COMPLEXITY: Physics of Life

Daniel Lieberman on Evolution and Exercise: The Science of Human Endurace

Episode Notes

Human beings are distinctly weird. We live for a very long time after we stop reproducing, move completely differently than all of our closest relatives, lack the power of chimpanzees and other primates but completely outdo most other terrestrial mammals in a contest of endurance. If we think about bodies as hypotheses about the stable features of their ancestral environments, what do the features of our unusual physiology say about what humans ARE, where we come from, the details of our origin story as a profoundly successful species? And what can we learn by telescoping that story forward to explain some of the most persistent puzzles and paradoxes about our health, the way we age, our need for physical exercise, and our nearly ubiquitous aversion to habits that are good for us?

Welcome to COMPLEXITY, the official podcast of the Santa Fe Institute. I’m your host, Michael Garfield, and every other week we’ll bring you with us for far-ranging conversations with our worldwide network of rigorous researchers developing new frameworks to explain the deepest mysteries of the universe.

This week, we sprint into the paleoanthropology, biomechanics, and physiology of exercise with Harvard evolutionary biologist Daniel Lieberman, author of several books including Exercised, The Story of the Human Body, and The Evolution of the Human Head. In our rapid-fire discussion we explore how millions of years as hunter-gatherers equipped hominids with a unique package of adaptations for endurance running, why exercise is so good for us but so generally undesirable, and how physical activity in old age helped shape us into the strongly intergenerational social apes we are today.

Be sure to check out our extensive show notes with links to all our references at Note that applications are now open for our 2023 Complexity Postdoctoral Fellowships! Tell a friend. And if you value our research and communication efforts, please subscribe, rate and review us at Apple Podcasts or Spotify, and consider making a donation — or finding other ways to engage with us — at

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Mentioned papers and other resources:

SFI Colloquium & Twitter thread on Daniel Lieberman’s “Active Grandparent Hypothesis”

The evolution of human fatigue resistance
by Frank E. Marino, Benjamin E. Sibson, Daniel E. Lieberman 

"What beer and running taught me about the scientific process"
Seminar by SFI Journalism Fellow Christie Aschwanden

Endurance running and the evolution of Homo
by Dennis Bramble & Daniel Lieberman in Nature

SFI Professor David Wolpert & the thermodynamics of computation

Complexity 64 - Reconstructing Ancient Superhighways with Stefani Crabtree and Devin White

3100: Run and Become (Documentary Film)

Why run unless something is chasing you?
by Daniel Lieberman at The Harvard Gazette

Hate Working Out? Blame Evolution
by Daniel LIeberman at The New York Times

The Aging of Wolff’s “Law”: Ontogeny and Responses to Mechanical Loading in Cortical Bone
by Osbjorn Pearson & DanielL Lieberman

Effects of footwear cushioning on leg and longitudinal arch stiffness during running
by Nicholas B.Holowkaab, Stephen M.Gillinovac, EmmanuelVirot, Daniel E.Lieberman

Episode Transcription

Daniel Lieberman (0s): Among the things that are important about being human, one that's been kind of overlooked is our ability to run long distances. And that actually played an important role in our evolution of history because starting around 2 million years ago, at least 2 million years ago, humans became hunter gatherers. Our ancestors, they started eating meat and that involved first scavenging and then hunting animals. And if you look at humans, we don't look really like a carnivore. We don't have the features that make carnivores successful. We're not fast. We don't have claws and fangs and fur to protect us.

And the technology that we use for hunting is actually pretty recent. The bow and arrow, it was invented less than a hundred thousand years ago. And in fact, just putting a sharpened point on the end of a spear was invented less than 500,000 years ago. And yet for 2 million years, our ancestors were hunting. So we got kinda interested in like, what was it that enabled our ancestors to hunt and why is it that humans can run marathons and are so good at it? And so we put together all the evidence that humans are exceptional long distance runners, and there's lots of evidence that we've been doing it for 2 million years and why and how it was important for our ancestors to becomes carnivores.  So it’s not what made us human, but it’s a part of that package, that hunting and gathering package that made us the creatures that we a
Michael Garfield (1m 38s): Human beings are distinctly weird. We live for a very long time after we stop reproducing, move completely differently than all of our closest relatives, lack the power of chimpanzees and other primates, but completely outdo most other terrestrial mammals in contests of endurance. If you think about bodies as hypotheses, about the stable features of their ancestral environments, what do the features of our unusual physiologies say about what humans are, where we come from, the details of our origin story as a profoundly successful species, and what can we learn by telescoping that story forward to explain some of the most persistent puzzles in paradoxes about our health, the way we age, our need for physical exercise, and our nearly ubiquitous aversion to habits that are good for us?

Welcome to Complexity, the official podcast of the Santa Fe Institute. I'm your host, Michael Garfield, and every other week, we'll bring you with us for far ranging conversations with our worldwide network of rigorous researchers, developing new frameworks to explain the deepest mysteries of the universe. This week, we sprint into paleo anthropology, biomechanics, and the physiology of exercise with Harvard evolutionary biologist, Daniel Lieberman, author of several books, including Exercised: The Story of the Human Body, andThe Evolution of the Human Head.

In our rapid fire discussion, we explore how millions of years as hunter gatherers equipped homage with a unique package of adaptations for endurance running, why exercise is so good for us, but so generally undesirable and how physical activity in old age helps shape us into the strongly intergenerational social apes we are today. Be sure to check out our extensive show notes with links to all of our references at Note that applications are now open for our 2023 complexity post-doctoral fellowships, tele friend.

And if you value our research and communication efforts, please subscribe, rate, and review us at apple podcasts and/or spotify and consider making a donation or finding other ways to engage with us at Thank you for listening. Daniel Lieberman. It's a pleasure to have you on Complexity podcast.

Daniel Lieberman (3m 56s): My pleasure to be here. Thank you.

Michael Garfield (3m 59s): So before we really rip the layers off the onion, why don't we have you talk a little bit about who you are and how you became interested in the work that you do?

