated repertoires, and medical systems, and technologies, depends on the size and interconnectedness of the population. Larger populations that are more interconnected runs-- this process runs faster and it runs farther.

[00:19:58.06] OK. And then finally, for the last point, I'm going to argue that this process-- collective brains and this cultural cumulus system-- has actually driven much of our species genetic evolution. So that if you want to understand human evolution, you have to understand it as a dual inheritance system, where culture is often the driving force in much of our species genetic evolution. Now the trick and in approaching this-- and I think one of the reasons why there were maybe delays in getting where I think I'm going-- is that for a long time, these two views were opposed. You had explanations that were based either on genetic evolution on the logic of natural selection to explain human behavior, or you had cultural, learning-based explanations.

[00:20:37.97] And in the book, I make the case that the trick is-- and this trick has been around for 30 years or so now, but-- is to take the logic of natural selection and say, how does it shape culture and cultural evolution? And specifically by thinking about, how does natural selection build learning machines? So how did it shape us to make us more adaptive learners, to better extract ideas, beliefs, and values from other members of our social group. And also when to say, well, now is the time to rely on instinct, or now is the time to rely on individual learning, on your own independent experience.

[00:21:07.67] And one of the things that's interesting about humans is we seem to have gone very far down this road. So we are much more inclined, than any other species that does social learning, to rely on the information that we acquire from others, versus our own direct experience and our own instincts. And I'll give you an example of that in the case of chili peppers.

[00:21:25.81] All right. And then the key move here is that back to genetic evolution. So this builds stuff that then affects this process. So you have a feedback loop here.

[00:21:34.23] I wanted to give you a little bit of sense about how you can take the logic of natural selection, of genetic evolution, and think about the evolution of our capacities for culture. So you want to think about the cues that a learner might use to shape what kinds of ideas, beliefs, and values they pay attention to, who they pay attention to in their social milieu, and how they integrate different kinds of information. And all of those are kind of semi-independent research projects that are going on in the cultural evolutionary world now.

[00:22:01.72] My favorite one is how people figure out who to pay attention to in their social milieu. So both theory and now quite a lot of evidence, evidence from babies, evidence from young children, as well as all adults, suggest that people rely on cues of skill and competence. So just-- if you're a young hunter-gatherer and you want to figure out how to be a good hunter, you might tend to pay attention to the people whose arrows tend to hit the target if they use bows and arrows.

[00:22:28.36] You might use cues of success. So in that case you might preferentially attend to and learn from the guy who tends to bring back the most big prey. So for example, I do field work in Fiji-- and this is a scene from my site-- and you can see as soon as Lakema, the best underwater fish spear fishermen in the village, came over to dismantle this sea turtle-- now this guy can't take apart this sea turtle because a hunter can't carve up his own prey. So someone else had to do it.

[00:22:54.57] He tried and failed. Turtles are tough to get apart. But Lakema comes over and he starts immediately tearing the thing apart like a pro, and the kids crowd around because they're going to watch whatever Lakema does because he's by far the most prestigious hunter and-- fishermen and hunter in the village.

[00:23:12.57] Prestige is next. So if everybody's kind of playing this game of trying to figure out who to learn from and who to pay attention to, then you can actually use what other people are doing as a way to figure out who you want to learn from. So if you see certain people receiving deference, people are giving them the floor, it means those other people think that someone is worthy of paying attention to. So you can use that as a cue to help refine who you're going to pay attention to. And we've done a bunch of experiments on this among three and four-year-old children.

[00:23:39.37] Age can also be a useful cue. So age is why five-year-olds tend to want to hang around and pay attention to the seven-year-olds because they can scaffold themselves up to increasingly more complex skills by not copying the oldest guy, the best member of their community, but rather by using age as a cue to scaffold themselves up to increasingly more success.

[00:24:02.81] Another kind of cue is old age. So in small-scale societies, not everyone gets to be old. So there's actually a filtering process going over the course of people's lifetimes, and if you get to be a senior member, a mature member of a small-scale society, you have actually shown something. There's an information content to the fact that you get to be an older member. So people pay attention to the wise, old elders because there are a few of them.

