英語語言學習：進化、健康與疾病

This is FRESH AIR. I'm Terry Gross. Acid reflux, acne, anxiety, asthma, certain cancers, depression, type 2 diabetes, flat feet, high blood pressure, irritable bowel syndrome, lower back pain and osteoporosis - that's just a partial list of non-infectious diseases that might be caused because our bodies haven't sufficiently adapted from the Paleolithic Era to modern life. This is what evolutionary biologists like my guest Daniel Lieberman are proposing.

Lieberman is the author of the new book "The Story of the Human Body: Evolution, Health and Disease." He admits that if you're sick, you would not go to an evolutionary biologist to get treated. However, evolutionary biology can help you understand why you got sick, and it can help you be more aware of healthy and harmful behaviors. Lieberman is a professor of evolutionary biology at Harvard University.

Daniel Lieberman, welcome to FRESH AIR. I want you to explain the basic premise of the book, about mismatch diseases.

DANIEL LIEBERMAN: Well, many people wonder how their bodies function today, and what's the relationship between how our bodies evolved and how we use them and how we get sick. And a key premise of the book is that many of the illnesses that we confront today are what evolutionary biologists call mismatch diseases. I didn't come up with the term. It's been around for a while.

But mismatch diseases are diseases that occur because our bodies are poorly or inadequately adapted to environments in which we now live. So an example would be eating large amounts of sugar or being very physically inactive leads to problems like diabetes or heart disease that then make us sick.

So mismatch diseases are really diseases that are modern, in the sense that they're more prevalent or even novel, or more severe because we don't live in the way in which our bodies are adapted.

GROSS: OK, let's look at diabetes 2, type 2 diabetes, which is, you know, in epidemic proportions in the United States and now much of the world. So this is a good example of a mismatch disease. Why are bodies ill-equipped for the kind of foods and fats and sugars and the quantity of food that we're eating now?

LIEBERMAN: Yeah, well, this is an important topic, obviously. You know, in India and China, for example, rates of diabetes are going up by almost an order of magnitude over the last generation, and they're going up very rapidly in the U.S., too. And the reason we're - they're mismatch diseases is that diabetes, for example, is caused by, essentially, the body being unable to cope with sugar in the bloodstream.

So we evolved to crave sugar, because sugar is an energy-rich food, but we didn't evolve to digest large amounts of it rapidly.

GROSS: So our bodies, the basic operating system of our bodies was designed for the hunter-gatherer era, right?

LIEBERMAN: To a large extent. I mean, until hunter-gatherers, and even before hunter-gatherers, chimpanzees, for example, eat plenty of foods that have carbohydrates. For example, chimpanzees eat almost all fruit. Right. You think, oh, if they eat all fruit, they must be getting a lot of sugar. But the sugar that they get is very different from the sugar that we get.

First of all, it's bound in cells. You have to digest it, and there's lots of fiber in and around the sugar. So you don't get access to that sugar really fast. And there's much less sugar in those sorts of foods. So we evolved to crave sweet foods. Everybody loves sweet foods. But if you try to eat foods that hunter-gatherers eat or chimpanzees eat, you'll be surprised at how un-sweet they are. Most wild fruits are about as sweet as a carrot.

So we love sweetness, but until recently, pretty much the only food that we got that was very sweet was honey, and honey, of course, was a special treat. That was pretty much the only form of dessert in the Paleolithic. But now we have access to abundant quantities of sugar and to simple carbohydrates, which we evolved to love because they're full of energy, but we don't have the metabolism, we don't have the bodies that are able to cope with those kinds of levels of sugar. And the result is that we get sick.

So, type 2 diabetes used to be exceedingly rare. I mean, it did happen occasionally, but it was an extraordinarily rare disease. And to my knowledge, there's not a single case of any hunter-gatherer ever diagnosed with the disease. And it was rare for our great-grandparents. But today, it's become very common because of the foods we eat for which we're not adapted. So, in that sense, it's a very classic mismatch disease.

GROSS: So, like our ancestors a zillion years ago, they didn't have sweets like we have sweets. The body needed - the body needs sugar, and it was designed to process sugar from foods like fruits, not designed to process sugar from foods like apple pie with icing on it or cinnamon rolls and stuff like that that are just, like, loaded with fat and sugar. Do I have that right?

LIEBERMAN: And all that good stuff. Yeah, exactly. So, sugar is good, right. Energy is good. Think about what natural selection's really all about.

GROSS: Right. Mr. Hunter-Gatherer needed that energy.