Daniel Lieberman (4m 12s): So I'm a professor at Harvard University in the Department of Human Evolutionary Biology. So the big question that motivates me is, how and why did humans evolve to be the way we are, but become over the years, increasingly interested in how that's relevant to health and disease. And although I started off my career as a head guy, studying skulls and heads and stuff like that, I became increasingly interested in the issue of endurance running because I was fascinated by how we stabilize our heads during running, which is interesting by-chemical sort of problem. And that led to interested in the evolution of running in general and humans, which led to you being more interested in the evolution of physical activity.

And before I knew it, we were studying barefoot running, cuz you study running and the evolution of running, humans ran barefoot for millions of years and before I knew it, I was studying injury and other sorts of things. And my career kind of took a bit of a change in orientation and I've become basically kind of someone who integrates physiology and biomechanics and paleontology with evolutionary medicine, how I sort of solve my midlife crisis. Like what am I doing? How can I make the world a better place? And that's kind how I sort of answered that question by trying to help people understand better the evolution of physical activity and how and why that can help us be healthier today in the modern world.

Michael Garfield (5m 23s): And how were you folded into the ambit of SFI?

Daniel Lieberman (5m 27s): Oh, well that was fun. I was invited by colleague Dan Shrek to come give a talk out there and I had a great time giving a talk on that topic and enjoyed myself tremendously. It's a really wonderful environment to have in depth, interesting conversations with interesting people

Michael Garfield (5m 41s): Right on. So the frame around this seems to be, "en medias res." Thinking about evolution, you and I, the body is a product. It's a kind of a memory of stable features of the environment. So we're gonna drop people directly into what am I doing here? Like you wake up in a burning building and you're a human in 2022 and compost is on fire in London and everything.

So let's telescope out from that and talk a little bit about what it is that makes us human. So like first, what I would like to do is have you unpack this extraordinary review that you did with Dennis Bramble, endurance running and the evolution of homo and give a little soil in which we can plant this conversation.

Daniel Lieberman (6m 36s): Well, first of all, there's a long history of people trying to say that what makes us human is fill in the blank, language, tool use, abstract thinking, our ability to deny reality, whatever. And I've never been one for that, cooking. There are many things that make us human or that are special about being human. The only thing that truly actually makes us human is if our parents are human and that's it, but beyond that to misquote George Orwell, all creatures are unique, but some creatures are more unique than others and humans are definitely up there.

That there are a lot of things that are special about being human and what Dennis Bramble and I wrote about in that Born to Run paper that you just mentioned from 2004 in nature, was that we made the argument that among the things that are important about being human one, that's been kind of overlooked is our ability to run long distances. And that actually played an important role in our evolution of history starting around 2 million years ago, at least 2 million years. Maybe more than that, actually humans became hunter gatherers. Our ancestors, they started eating meat and that involved first scavenging and then hunting animals.

And if you look at humans, we don't look really like a carnivore. We don't have the features that make carnivores successful. We're not fast. We don't have natural weapons. We don't have claws and fangs and fur to protect us. And the technology that we use for hunting is actually pretty recent. The bow and arrow, it was invented less than a hundred thousand years ago. Hunting dogs, nets, all of these are extremely recent. And in fact, just putting a sharpened point on the end of a spear was invented less than 500,000 years ago. And yet for 2 million years, our ancestors were hunting.

So we got kinda interested in like, what was it that enabled our ancestors to hunt and why is it that humans can run marathons and are so good at? And so we kinda built up on a paper that had been written by a student of Brambles named Dennis Carrier who wrote about sort of the regulatory advantages of humans, that he really able to sweat really well and dump heat in a way that other animals can. And he related that to running and he published that paper what, 1984. And it basically died. You know, nobody paid any attention to that paper.

In fact, I remember my professor, I had very famous physiologist at Harvard named Dick Taylor. And I remember mentioning Carrier’s his paper on running in human evolution. He said, oh, that's just silly. You know, humans suck it running. We're slow, we're inefficient, we're awkward. We're just obviously not good at running. It's just a silly idea. And, you know, put my tail between my legs and went off. And, but I was a bit of a runner myself. And I remember thinking about that paper for years and years and years. And then eventually Dennis and I started working on this paper because we're interested again, this problem of head stabilization.

And we started thinking about how humans keep their head still. And we spent many, many years thinking about it and writing. We spent three years writing the paper. You know, we kept calling each other up and saying like, are we crazy? How come nobody's made this argument before? But we basically put together all the evidence that humans are exceptional, long distance runners. And there's lots of evidence that we've been doing it for 2 million years and why and how it was extremely important for our ancestors to become carnivores. So it's not what made us human, but it's a part of that kinda package that hunting and gathering package that helped us become the amazing creatures that we are.

Michael Garfield (9m 46s): One of the things you talk about in this review is that running is more costly for humans than it is for most mammals. So talking about like cost of transportation per unit mass, I'd love to hear you riff on that a bit.

Daniel Lieberman (10m 2s): If you do the calculations correctly and you look at the scaling relationships, humans are actually just as expensive as we should be for a typical animal of our body size. So once you're correct for body mass. So back in the day, it was thought that humans were really costly. In fact, there was a study, the only good study that had been done or a study that been done was done on a single individual, I think he was from Italy and he, he must have been a really bad runner because in that study, this one guy came out as about as efficient as a penguin. And I think you can imagine that penguins aren't really great runners, but more recent studies of shown that actually if your body mass on the X axis against the cost of transport humans fall right on the line for mammals.

So we're actually just where we should be. We're not inefficient. We are very costly though. And this walking what's interesting is that the animals that we evolve from chimpanzees, they're very costly. So chimpanzees spend about twice as much energy to move a kilo of their body, a given distance as a human.

Michael Garfield (10m 57s): So there's an argument here for small feet and big hips making running cheaper.