[00:24:28.88] And finally, self-similarity can help you hone your learning into things that might be useful to you later in life. So if the sexual division of labor is at all old, and at least some paleoanthropologists think it is, then males should be inclined to copy males, and females should be inclined to copy females. And if there were ethnically-marked differences for over the course of human evolutionary history, then we should use cues of say dialect or dress to figure out who to learn from. And there's good evidence for that as well, actually done here in psychology.

[00:25:00.96] Now we know that these things affect what a wide range of domain. So this is just a small sampling of the domains that are important. Food preferences, so which food do you like? If you want to get a kid to eat a certain kind of food, put him at a table with slightly older same-sex kids who love the food that you want to get him to eat, and then he'll change his ranking. And there are some interesting experiments on it. Telling him that he should eat that food, not a good strategy.

[00:25:24.77] It affects things like mate choice. My favorite is suicide. So people-- when a celebrity commits suicide, you get a-- you can get a rash of suicides. The more prestigious the celebrity, the larger the spike in suicides. And people match on sex, and they match on ethnicity. So you can see the cues even on suicide. Very bad for your fitness, suicide is. All kinds of domains.

[00:25:46.47] So this seems to be something that develops reliably. We found it in Fiji. We find in other-- lots of other societies.

[00:25:51.73] Develops early. You can see it already operative in young children. And it's automatic and unconscious. So people are doing it all the time and they don't know they're doing it.

[00:26:01.38] Now this can give rise to cultural adaptation. So individuals are selective about who they're paying attention to, so it's selection-- selection and transmission. So you have these psychological capacities that are selective, and that can build cultural adaptations over time without anybody realizing it, so zero intelligence adaptations, no intention, no design.

[00:26:21.46] I'm going to talk about spices in a minute, but let me just mention one other one. So in pre-Colombian-- in the Americas, corn was the staple in many populations. But the populations that relied heavily on corn had an odd custom of putting a non-food substance into the recipe. So they would include burnt sea shells, or ash from the fire, or natural lime sources. And this is important because corn, if you just use it as a staple and don't treat it with this chemical process that creates a chemical reaction that releases the otherwise unavailable niacin, you get a horrible disease called pellagra, and so these populations were able to able to be reliance on corn and not get pellagra.

[00:27:02.16] Now you might think, well people can figure that out, that they're going to pellagra, they're going to kind of work through it. We know that's false because we have historical experiments, both in the US and in Europe. So the Europeans take corn over to Europe, and corn spreads widely, people become dependent on corn.

[00:27:16.65] And of course, the lower classes become entirely dependent on corn because it's the cheap staple. They all get pellagra. And this continues for decades and decades before finally, a physician named Joseph Goldberger figured it out. And he got-- he was massively opposed by the medical authorities of the time who all thought it was pathogenic, not this missing niacin issue.

[00:27:38.01] OK. And then you would also get to read about how in Fuji in food taboos-- this is my own research-- project pregnant and breastfeeding women from ciguatera toxin that you find in reef fish. So these are all non-conscious. People don't have a causal understanding of how they work, but yet they're highly adaptive to local environmental challenges.

[00:27:57.14] OK. Now spices. Spices are an odd thing. As far as I know, other animals don't do it. And we seem to use chemicals that plants produce-- and usually these chemicals are to keep away mammals, or insects, or fungi, or bacteria-- and we put it on our foods.

[00:28:12.40] Now it turns out-- so this is work by Paul Sherman and Jennifer Billing. And they studied these spices, and they looked at the most common spices, things like garlic and onions, and they looked at in the published literature, how much they killed in terms of bacteria. And the most commonly used spices are also the most effective in killing bacteria. And lots and lots of spices have some bacterial killing properties. And things like lime and lemon, they're not very good at killing bacteria on their own, but they turn out to be catalysts for other kinds of spices. So it looks like our recipes are chemical concoctions that can reduce the pathogen in meat.

[00:28:50.31] Now one piece of evidence that's interesting is that if you look at the mean annual temperature of different countries, and you look at the number of these pathogen-killing spices, the hotter the place, the more the cuisine has pathogen-killing spices in each recipe. And in fact, whether it has one of these spices are not, in the hot climates, every single meat recipe has some of the pathogen-killing spices. Norway, not so many.