LIEBERMAN: Right. Well, the more energy you have, the more successful you'll be. Most hunter-gatherers, most animals live on the margin of energy balance, right? They have just enough energy to do what they need to do, and any more energy is a good thing, right. It means that you can put more of that energy into growing a better body, or put more of that energy into reproducing and put some of that energy into coping with periods when food is scarce.

And one of the interesting things about humans is that we are specially tuned to energy, compared to other animals. We're actually a very fat species, compared to other primates and most other mammals. The average primate, a chimpanzee or a monkey, has maybe about 5 or 6 percent body fat, whereas a thin human being, a very - you know, a supermodel has more, much more body fat than that.

You know, it's normal to have about 10 to 20 percent body fat for a normal, average, healthy human being. So we're already a very fatty species, in a way, and that's because we have a very - we evolved a very energy-intensive way of reproducing. And that part of our story is incredibly important to know, because we pulled off kind of a hat trick in the Paleolithic, right.

We figured out how to grow big bodies with really big brains. And brains are very expensive, and they always need lots of sugar all the time. And we also figured out a way to have babies about twice as fast as our ape ancestors. So a typical chimpanzee has a baby every three years, and she waits until that baby is pretty much fully grown and able to feed on its own before it has another baby.

But humans are able to have - in the - you know, today, of course, we can have babies every year or so. But hunter-gatherers managed to have babies every three years, and that's well before a baby is able to eat on its own, forage on its own. So mothers have to have lots of energy to grow babies in the womb. They have to have lots of energy to breastfeed them, and that costs a lot of energy to synthesize, to make breast milk. And they also have to have extra energy to feed those toddlers and young kids around who can't fully feed themselves.

That reproductive strategy that we evolved at some point during the Ice Age requires enormous quantities of energy, and it requires energy reserves. We have to be able to store it in our bodies. We have to have access to a lot of it, and so we're like gas guzzlers of the primate world.

GROSS: You know, in your book, "The Story of the Human Body," about evolutionary medicine, you write about how when farming evolved, then industrialization, the nature of what we were eating changed, and the nature of how we spent our time changed, too. You know, with farming, we started to eat more grains, more carbohydrates, and you write about how that can be very taxing on the body.

How come our bodies haven't evolved more effectively to deal with more modern life, to deal with the kind of foods we actually do eat now?

LIEBERMAN: That's a wonderful question. Well, to some extent, we have. But some populations have done it a little bit better than others. So going back to type 2 diabetes, for example, there are genes that protect some people more than others from the disease. These are genes that probably evolved fairly recently that give some populations - it tends to be Europeans - better ability to cope with high-glycemic foods, and that means foods that cause more sugar in your bloodstream.

But some populations, for various reasons, probably chance, never evolved as many of those adaptations. So they tend to be more prone to developing diabetes and obesity from the same kinds of diets. So India and China are examples of countries where these epidemics seem to be growing faster because of a lack of selection that somehow occurred, or just the genetic variation that made it possible for evolution to give some protection.

So we did actually evolve, to some extent, to cope with some of these changes in our diet. Another factor is that a lot of these diseases don't occur until after people have reproduced. So most people who get diabetes, for example - I know we're harping on this disease - but most people who get many of these diseases, heart disease, diabetes, hypertension, often get them after they've had their kids and after their kids are grown up.

And so the ill effects of these diseases don't have a very strong selective effect. So natural selection is not going to really operate that strongly.

GROSS: Oh. I see what you're saying. Because you've already reproduced, so any adaptation that you make as a result of the disease, it's too late to have an effect on reproduction.

LIEBERMAN: That's right. And, you know, it can have some effect. So we know, for example, that there's a strong selective benefit to being a good grandparent. Grandparents who do well - particularly among hunter-gatherers - their offspring do better. But with the origins of farming and industrialization, that role of grandparents in terms of providing food has declined in importance, and so those grandparenting-selective effects have probably also declined, as well.

(BREAK)

GROSS: Let me ask you about comfort food. Many of us, when we're stressed out, we crave carbohydrates or sweets, and you say there's actually an evolutionary reason for that.

LIEBERMAN: Yeah. So, the hormone that gets elicited when you get stressed is cortisol. That's the chief one. So when - if you jump out an airplane, or if something really bad happens, a loved one dies, our body goes into a flight-or-fright state. We're ready for danger. And cortisol plays an important role, because it releases sugar into the bloodstream. So you can run from that lion or deal with whatever that bad situation is. It makes you more alert.

But it also - cortisol also makes us want to bring in more energy to cope again with that - those energetic needs. So stress activates basic primal urges to eat calorie-rich food, which is, of course, useful to cope with those kinds of situations from an evolutionary perspective.