Daniel Lieberman (11m 4s): Well, it's not big hips, but it's well, by mechanical scale it gets little bit complicated, but most animals run on their toes. If you think about dogs, horses, they run on their toes, but we're primates. Primates have these big ugly, flat plantigrade feet. And so a big feed are a problem cuz you can break them when you run because they, you have really high torques. You have high forces that act over large moment, arms too. So, and chimpanzees have really long toes and Australia pets like Lucy have really long toes, but once you get to the genus homo, the toe length gets really shorter.

And like, why would that be the case? Well, actually turns out has very no effect on walking. You can walk with long toes and it's just fine. But when you start running with long toes, it's a real serious problem. So we shortened our toes and that's clearly an adaptation for running again because we've proved biomechanically. It's not really very relevant to walking. And also the other thing is we added an arch to our foot. So chimpanzees have flat feet and there are humans today who have flat feet about a third of Americans, for example, have reasonably flat feet. And the problem with flat feet is not so much walking.

As you can tell, most of those Americans are able to walk just fine, but the arch of the foot is a spring that's used in running and running is a spring-like gate. And walking is not walking is basically a pendulum gate. So adding that spring to our feet was also a really key adaptation that really only makes sense for running. It has really nothing to do with walking. Hips are another interesting issue. I mean, I could go on, look there are adaptations literally from our toes all the way up to our heads that help us become better runners, long Achilles tendons, for example, which are key springs. Chimpanzees have Achilles tendons that are like centimeter long, maybe less.

I mean there's and whereas humans, we have these really half the length of your shank. And they act as an important spring. We have hips that are mechanically advantageous, not just for walking, but also for running. It reduces the torques and the hip. But importantly, we also have wastes that enable us to move our thorax, our chest independently of our hips. So when you run right, you twist your upper body. Chimpanzees can't do that when they rotate their hips, their whole upper body has to rotate with them. But we have this zone of separation between our hips and our thorax that enables us to twist so that our bodies can basically point forward as we're rotating our legs as we're running.

And we also have another zone of separation between our thorax, our shoulders and our head, which also chimps can't do. So when that chips rotate their thorax, their head rotates with their thorax. So we've dropped down our we've lost a lot of the muscles that connect our body with our heads. Those are great muscles for climbing. We lost those. But they enable us to keep our heads still while we rotate our thorax when we're running.

Michael Garfield (13m 34s): So thinking back into context, why are we running? Like what other animals are endurance runners? What's going on here?

Daniel Lieberman (13m 43s): Well, the only animals that tend to run long distances tend to be carnivores. And there are only a few cuz the vast majority of animals are adapted for sprinting for short, quick bursts. And the only animals that tend to run long distances naturally are animals like hunting dogs, wolves, hyenas. These are animals that will run their prey down. But they tend to do their hunting at night. Or dawn or dust when it's not too hot. And what's exceptional about us, is that not only we good at long distance running, which is again a very unusual behavior among animals, but we're also able to do it in the heat when it's hot because we're glorious sweaters. You know, most animals cool by panting, which is basically evaporation of moisture in the mouth and the nose. We basically turn our entire bodies into a tongue by squirting water all over our bodies. And that gives us enormous amount of surface area to cool. So we pump our blood to our surface, to our skin. So just below the skin cool that blood through evaporations and that cools are whole bodies, no other animal can come close to humans in ability, vigorous exercise in the heat to keep cool in the heat.  We’re the unquestionable champions of mammals for that.

Michael Garfield (14m 52s): So there's competing hypotheses though about this, about whether this was about us actually running down prey or whether it was about us scavenging. I'd love to hear you unpack that.

Daniel Lieberman (15m 4s): They're not competing hypotheses. No, I think they're both important. So, and in fact, scavenging certainly predates hunting. Look, you can't be any kind of carnivore without being able to run. So Dennis and I both agree that the most likely scenario is that and of course there has to be an advantage for some kind of intermediate feature for selection to operate on it. So probably what initially happened was that there were scavenging opportunities. If you ever out on the safari and you see a bunch of vultures in the distance, what's underneath those filters. There's a prey item. But if you don't get to it quickly, it's gonna be gone.

Cause scavenger carcasses are opportunities for all kinds of animals out there for vultures, for hyenas, for foxes and you know, all kinds of critters. But if you can, it's the middle of the day and you can run to get to it. You have a big advantage. You can get there before others do and have access to what's left in that carcass or the bones, et cetera. So it makes complete sense that the initially running evolved for scavenging. But at some point we also became hunters and we have unquestionable data that by 2 million years ago, our ancestors were hunting. I mean, there's clear evidence, there's large carcasses of wildebeest.

And you can actually see from the cut marks on them and things like that, that the humans got there before other creatures. So by 2 million years ago, we added hunting to scavenging and all hunter gatherers, scavenge, and hunt.  They don’t do both. So we added hunting to the mix.  And so once you hunt, you don’t just have to rely on hunt.  You can become a real predator.  You can become a real carnivore.  So, I think it’s a two stage process.

Michael Garfield (16m 30s): I would like to talk about shock absorption cause there's something funny going on with people in terms of the way that we move from one gate to another gate. And I mean, unless someone is into exercise literature or really like deliberately thinking about this, you may not like notice it's, it's certainly the case that at some point you kinda can lever over into another thing. So I'd like to talk about our joints surface area of, and why is that were naubbier than other primates?

Daniel Lieberman (17m 3s): Well, there are a number of demands that running imposes on the body. So there a regulatory demand, there's a metabolic demand, but there's also a stress problem, right? Cause running involves higher forces than walking right in walking. You're never in the air. You're always on the ground. But in running, you actually have an aerial phase and then you collide with the ground. And when you collide with the ground, you have like an impact with the ground. And that sends a shock wave up your body running also involves more force because you're moving faster. And so your muscles are also generating more force. So there's a lot more force that is involved in running and runners tend to have adaptations to deal with those stresses right though, cause force is applied to a unit area that causes a stress and that stress causes deformation in a tissue and that's strain. So one of the ways to cope with those higher stresses, there are a number of ways. One of them is learning to run properly. So they don't crash into the ground, like run like an elephant. But the other is to increase the size of your joints because if you have given force, but you have spread it over a larger area, you decrease the stress. It's very simple math. And so if you actually look at the scaling relationship between joint size and body size and most animals as body size gets bigger, joints get bigger.