[00:29:15.57] So and I-- and what I like particularly about this is some of these chemicals produced by plants are actually to keep mammals away, so mammals like us. In the case of chili peppers, it produces this capsaicin which is designed to get mammals not to eat it so that birds can take it and then disseminate the seeds further. So it's trying to keep mammals away. So chimps won't eat these chili peppers, and physicians recommend to nursing mothers not to eat chili peppers because the baby won't like the milk as much if some of the capsaicin gets into the milk.

[00:29:48.69] But if you grow up in a place where they eat lots of chili peppers, and you observe others eating and enjoying chili peppers, you can turn the same physiological sensation-- which taps directly into some of our pain system-- turn it into pleasure. And you can come to enjoy what you would otherwise experience as pain. There's probably a lot of things like that.

[00:30:08.61] All right. So once [INAUDIBLE] this is when-- these things can build cultural adaptations, but the ability of the system to build cultural adaptations turns out to depend much more on the things about the population-- the size of the population and the interconnectedness-- that it does on the individual intelligence of the agents. And I mean one of the ways we explore this is by building simple mathematical models. And you can increase the intelligence of the agents by an order of magnitude, and you get, instead of 1% of the population doing it, you'll get 10% of the population doing it.

[00:30:38.49] But if you turn up the interconnectedness, the whole thing explodes because it has this kind of magnifying power law-type effect. So you get this interconnectedness. Size and interconnectedness is important for building this fancy cultural repertoire. But one interesting implication of these models is that, if suddenly your collective brain gets shrunk, somehow your population shrinks, or it gets cut off, or your interconnectedness goes down, you can actually begin to lose some of this adaptive know-how.

[00:31:07.63] OK. So I want to show you a little bit of evidence for these ideas. This is research done by Rob Boyd and Michele Klein, and they wanted to look at the relationship between population size and technological complexity. But the problem is on continents, it can be tricky because it's hard to say what the population is. Because continental populations are all interconnected, especially in small-scale societies, especially hunter-gathers.

[00:31:29.93] So they went to the Oceania, where the island and the island group provides a kind of natural way to carve off and say, that's a population. You can assign a population size to that. Now, they're not saying that these aren't interconnected populations. They're going to try to measure that as a separate variable.

[00:31:44.45] So they went back to the ethnographies and tried to calculate the complexity of the marine foraging tool. So things for getting fish and-- my guess is mostly fishing-- and they counted up the tools. So this is the population size of the island or island group, and this is the number of tools they found. And the larger the population, the more tools-- more different kinds of marine foraging tools they had-- and the more the fancier those tools were.

[00:32:11.69] So they made this measurement which is a borderline standard measurement of techno units. It's basically counting up the separable parts of the tool. It's not the best measure, but it's the best that anybody has come up with. And so, not only do those societies have more tools, but they're tools they have are more complex. This is robust, all kinds of ecological variables that you can try to pop in there.

[00:32:34.80] You'll also notice that they measured a low-contact-- so low amount of interconnectedness with other islands-- and high interconnectedness. And the high-- the ones with high interconnectedness tend to be above the line, which means that they have even more tools, or more complex tools, than you would expect for their population. So the interconnectedness is also doing some work. They're getting re-energized by the other islands. Oh so that fits the basic story.

[00:32:58.07] Now of course, if fits the basic story, but could it fit lots of other potential stories too. So I want to kind of get a grip on this, so I worked with someone who was a graduate student and then a post-doc here, Michael Muthukrishna. And what we did was we had 100 undergraduates, and they came into the lab-- we're trying to do a controlled experiment-- and their job was to use a difficult-to-use freeware, free software for editing images called GIMP.

[00:33:25.10] And they had to reproduce this supposedly complex image using the software. And they had a time limit. They got paid for their performance. So the closer the image that they produced was to this image, the more money they got paid. They also had incentives if their student-- someone who they could transmit information to-- if their student did well.

[00:33:44.87] Now we had two treatments. And this is the key part. So this is the social interconnectedness part.