So people who have lives that contribute to higher basal levels of cortisol are more likely to suffer the ill consequences of that stress.

GROSS: So when we're stressed, we secrete more cortisol, which gets us to crave fats and sweets. Another part of the problem here is that just because we're stressed doesn't mean that we're actually running from a beast that's chasing us. We're probably sitting in a chair or standing. So how does that affect how our bodies respond to both the cortisol, and also to the sweets and carbs that we're eating?

LIEBERMAN: Well, stress creates this sort of vicious cycle, this positive feedback loop, because when you're stressed, you crave unhealthy foods. But when you're stressed, you also have a harder time sleeping. And when you have a harder time sleeping, that elevates your levels of stress. It just sets off this chain reaction that keeps going on and on.

Now, normally when we're stressed, that's - the stimulus that causes the stress should be a short-lived one. That's what evolution predicted, right. A lion chases you, that's a very stressful event, obviously. Hopefully you manage to run away from the lion. You climb into a tree. You calm down. You tell everybody the story. Life goes on.

But much of the stress that we create today results from social conditions. If you think about, you know, the most stressful things that we experience, they're often our lives. They're our jobs, our commutes, having - not having enough money, the list goes on. And those, of course, elicit chronic levels of stress.

And when stress becomes chronic, then it helps feed a variety of mismatch diseases that make us ill, that make us depressed, that make us anxious, that make us overweight, which then cause more stress, which then keep the cycle going.

GROSS: What are some of those mismatch diseases that arise from the stress response?

LIEBERMAN: Oh, gosh. Many of them are metabolic diseases. So anything that has to do with high blood pressure, so hypertension, strokes. Stress certainly has effects on cardiovascular disease. Stress has effects on many mental health diseases and illnesses. In fact, the list is so long, I'm not sure it would be a good idea to list them all.

GROSS: Oh, OK. But - well, let's get to digestion, since we're talking about food and stress. You describe the gut as a second brain, having more nerve endings than our spinal cord or our peripheral nervous system. What are all those nerve endings doing in our gut?

LIEBERMAN: Oh, gosh. Well, they're helping move food around. They're - they help with just basic process of digestion, your - for example, when you eat a meal, you have to push all those - all that food through your digestive system, all the way out to the outside world. And so you have all these contractions that your body is completely unaware of that are squeezing the food along.

There are other nerve responses that sense what kind of enzymes need to be added to the system. It's - digestion is a very complex phenomenon that has to be regulated, hence has a lot of neural control.

GROSS: Why does our gut know when we're nervous?

LIEBERMAN: That's a good question. I don't actually know the answer to that. Certainly - well, I know partly the answer to that. Certainly, much of our digestive system is regulated by what's called the autonomic nervous system. It's the involuntary nervous system that you have. And the involuntary nervous system, of course, is heavily regulated by hormones. And among the chief hormones that regulate that system is cortisol.

So, when we get stressed, or when we get excited, hormones then affect pretty much every aspect of our body, including our digestive system. So sometimes we want to activate energy. Sometimes we want to store it away for future use. So, you know, one of the things that one learns when studying and teaching human biology and physiology is just how interconnected every system of the body is with other systems of the body.

It's - you can't change one thing without changing many things. And the hormonal system is - its job, actually, is to help make all those systems work together.

GROSS: So while we're talking about digestion, you write about how irritable bowel disease and Crohn's disease, which lead to all kinds of intestinal distress, are mismatch diseases. So put that into context for us.

LIEBERMAN: Well, many of the mismatch diseases that we think are going on - or actually, they're hypothesized, because we don't totally understand their origin. But Crohn's disease, for example, is a recent disease. It used to be extraordinarily rare, and today it's cropping up more and more as - and it's an autoimmune disease. It's a dysfunction of the digestive system that starts essentially attacking itself when - often in response to certain kinds of food.

And celiac disease would be another. And we think that - we're not quite sure, of course, what causes Crohn's disease. And we're not entirely sure what causes the immune responses that lie behind celiac disease or irritable bowel syndrome. But the fact that these diseases are recent in origin, they're becoming more common, they're becoming more severe, particularly in developed countries, are all the hallmarks of mismatch diseases.

And there's enough evidence to suggest that the way in which we live today - and that can be anything from antibiotics to our heavy reliance on cereals and grains - are promoting those diseases.

GROSS: How are they promoting them?

LIEBERMAN: Well, we don't know exactly. Allergies are a fascinating example. I mean, the rate of allergies to gluten in wheat, or peanuts, or all kinds of proteins and substances that used to be very common and harmless raises a fascinating question, because there's no gene sort of sweeping through America causing peanut allergies. It's clearly an environmental cause.