But in humans you can see that for our upper body, we have ape size joints, just what you predict from a chimpanzees scaled up to our size or a kibbit scaled our size. But from the hip down, our joints are much, much larger than you would predict actually about three times larger. And you don't see that in Australia pets who we also know are walking. You only see that in the genus homo. And again, that suggests that that's the forces added to right knees. You need big ankles, you need big hips so that you spread those forces over and not cause damage.

Michael Garfield (18m 44s): We had a SFI Journalism Fellow, Christie Aschwanden, who talked about comparison of male and female endurance runners. And thinking about the stuff, thinking about the way that our toes are getting shorter and our feet are getting more compact. I've heard people project into the future of evolution that we're gonna lose a toe at some point. Do you think that we're…

Daniel Lieberman (19m 10s): I mean, and that's all kind of fantasy evolution. I mean, look, the only way evolution is gonna cause any changes, if there are heritable variations that affect our reproductive success. And I don't know anybody today who needs to run down their prey to get dinner. So, and furthermore, the kind of anatomy that we have seems to do quite well for running. I think we're pretty well adapted so that doesn't hold a lot of water for me.

Michael Garfield (19m 31s): Fair enough. I wanna double back to heat dissipation because I'm thinking about the metabolic costs of things like David Wolpert and his work on the thermodynamics of computation. How much of the brain physiology of human beings is derived due to the fact that we are runners?

Daniel Lieberman (19m 49s): That's a great question. So of all the parts of your body, you need to keep cool when you're running, nothing matters more than your brain. You know, an overcooked brain is death. You know, you're using your brain of course, when you've run a lot too. So we think there's a variety of adaptations in our heads to help us keep our head school in our brains to keep cool. And one of them is that we of course sweat more in our heads than anywhere else in our body. So if you go outside on a hot day, you know, you'll sweat profusely in your forehead and your scalp and everywhere else.

And one of the things that does is actually cools blood in your head. And it turns out that we have a really interesting countercurrent exchange system. That seems to be unique in humans, whereby blood from the facial vein actually drains into what's called the cavernous sinus, which is where the carotid artery comes up to supply blood for the head. And so you have this cool blood that completely surrounds the rising blood from the core and it creates a countercurrent exchange system. And the cavernous sinus is enlarged in humans in a way that is not in any other animal. Of course we sweat more than chimpanzees and most animals don't sweat at all and certainly not in their heads.

So we have this elaborated countercurrent exchange system that keeps our brains cool. We're able to increase blood flow to our brains enormously. So when you exercise, we have a reticulated system that increases blood flow to the brain. It seems to be elaborated in humans. If you cool the blood in your core, that cooled blood from the core is gonna keep your head cool. And then finally, we may have some interesting adaptations to cool blood from the scalp that actually cool the brain itself. So again, another kind of radiator system in the diplo that, oh, and finally we have noses.

So noses are also cooling systems because we exchange not just moisture, but also heat in our noses cause of the airflow and the nose. And so, and we dump heat through our mouth. So when you're running, you're not using your nose so much, but you're also breathing out with your mouth and that's a heat dump. So on a cold day, if you breathe out of your nose versus your mouth, you'll see much less moisture leaving your nose in your mouth. The same would you be true if you put a heat sensor because you're actually capturing more moisture and heat in your nose, but when you breathe out through your mouth, you're just dumping it.

And so we become mouth breathers and vast majority of animals. When they run, don't breathe through their mouths, they breathe through their noses. The only other creature I know that is a mouth breather when they run are horses, although sheep do it to some extent, there's probably some other animals as well. So we just open our throats and bam, we just dump heat when we run.

Michael Garfield (22m 13s): Let's move on to this paper that you wrote with Frank Marino and Benjamin Sibson, The Evolution of Human Fatigue Resistance and the journal of Comparative Physiology, because this is another thing that, most people have had that, which was  would you rather fight Joe Rogan or at chimpanzee? It's like, oh, you'd rather fight Joe Rogan, but there are strength, power in stamina tradeoffs. So what are we talking about here? And why is that lens in the way that it is through the body of the human being?

Daniel Lieberman (22m 45s): We wrote this paper because there's been very little research thinking about fatigue, the evolution of fatigue. And of course, fatigue is really important if you're doing kind of physical activity and there are sort of many dimensions to what makes us exceptional endurance because essentially most creatures that's implied in your Joe Rogan, chimpanzee comparison, most creatures are adapted for strength and power and not for endurance, So chimpanzees are typical. They can sprint for about a hundred meters and then they run outta gas. They rarely walk more than three or four kilometers a day.

And when they do, they're like exhausted and basically do nothing the next day. They're power animals. But we can run marathons and go hiking all day long and be fine the next day. And part of that comes from classic trade off because we have different muscle fibers that are good for producing power and strength. Those are type two muscle fibers versus muscle fibers that are really good for during that are metabolic less costly and more aerobic. And those are type one or slow switch, muscle fibers and humans have a preponderance of course there's a lot of variation and depends on muscle to muscle, but in general, the average human being is much, much more endurance adapted than power adapted.

And because of the way those muscle fibers work, those lead to less fatigue. So it's evolution again, an evolutionary signal of selection for endurance and fatigue resistance, and there whole bunch of other features of the human physiology, which we think again, sort of play into that general argument. But of course there's variation in humans and people like Usain Bolt are more power adapted, speed adapted, and people like me are more endurance adapted and you know, variation is the stuff of life. Humans are no exception.

Michael Garfield (24m 18s): I wanna hear you talk a little bit about the difference between the different kinds of muscle fibers because not being so specific to human beings. I think about the evolution of homeothermian fish, underappreciated fact that the red meat of a tuna is, because they are endurance swimmers. And in fact, leatherback, sea turtles also have some amount of regulation.