[00:33:49.37] In one treatment, information could just pass down a transmission chain from one individual to another. So this individual could learn from him, he could learn from him, but that's it. In this treatment, any of these guys could learn from any of these guys. So this is a fully interconnected population. So it's like you can learn from anybody your village, or you can just learn from your parents.

[00:34:10.47] And then after they were done, they could create the best image they could in the time they had, they could write it up to two pages that would then get passed to the next person down the line. So then the next generation would get the target image, this, the model's product-- something that hopefully looks kind of like this, but as you'll see, often doesn't-- and then the write up-- the tips, the tricks, any suggestions that the teacher had. And then we can measure their skill by similarity to the target.

[00:34:37.23] So this is 10 generations, each laboratory generation, so one's learning from the previous generation. This blue line is the mean image rating. So that's how good the image is over time.

[00:34:48.46] And you can see that the one group, the group where you can only learn from one model, they had a really good first round, so they're way up here, but then they drop. These guys had a bad first round, but they eventually take off, and they end up with a much higher skill in this than these guys. Remember, everybody is randomly assigned to a treatment groups, so there's no reason to expect there to be any intelligence or skill differences.

[00:35:08.31] But these guys get this cumulative effect, and nothing in this group. So that seems to suggest that there is some causality here that if you interconnect groups, you get more rapid evolution. So it's consistent with the story that the Polynesian, or the Oceania data fits.

[00:35:23.99] Now one of things-- I'm showing this experiment-- there's actually a whole class of experiments like this-- but I like this one because you can look at a lot of the data right here on one screen. So this is where you can only see one person [? priority. ?] And this is where you can see five.

[00:35:37.75] And this first one is the first round. So these guys don't have any teaching at all, right? They're just trying to figure it out. And then these guys learn from these guys, and these guys learn from those guys, et cetera, down the way.

[00:35:46.92] And you can see these guys had a good first round, right? But then the second round loses it. So these guys had some good tips, but they didn't go anywhere. Remember, you're trying to match this.

[00:35:55.13] These guys had a terrible first one, these guys didn't even turn anything. Not too good on the second round, but then this guy gets it, and then everybody gets it, and then they start-- they gradually start rashing their way up and things kick in, where these guys never seem to go anywhere.

[00:36:07.95] By the time you get here, the worst person in round 10 is better than the best person in around 10 here. So you completely exceeded the individual's ability to figure this problem out. So this-- because this is an experiment, we can say the sociality, at least in this experiment, caused an evolution of greater skill over the laboratory generations.

[00:36:30.69] All right, now I want to enter into thinking about how a shrinking population might affect technology, and know-how, and this kind of culture cultural adaptations. So I like to show this to my undergraduates. It's from an archaeologist named Rhys Jones.

[00:36:48.96] And I ask him to say, which of these stone-- these are collections of stone tools made by different groups of hunter-gatherers-- and so which one most recent in time, closer to us in time, and which is the oldest? And what they typically say is that these top ones are the most recent in time, although these are quite different. So this is upper paleolithic, 35,000 years ago, and this is Australian aboriginal stone tools, about 1700. So they say these are the most recent, and they always say these two are the oldest. Sometimes they're not sure which one's the oldest.

[00:37:17.82] What's interesting is that-- oh, sorry-- these are very old. So that's an old one chopper. These are Mousterian tools often associated with Neanderthals.

[00:37:28.27] And these tools are Tasmanians. So these are contemporary. So they're made by anatomically modern humans, who are only separated by 150 miles of the Bass Strait. These guys are in Tasmania. These guys are in the mainlands.

[00:37:44.13] So there's this interesting pattern of tool technology. How can we possibly explain it? So let's go into a little bit about Tasmania.

[00:37:52.22] So the Europeans arrived in Tasmania about 1642. Abel Tasman kind of sails by the southern end. It's an island of all hunter-gatherers just like the rest of Australia is, about 4,000 Tasmanians, 2/3 the size of Ireland. And it's by far the simplest technology that the Europeans ever encountered as they expanded, so simpler than the Fuegians in tip of Tierra del Fuego, simpler than the folks in the Chathams-- an island not too far from New Zealand.

[00:38:23.83] So it got even more interesting when the archaeologist Rhys Jones began to dig back into the history of Tasmania and look at the archaeological records. So it seems that the Tasmanians actually lost a lot of valuable tools and technologies over the last 10,000 years. So 10,000 years ago, Tasmanian archeology looks like mainland Australia. But then the two begin to diverge with Australia getting more complex and Tasmania getting simpler.

[00:38:50.23] So the Tasmania either never developed or lost bone tools, cold-weather clothing, hafted tools, nets, fishing spears, barb spears, durable watercraft, and boomerangs. So you can see this is an image of a Tasmanian raft. They actually had no paddles.

[00:39:04.67] And to cross rivers, women would grease themselves down. And the husband and the kids, I guess, would get on to the raft and she would swim it across. The grease protects her from the cold in the water.

[00:39:15.67] And they use one-piece wallaby skins-- you saw that on the previous slide-- one-piece spears, and clubs for hunting. So they drank from skulls pictured here. That's from-- that's from the Adelaide Museum actually. OK, so Rhys Jones estimates about 24 items in their toolkit.

[00:39:33.01] Now, right just across the Bass Strait in the control group, we have other group that has the entire Tasmanian toolkit plus multi-prong fishing spears, spear throwers, boomerangs, mounted adzes, composite tools, a variety of nets for different kinds of birds, fish, and wallabies, sewn bark canoe, string bags, ground edge axes, wooden bowls for drinking. So you can see some of these hafted tools. There's their boomerang. These are different kinds of nets they have for different kinds of fish and depending on the ecological conditions.

[00:40:01.50] OK. So it's a puzzle because it's very non-intuitive to realize that groups could lose valuable stuff. How could that happen? How could you lose something that's useful?

[00:40:09.48] Well if we look back into the kind of climatic history, a picture begins to emerge. So the last glacial maximum, about 18,000 years ago-- takes want to start warming up. By 12,000 years ago, the seas are rising, and Tasmania is going from a peninsula of Australia, where it's attached to the rest of Australia, to becoming its own island. By about 8,000 Tasmania's got quite a bit of ocean between itself and mainland Australia, and contact ends between the two. And it's over that next period of 8,000 to 10,000 years that all these losses occur, that you have this shrunken population that's no longer attached to mainland Australia that just-- it all begins to ebb away.

[00:40:49.10] Meanwhile, in Victoria, not only are things staying the same, but a group called the Pama-Nyungan-- because it's a language group-- they're expanding out of the North. And they have a fancy set of rituals and institutions that allow them to stay very interconnected. So this spreads down South and technology begins to really take off because they have new institutions that make them more interconnected.

[00:41:10.86] But again, this is an interesting bit of ethno-history, but is there any causality? Can we get to the causality? So back to the lab.

[00:41:18.32] And this time I wanted to give the 100 undergraduates something a little bit more ecologically realistic. So it's knot time. So we developed a complex system of knots where they had to use these knots, and assemble them, and then use it to lift a chair successfully up into the air.

[00:41:33.17] Same setup as before, so either one-to-one transmission or the group condition, so interconnectedness. But we started with experts. So before the first group started, we train them up, and we got them to a high level of skill. And then we started the transmission chains. And again, everybody's paid here for performance.

[00:41:52.18] Instead of passing down written tips, I wanted something again, more ecologically valid, but I want to control over it. So they-- after they did they're knot making, they could make a video, kind of a how-to video. And so the students get the how-to video. And then again, we can measure similarity to this original.

[00:42:10.62] And what we find is that when there's just one-to-one transmission, so not very good interconnectedness-- remember, they're initially skilled, so they have high knot skill-- you get a drop in the skill quickly, and then it begins to level off here. The decline is slower, and then it levels off here. And so you have a Tasmanian-like effect, and it looks like this-- you have differences in steady-state skills. So it looks like-- you can't say for sure-- but it looks like this group is going to pretty much stick here at about a skill of 60, and this group is going to stick here at about 30, 32, or something like that. So this is consistent with the idea that there are these steady-state maximum that depend on your group's-- your group size an interconnectedness.