Daniel Lieberman (24m 42s): That's right. And even with the mammalian world, like skunks, if you ever meet a skunk and actually it doesn't spray you whatever, and you get a chance to look at his muscle fiber types, skunks are almost entirely muscle fibers. Skunks don't have to run away from anyone, right. They stand their ground and everybody runs away from them. So skunks are like totally endurance, slow twitch dominates. So there's a lot of variation out there in the animal world. You just mentioned some other great examples, certain birds, et cetera, or do adapted. So there's, you know, selection goes back and forth and back and forth across the tree of life in terms of adapting animals to different kinds of functional tasks.

What's interesting about humans of course, is we have all from apes. Not from tuna or skunks or turtles. And so our ancestors  our ape ancestors are clearly power adapted. They're not adapted for long distance running. They're not adapted to be metabolically very efficient in terms of locomotion. So that's the key evidence for selection, but then once humans evolved, there's been recent selection for variation within human beings. And I think a lot of that actually started with the origins of agriculture. So when farming comes along, all of a sudden endurance becomes less important and power becomes more important again.

And so I think there's compelling evidence as a gene, for example called actinium three, which is a gene that affects your ability to produce speed and power. And that seems to be a fairly recent selection, probably post to agricultural and a higher percentage in some populations which are more power adapted. So, or at least individuals that are more power adapted. There's again, as within all human things, there's much more variation within any population than variation between populations. We have to be very careful about not exaggerating population level differences.

Michael Garfield (26m 22s): So there's a passage in this paper where you say given muscles with high proportions of type one fibers generally function to maintain posture and stabilized joints. And these functions stay vital with aging. “It may be the case that the increase in the size of the slow motor unit pool functions to preserve mobility and physical activity into old age. This makes sense from an evolutionary anthropological perspective, as older adults and hunter gatherer and subsistence farming societies stay active throughout their lifespans, continuing to walk, dig, and carry into their sixties and sometimes seventies and eighties.”

So this is where I want to get into the meat of the conversation, a conversation about meat, but the talk that you gave at SFI was about this active grandparent hypothesis. So it's not just that we are adapted for running, but that within an individual, that there appears to be a development in the amount of running that we seem to be adapted for or a physical activity that is good for us as we get older or what's going on here?

Daniel Lieberman (27m 29s): So first of all, let's be clear about running. So although running, I think played an important role in human evolution, we evolved to be occasional runners. We're mostly walkers and the endurance running hypothesis is not this idea that we evolved to just basically run around the landscape nonstop 24/7. If you go out and look at any population on the planet, hunter gatherers, you name it, who most of the time when they're physically active, they're gonna be walking. So walking is the most fundamental form of human, physical activity. And occasionally we would run, and have enough further be selection for running to be added on top of walking.

There's no compromise between running and walking. You can do both. And clearly hunter gathers, especially as they get older, one of the things that makes humans special is that we have this long life history. Most animals don't live beyond the age of reproduction, but humans are unusual in that we tend to do so. You know, humans who survive childhood in hunter gather societies tend to live to dream about 68 and 78. So they spend about two decades, post reproductive life span on average. And those two decades are not spent in retirement.

They don't move down to Florida and, sit on a beach or play golf with a golf cart every day. They're out there hunting and gathering. And they're collecting a surplus of food, plentiful data, which shows that and what they're doing with that surplus, they're giving it to their children and their grandchildren. They're helping pass on energy as well as, you know, wisdom and knowledge and all that. But they're also passing on calories to their offspring, which of course increases their reproductive success. And most of that physical activity, which we evolve to do and continue to do as we get age is endurance physical activity, you know, walking, digging, carrying all those sorts of things involve acquire endurance.

And as you get older, those kinds of tasks become more important, not less important. Selection's operating them. So we evolve to be physically active in general, we evolve to be endurance physical athletes in general. And as we evolve to do that as we age and what we've argued in this endurance active grandparent hypothesis is that we not only evolve to be physically active as we age, but that the physical activity itself helps us age well, because what physical activity does is it stresses the body and it turns on repair and maintenance mechanisms that help us age well.

And it turns on mechanisms that prevent metabolic damage and mitochondrial damage and keep us from getting high blood pressure and repair DNA and all kinds of other repair maintenance mechanisms. And of course we know that one of the best ways to age in a healthy way is to stay active. You don't need me to tell you that. That's kinda well known, but we're arguing that there's an evolutionary origin to that. And we tend not to think about that much because a, we tend to think, you know, a lot of people focus on lifespan, not healthspan. And until recently until the modern medical era healthspan, which is the years you live without any sort of major disease equals your lifespan.

Cause once you got sick, you died. There was nobody, there were no pills to keep you alive. Once you had heart failure or osteoporosis or whatever. So we evolved to not only be physically active and that physical activity itself turns on repair and maintenance mechanisms, which also help us to live long. So it's a kinda a feedback system here. And of course, as we get older, the kinds of physical activities that are important for grandparents tend to be endurance, physical activities. They're not out, wrestling bulls and racing lions and stuff like that. They're doing standard good old fashioned walking and carrying and digging that are just so essential and so fundamental to being a human until the age of machines.

Michael Garfield (30m 49s): You noted in your talk that very few chimpanzees in the wild live past the age of 30 or 40, but on average, even a hundred gatherers. And this is again to poke a hole in the widely misinterpreted average life expectancy due to like child mortality rates. That even in hunter gatherer societies, we're still talking about life expectancies of 68, 78 years. You need to talk about burning. I don't know if this is fair, but burning twice as bright and lasting half as long.

It seems like the power endurance trade-off between humans and chimps.

Daniel Lieberman (31m 25s): I'm not sure if that's what it's about. Humans just basically are energetically amazing species. And we acquire a lot of energy when we eat really high quality foods and we devote a lot of that energy to our resting metabolism. And we use that energy to live our long lives and be physically active without all that cost energy. So we're just kind gas guzzlers of the animal world, right. And compared to our ancestors, and that helps us have this extraordinary life history. There's this idea out there, this misconception that you just mentioned that, you know, life in the paleolithic used to be nasty and short. People would die young, but that's kind of a confusion that occurs because of high infant mortality.

So life expectancy is low among hunter gatherers because of a lot of hunter gatherers die in the first few years of life. If they make it through the first few years of life, they're likely to live long and healthy lives. And in fact, if you go to your typical hunter gatherer population, a quarter of the population are grandparents, and that tells you just, you know, how impressive human longevity is. And remember those are contexts. Those are environments without any medical care. There's no doctors keeping these people alive. And healthy. They're staying healthy because they have healthy diets, but also they're staying healthy because they're staying active and you wanna increase your risk of dying young or being sick and having to take pills for several decades before you shuffle off this mortal coil. Becoming sedentary is probably one of the best ways to do that.

Michael Garfield (32m 47s): So in so much as the active grandparent thing, I have two small children and my wife seems to be considerably hungrier than I am at all times. And that's a, you know, that should be obvious. You mentioned that female hunter gatherers are in a deficit until they become grandparents. And then at that point, they're consuming more than they produce. But then things flip. This is the crux it seems like right upon which this hypothesis is all resting.

Daniel Lieberman (33m 18s): It's an important component of it. I mean, this is not my insight. This is research that was published by Hillard Kaplan at all many years ago, but it's an important component of the hunter gatherer system which is that females, we usually call the ecological species. They're bearing the cost of reproduction. And so reproduction's expensive, especially lactation. Producing milk is metabolically very costly. Furthermore, if you're a female in most of these societies, you're doing more of the childcare. And so you're taking care of kids and you're busy, taking care of small ones.

You're also out there foraging and it's very hard to get enough energy to pay for your needs plus the needs of your children. And so in general, most of these societies where people have collected good quality data, the females tend to be in a deficit. They're not able to get enough energy to pay for their needs and their offsprings needs. And so they need help. And where does help come from? Well, help comes from their grandmothers who are gathering a surplus. So they've got more than they need. So they pass that on. And also they're getting it from husbands and cousins and aunts and uncles and others.

So that kind of cooperative society is utterly fundamental to the hunter gathering way of life. Surplus doesn't just come outta nowhere. That surplus comes out of physical activity. They're not just sitting around, ordering stuff on Amazon. They're out there, getting food and getting food until recently required being physically active. That's why we evolved to be physically active, either get dinner or avoid being somebody else's dinner. And for the most part, it's get to get dinner. And you do that every day of the year. There's no retirement, there's no weekends. That's what we evolved to do.

And now we live in this bizarre modern world where machines do everything for us and we're paying a huge price for it.

Michael Garfield (34m 56s): I mean, certainly the literature is thick on walkability and staving off a senile dementia. That kind of thing. Although I've mostly heard it more in terms of it being a matter of mapping one's environment like strengthening the hippocampus.

Daniel Lieberman (35m 15s): If you look at the data, there's nothing, nothing comes close to physical activity for promoting neurogenesis and for preventing dementia. Nothing comes even remotely close in terms of the epidemiological and the mechanistic data. For example, the effects of physical activity, especially vigorous physical activity on reducing risk of Alzheimer's. It's like an order of magnitude greater than any kind of mental acuity or whatever, test or whatever. If you wanna save off, Alzheimer's one thing you can wanna do is stay active. And by the way, the more vigorous it is, the better it is by the way.

So walking is helpful, adding some vigorous physical activities like running or swimming or using the elliptical or whatever the hell you like to do has huge effects. Nothing comes close, nothing, nothing by order of magnitude.

Michael Garfield (35m 57s): This seems like an opportunity to ask you to unpack the story of the brain derived neurotrophic factor.

Daniel Lieberman (36m 5s): BDNF. Totally cool. So there are quite a bunch of molecules that are important for the brain and one that's received enormous amount of attention is brain derived, neurotrophic growth factor BDNF, and the fascinating thing about BDNF. So it's sometimes some people call it miracle grow for the brain, but it has a number of functions. But one of the things it does is it promotes neurogenesis. It promotes stability of synapses. It keeps astrocytes, which are glial cells that kind of maintain the neuroconnections and the brain healthy probably reduces inflammation that maybe astrocyte derived and it's operated by physical activity, like nothing else.

And when we think the reason for that is again, evolutionary because BDNF actually didn't evolve as a neurotropic growth factor. It actually evolved as a metabolic signal in muscle. So BDNF is turned on in skeletal muscle that activates AMPK and which is basically releases energy. And that system was then co-opted by the brain later on in evolution, but it's still up regulated by physical activity. The thing about humans is that we never evolved to turn on that system in the absence of physical activity, for the simple reason that we never had the chance not to be physically active. None of your ancestors were able, able to sit around and be couch potatoes and not have high levels of BDNF turned on by their physical activity.

So there never was this kind of modern mismatch of sedentism and low levels of BDNF until we can find some pharmaceutical way to turn up BDNF levels. If you wanna keep your brain happy, that's why physical activity is so important.

Michael Garfield (37m 34s): So with respect to that mismatch between sedentism and physiology cancer. This is something that comes up on the show a lot, you know, cancer being apparently quite a metabolic problem. And obviously we don't have excellent records stating back all the way, but it doesn't seem like cancer has historically been quite the problem that it is now. And why is that?

Daniel Lieberman (38m 1s): Look, cancer is a result of multicellular life and all our multicellular organisms have cancer. So it's not a new thing, cancer. It's a byproduct of evolution and it's a kind of evolution that's gone kind of awry in the body, but there's compelling multiple lines of evidence that cancer rates are increasing. And you can just look at it just the last few decades. It's as countries become wealthier, their cancer rates go up. And if you look at the data on physical activity, people who are less physically active, have much higher cancer rates of almost every kind of cancer you can think of than people who are more physically active.