[00:42:53.09] OK. So again, we know that it's not intelligence. This incentives are the same in the two groups. The only thing different is the interconnectedness, the sociality.

[00:43:04.03] So that's relevant using data from-- well, from the laboratory and from ethnographic and historically known cases, but I think this picture extends back into human history. Now how far back is a big question. In the book, I do my best effort to attach it to the available evidence and anchor it in what we know about human evolutionary history. But I think that these cultural-- the products of this cultural evolution actually drove the expansion of our brains and shaped our anatomy in lots of ways. So let me sketch it out for you.

[00:43:40.23] So first you have genetic evolution. It's the first move. It develops sufficiently good cultural learning capacities, or something about sociality, that allows us to cross this threshold and begin to accumulate cultural evolution.

[00:43:52.99] Once cultural evolution begins the accumulated process, it is going to-- so it's a separate inheritance system-- it's going to produce say tools, so say cooking and fire and some cool looking stone tools, maybe some good wooden tools. But that means that if you're a learner in this world, there's this really great stuff you could get if you can only learn from other people. So that puts a pressure on those who can learn better from other people. So it's going to create brains that are better at acquiring, storing, and organizing cultural information. Once you have those kinds of brains, that's going to make this whole system work better. So it's going to turn up the juice on the ability of this system to generate stuff.

[00:44:28.93] Then you're going to get, say, tracking knowledge, water containers, and better food processing. And that means that the learner is faced with this world where this is a growing body of really good stuff in the minds of other members of his social group, if he can only acquire it. So pressure for brains are even better at acquiring, storing, and organizing. So this can create an autocatalytic runaway process which I think explains the rapid expansion of human brains in about the last two million years or so.

[00:44:56.61] Now, this system eventually hits the stops because there are some constraints in a primate's physiology. Baby's-- the head can only get so big. Natural selection uses all these tricks to keep the baby's head soft, make it get born prematurely, get that baby's head out of there before it gets too big. So this hits the stops there.

[00:45:15.80] This process, of course, can keep going-- and it's still going now, ever faster and faster as it generates cultural information-- but then other things start to happen. So the division of labor between males and females, I actually think is a division of information. They're splitting up the info that one has to know in order to survive. Then, of course, you eventually get a larger divisions of labor, and then extra somatic storage of information, then computers. There's a few steps in between, but-- So there's that.

[00:45:45.09] Now that's the information that's creating a selection pressure for brains to do this. But there's also other cultural products besides just the information that do this. So let me give you some examples. So the idea here is that these are the cultural products, and then these creates selections.

[00:46:01.34] So fire and cooking. Now my colleague, Richard Wrangham, who's right in the back there, and his colleagues have persuasively argued that humans are unusual for a primate of our body size. Our digestive tract seems underwhelming. So our gape's are too small, our teeth are too small, or stomachs too small, our colons are too small. Our intestines are probably about right, but there's another-- there's a story about that.

[00:46:25.47] The thing that's interesting though, is we don't innately know how to make fire or cook. So if I was to take all of you very smart people and deposit you in Western Massachusetts, there'd be no technology and say, make fire and cook. You would not-- nothing would happen you unless you had some training in how to make fire from natural materials, you would just flounder.

[00:46:43.42] And same thing with cocking. There's lots of ways to cook poorly that actually makes the food less good to digest. So that seems to be a case where the cultural spread of fire and cooking then generates this downstream that Richard has documented so effectively.

[00:46:57.94] My other colleague, Dan Lieberman, has argued persuasively, I think, that humans are full of running adaptations. So we have springy arches, and strong calves, and nuchal ligaments that allow our head to move independent of our body that we don't see other primates. And in particular, we have this powerful sweating system which can cool us, cool us down.

[00:47:17.23] But if you look at this system like an engineer, there's a flaw. There's a big gaping hole in the system that there's no water tank because we have to cool ourselves with sweat, but we need water to run that system. And unlike camels and horses, our ability to store water is quite limited.

[00:47:34.24] But if you look at the way hunter-gathers who engage in persistence hunting and chase down animals do it, they use cultural know-how. So they have water containers. So you actually saw some water containers earlier in the talk.