And that makes total sense because what feeds cancer cells well, energy. Cancer cells are cells that are competing for energy from other cells. And so when you don't spend physical energy on physical activity, that energy goes to other processes and one of them is cancer. So for example, high levels of insulin, which is your basic anabolic hormone are closely related to levels of cancer. So physical activity drops your insulin levels because you're producing glucagon when you're physically active and high levels of mitotic hormones like estrogen and progesterone and testosterone are all increased cancer rates.

Most breast cancers are strongly related to higher levels of progesterone and estrogen. When you're not active, what does the body do says, ah, got more energy available. Let's shunt it towards reproduction cuz after all that's all that natural suction cares about. If you have any extra energy, you win from a selector perspective if you shunt that towards energy. So people are less active, their heart estrogen levels shoot up. Their progesterone levels shoot up, which in course increases their risk of breast cancer. We have multiple, multiple, multiple studies, which show that physical activity dramatically lowers a woman's lifetime risk of estrogen progesterone sensitive breast cancers by 30, 40, by some estimates 50 percent. It lowers colon cancer rates.

It lowers almost every kind of cancer you can think of. And there are other effects. One is to reduce the amount of circulating glucose. Most cancer cells are sugar hungrier. They don't engage in aerobic respiration. It's called the Warburg Effect. And so physical activity lowers those levels. So there are many, many pathways by which physical activity decreases cancer rates. Now of course it's not the only factor that is a responsible for increased cancer rates in the modern Western world. But it is an important one. And I mean, you just have to be a flat earth, not to appreciate the many, many, many, many studies that show that physical activity rates are strongly associated with lower rates of cancer.

And strangely enough, when we talk about cancer, we rarely as a society talk about how to prevent cancer and how physical activity is a very clear unquestionable form of prevention and from an evolutionary perspective, it makes sense. And from a mechanistic perspective, it makes sense. And of course there's abundant data, both epidemiological as well as mechanistic to explain why that's the case. It's sadly under discussed.

Michael Garfield (40m 48s): And yet one of the funnier parts of the colloquium you gave was on how you would think perhaps that physical exercise is good. Therefore we should be putting zoo animals on treadmills and so on. And yet what we find is not that.

Daniel Lieberman (41m 6s): So, well again, that's kind of a prediction of the active grandparent hypothesis is that there's been special selection in humans. And again, remember humans are different from zoo animals because we have this long post reproductive lifespan. Now we've been selected to be a very physically active and do a lot of endurance physical activity, but also to do that as we age and that physical activity as we age is maybe an important mechanism by which we age in a healthy way increase our health span. And so one prediction is that physical activity may play a more important role in maintaining health span in humans than in other creatures.

And sadly, there's just not a lot of good data on this cause it's just not something that people are studying, but there are quite a few studies where people that put mice and rats on treadmills or measure their wheel running and all that kinda stuff, it doesn't have the benefit in rodents as it does in humans. There's a mouse selection experiment that's been done by a guy named Ted Garland in them. You see Irvine, I believe is that really cool. He's bred these mice to be super runners, marathon mice. And these mice, they actually live less than the mice who are running less and they wanna run. They have this intrinsic desire to run and they run ridiculous amounts.

It's also true that zoo animals who you'd think would get less physical activity and once you know, one's correct for diet, et cetera, because they're more sedentary. They'd pay a price for it, but there doesn't seem to be any evidence for that. They're not senescing at a slower rate. So, so far the evidence suggests that this is really pay a phenomenon. It may special to humans, or it may be more intensely, maybe some shifted up in humans. We don't know, we need to do more research on that topic. So that's really kinda more hypothetical, but it's an interesting sort of glimmer of data.

Michael Garfield (42m 41s): So I mean, based on all of this, what is the prescription? Obviously, it's fairly common for folks at SFI to have standing desks, to go on walking meetings around campus. What are the ways in which you can imagine our life as modern human beings adjusting within the constraints of the institutions and the infrastructure that we've enveloped ourselves with that can adjust and actually accommodate to this and make sense of it.

I mean, it's not Peloton, right?

Daniel Lieberman (43m 13s): You know, this is the 64 gazillion dollar question because one of the problems with exercise is that exercise is discretionary voluntary physical activity for the sake of health and fitness. And nobody did that until recently. When you live in an environment such as our ancestors did where energy is limited and you have to be physically active in order to get food in the first place, any extra physical activity is a really stupid thing to do. It's gonna decrease your reproductive success. So we evolved to be inactive whenever possible. We evolved to be physically active for two reasons and two reasons only when it's necessary or when it's rewarding.

And so if we're gonna improve people's physical activity today, we need to understand that those instincts are deep and fundamental. And if you ever have an escalator and extra stairway, people are gonna take the escalator even though there were no escalators in the stone age, right. It's just an instinct to save energy. And we tell those people they're lazy, but they're not lazy. They're just normal. They're just acting on basic and fundamental instincts. So in the modern world, we need to find ways to help people be more physically active when we no longer have to do it because machines are replaced human labor with just press a button and dinner arrives.

So we need to find ways to do it. But I think using evolutionary logic, the only two ways to do that are to make it necessary and rewarding. That's gonna involve some social engineering. So in schools, for example, we need to promote physical education more. I mean, most schools are pathetic in terms of the amount of physical education that kids get in schools. And of course the habits that you develop in school tend to be the ones that you live and retain pain for your life. Especially in college. Most universities are completely given up on physical education. Harvard is no exception. We serve a physical education requirement and it was abandoned in 1970.

That's true of almost every American university and the ones that are left tend to be pathetic. So we don't do that. And also we live in environments where it's great that you have walking meetings at SFI, but most places don't do that. So we need to find other ways to do it. And again, we're gonna have to be creative. And again, make it necessary and make it rewarding. And there are ways to do that. I think the way to do that is to treat exercise like education, another abnormal, modern behavior that we never evolve to do. And how do we make it work? We make it necessary.