[00:47:46.21] Australian Aborigines keep these bamboo tubes, and they walk with or run with the tubes of water. And there's also know-how about how to find roots that bear water, and how to use little cues on the surface to find water. So that's the fuel for that. So it's a culture-gene co-evolutionary package that allows that running stuff to go.

[00:48:04.78] And then what I'm particularly interested in, and what I spend a lot of time in the book on, is the psychological stuff. So humans seem to have a particular system for acquiring knowledge about plants and animals. We organize in a taxonomy.

[00:48:17.97] We can readily make inferences. So if you hear a story that one tiger-- that someone saw a tiger at night hunting, you might naively assume that well, only that's just that one particular tiger-- Tigger, the tiger, that hunts at night. But because you have the system, you immediately extend it to all tigers.

[00:48:35.39] It's a good bet that if you saw one tiger hunting a night, that all tigers hunt at night. And in fact, lions might hunt at night too. So you can make a kind of jump.

[00:48:42.47] But it probably doesn't tell me anything about mice or ants. That's all because you built this kind of complex system. And there's a bunch of other cool features of that.

[00:48:51.88] We seem to have a specialized system for thinking about artifacts as separate from other non-living kind. So when little kids ask questions about artifacts, they don't know what it does, but they kind of have a sense that it's an artifact, and so they want to know what it does. Which is different than if you show them something they don't identify as an artifact, they're not interested in its function. And in this case, they tend to over imitate. So they copy be all the details of how to use these things called artifacts.

[00:49:17.29] I've argued that to understand human status psychology, that we have a dominant psychology. That it's kind of similar to chimps so, you have high status because you control force and force-threat. You can use coercion. You're a subordinate in a dominance hierarchy because you recognize that someone controls force, force-threat, and coercion.

[00:49:35.20] But in humans, because we can learn from each other, we also have prestige. And this is because knowledge and know-how is unequally distributed amongst the minds in your group. Some people know lots of stuff, and so they tend to receive deference because people are trying to learn from them, and that gives rise to different emotional packages and different kinds of status. So in order to understand human status psychology, you need to understand the gene culture stuff.

[00:49:57.76] This is actually quite a long list. This is the last one I'll do, but cultural evolution also produces social norms. So once we can transmit ideas about reputation, about how to judge other's behaviors and not just transmit the behaviors themselves, then you get this sticky situation where groups will start doing something and their judging each other based on how they do it, and the group can get stuck there. And then as a learner, you've got to do whatever it is the group does because otherwise you're going to get a bad reputation or get punished in some way. And this gives rise to a norm psychology where you expect the world to be full of social rules, even if we don't know what they are. And we're good at inferring them. Even when sometimes rules don't exist, we kind of have a hair trigger rule inference machinery.

[00:50:35.92] And this can also lead to prosociality. So it's the institutions that are actually shaping our social psychology. I call that process self-domestication.

[00:50:44.96] All right. Let me just wrap it up here. I've just given you a quick tour of some of the ideas that I developed in the book here. I'll go through a few key ones.

[00:50:53.21] So we have to think of our learning abilities as adaptations themselves. They're honed to allow us to effectively learn what we need to know in the world. But this gives rise to the second system of culture and inheritance which then becomes intertwined with our genetic inheritance system, and you get gene culture co-evolution. To understand our ability species to adapt, you have to realize that we have collective brains, that we solve these problems over generations as a group because of our interconnectedness, because it's that system that generates our ability to adapt to environments.

[00:51:23.44] And I mentioned some stuff about how this can make us individually smarter. We produce these cultural products which shape how we think. And it actually shapes our brains. So for example, when you learn to read, you get specialization in your left hemisphere that non-literate people don't have. So your brain is different if you come from a literate society, and you've learned to read.

[00:51:43.83] Culture-driven genetic evolution. So culture is part of our biology in two different ways. First is the way I just mentioned with the reading, where culture shapes the environment we develop in, so through that, it shapes our brains and our hormones. But of course, there's also the other way, which is by shaping our genetic evolution. And that's kind of the long-term take on the book. And then finally, so to really understand humans, you have to recognize that we're a culture species and that this goes deep into our evolutionary history. Thanks.