Make it rewarding. School is fun. We meet friends there. We do all kinds of things apart from learning math and biology and whatever it is you learn in school. And we need to find ways to do the same thing for exercise. And sky's the limit really. Or things like dancing. How many people think of dancing as exercise? But it's physical activity that's rewarding, right. And why not have more dancing? I mean, I could go on. But that's what we're gonna have to do. Standing desks are great. You know, they prevent you from being completely inert, but standing desks are not forms of exercise. So let's not equate that with.

And actually there's yet to be any good data showing that standing desk have really major benefits. I suspect they do, but there's no data on that.

Michael Garfield (45m 56s): The spirit of this show is one in which this often links out from the central topic into these other things and listening to all of this, I can't help but think of the conversation I had with Stephanie Crabtree and Devon White on their work reconstructing the migratory pathways that the peopling of the continent that is now Australia. And they started out thinking, okay, let's look at satellite data and let's map least effort paths across the landscape. That's the laziness piece.

You would expect that that's how it worked. But what they realized is that by talking to people that actually live in these spaces, that these people orient themselves with visual landmarks, kinda like ritual or sacred monuments, like Ayers Rock being the most well-known among them. And that as soon as they included line of site visibility and orientation into their model and included this piece about the meaning or the significance of going out of your way to visit a particularly interesting spot, their pathways that came out of the model, much more closely resembled the actual Aboriginal song lines and what people that live there say is the actual map, the foot network for Australia.

And so when you're talking about social engineering, I think a lot about making things intentionally difficult for the fun of it and the importance of that. And I don't know if you've seen this film, spectacular documentary 3,100 Run and Become by Sanjay Rawal. I spoke with him a while back and it was an interesting conversation about just the persistence of endurance running as a kind of ritual or sacred activity across human cultures. 

Daniel Lieberman (47m 46s): Oh,absolutely.

Michael Garfield (47m 47s): So I'd love to hear you riff on that, cuz it seems like that's really what we're getting at here that it's not just the empty like hamster wheel thing, but it's about having a reason.

Daniel Lieberman (47m 58s): So I said, we evolve to be active for two reasons when it's necessary and awarding. And a good example of that, that I've written about are the Tara that Native Americans are well known for running long distances, turns out every native American population, every native American peoples have long distance running traditions. It's just that most of them have been lost as they become westernized and the Tarahumara because they're still very isolated in the Sierra. Tarahumara have sort of maintained a lot of their old traditions in ways that a lot of other groups have struggling to keep up today.

And one of the things that you'll learn, if you look at every Native American running tradition is that running has a spiritual dimension. One of the things that, one of my things I really dislike about the book Born to Run, which have popularized the Tarahumara, is that it never mentions that the main reason that they do these long races is that it's a form of prayer to the Tarahumara completely left out of the book. And it's really spiritually very important to them. And that's true for Navajo and Hopi. And I could go on. Every population it's true for wherever you look around the world.

And I think that makes sense for just the reason that you've articulated. It's part of who we are and it plays important roles and it helps train people for multiple reasons, but it also helps us thrive and endure in various ways. And furthermore, there are other interesting spiritual dimensions that come from endurance. One of them is that you can enter a kind trance-state. There's a really interesting hypothesis that some of the rock art from Southern Africa, who much of it depicts hunting scenes, but a lot of those hunting scenes involve animal human transformations and evidence for sort of trans like states that is a consequence of long distance running.

Anyone who's had a runner's high can tell you that when you run, it becomes a kind of a, I'm not a very religious person. I'm not a religious person, but I've experienced runner's highs. And it is a very kind of spiritual trance-like feeling that you get from doing endurance. It's part of the same phenomenon. And I don't think it's not coincidental, this relationship. And of course also in, in most societies, a lot of endurance acts are very social. As an example, think about the modern marathon movement, right? You know, the biggest single day charity in the world is the London marathon. It's hard to get into these marathons.

I mean, you try to get into Tokyo or Berlin or New York or whatever, Boston Marathon. I mean, you have to get on waiting lists for years or pay ridiculous amounts of money. And there's so many people who wanna do it and, but they're doing it as a group and you're doing it with other people. And there are millions of people watching you and you're raising money for charity and they're giant community events and anybody who's participating in a big city marathon knows that you're not just running for yourself, you're running for the community around you. And you're doing that to help people as well as yourself. And that is a part of the ancient history of physical activity because we aren't physically active just for ourselves.

We're physically active in order to help each other. And running is a part of it, not the only part of it, but as a part.

Michael Garfield (50m 46s): It strikes me that one of my daughter's favorite films is Wall-Ewhere humans go into space and they all become cybernetic couch potatoes and bone density drops. But the motto, the slogan of the state of Kansas is Ad Astra Per Aspera. 

It's not Ad Astra on your ass, so it's like there is something kind of tragic and yet wonderful about the fact that we are destined to make things difficult against our own drive for laziness.

I don't know. It's just curious paradox that I find your work really speaks to. And I really appreciate it. Is there anything else that we haven't asked? Is there a big question?

Daniel Lieberman (51m 30s): You could go on for hours. I mean, obviously there's tons to talk about, but lemme conclude on the following thought, which is that we have this idea that's been common in Western thought for millennia. St. Augustine, for example, that there's this kinda separation between the mind and the body. My colleagues here at Harvard certainly think that right where our job is to educate our students' mind, to help with their bodies. Fear them crap. And who cares if they exercise or not. But that's just a completely false dichotomy. And the more I study the evolution of physical activity, the more I realize that those connections between the mind and the body are completely artificial. They're part and parcel of the same integrated systems.

And we are in trouble when we forget that.

Michael Garfield (52m 10s): Awesome. Thank you, Daniel. This has been a, a fun, conversational sprint with you, and I really appreciate having you on the show. Thanks a lot.

Daniel Lieberman (52m 17s): My, my pleasure. Good. All right. Take care and apologies for rushing off.

Michael Garfield (52m 21s): Oh, no problem. It's okay. All thank you for listening. Complexities produced by the Santa Fe Institute, a nonprofit hub for complex systems science located in the high desert of New Mexico. For more information, including transcripts research links and educational resources, or to support our science and communication efforts. Visit