And the immune system - if there's any system in the body that's very complex, it's the immune system. And we don't really know yet what it is that's causing our immune systems to react as they do in this abnormal fashion. But one hypothesis that's gaining a lot of traction recently is the idea that somehow, we are knocking out our immune system or causing malfunctions through a combination of overuse of antibiotics and also not using our immune systems in the ways for which they're evolved.

So this is often called the hygiene hypothesis or the "old friends" hypothesis, that our immune systems evolved to be active. Just like our muscles and skeletons evolved to be used and stressed, our immune systems evolved to cope with all those germs in the outside world. And we've now created environments where - that are very sterile. They're extremely clean. We have very few pathogens that we have to deal with. And when we do get them, we nuke them with antibiotics.

But in so doing, we are now affecting how our immune system functions. It's still there, and it's primed and ready and waiting to attack all those nasty germs and worms that used to make us sick, but now those pathogens are absent. So it's - sometimes by chance, it finds the wrong targets.

And so that's a hypothesis for why so many allergies are on the rise and so many autoimmune diseases are on the rise, is that our immune systems are essentially not being used properly, and as a result, they go into overdrive. They attack ourselves through accident. And the result, of course, is very unfortunate. It's horrible.

GROSS: Daniel Lieberman will be back in the second half of the show. His new book is called "The Story of the Human Body: Evolution, Health and Disease." He's a professor of human evolutionary biology at Harvard. I'm Terry Gross, and this is FRESH AIR.

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GROSS: What would happen if we all read your book and thought, oh, mismatch diseases; we're eating the wrong foods and we're not doing what our bodies were designed to do, so let's go back to a Paleolithic diet?

LIEBERMAN: Ah, well, there are a bunch of people who are trying that experiment right now. It's...

GROSS: Oh, please don't recommend it.

(LAUGHTER)

LIEBERMAN: No, I'm not a...

GROSS: I really don't want to do that.

(LAUGHTER)

LIEBERMAN: I'm not a - I'm a bit of a skeptic about the Paleo diet. One of the things that's interesting about the health movement today is that just as there is a lot of polarization in American in terms of politics and class, that's also happening with how we use our bodies. And it used to be that only rich people could afford to be unhealthy and overweight and inactive, and now it's, it's almost reversed and there are a lot of well-off, educated people who are now really interested in using their bodies better and they're exercising and they're running marathons and they're trying various kinds of diets.

And one of them that's become very popular recently, of course, is the Paleo diet. And I think that the Paleo diet has some germs of truth to it. It is true that an evolutionary perspective does help us understand that there are certain foods for which we're better adapted and there are other foods for which we're less adapted. But the Paleo diet also has a kind of simplistic approach to this question, almost creating a bunch of rules that don't necessarily make any sense from an evolutionary perspective. After all...

GROSS: So before you describe some of those rules, what's the diet?

LIEBERMAN: So, people who eat the Paleo tend to, they avoid all cereals and grains. They eat a lot of fat and meat because they have a, there's a notion that lots - our ancestors ate huge amounts of meat.

GROSS: And this is the pre-farming era, so there's no grains yet.

LIEBERMAN: That's right. Which is actually not true, because we know from various archaeological sites that hunter-gatherers, when they had grains available to them, did eat grains in probably very large quantities - although not, of course, as much as farmers.

They tend to avoid milk products because after all, we evolved to drink milk only when we're young and not to drink milk when we're older, after weaning. Many of them avoid legumes - things like peanuts, etcetera, which were not eaten by hunter-gatherers. At least so they think.

So it's a kind of diet by analogy. If hunter-gatherers ate it, it must be good; if we eat it - if it's more recent than hunter-gatherers, it must be bad. And you can, of course, quickly appreciate that some of that logic is a little bit flawed because just because something is recent doesn't mean it's bad. And just because something is old doesn't mean it's good. And furthermore, hunter-gatherers didn't evolve necessarily to be healthy. They evolved to have lots of babies, and health was only selected for in so far as it helped people have more babies. That's after all, what natural selection's really about.

And hunter-gatherers, in fact, aren't always healthy. There's I think a kind of a little bit of romanticism applied to hunter-gatherers. And there's also no one kind of hunter-gatherers. After all, our ancestors who are hunter-gatherers lived in environments as diverse as the African savanna to in rainforests and in the Arctic and everywhere else, they managed to eke out a living in all kinds of different habitats. And there was no one Paleo diet, there were many Paleo diets. And so it's a complex problem.

GROSS: And probably they didn't live to celebrate their 80th birthdays very often.

LIEBERMAN: Well, you know, actually you'd be surprised.

GROSS: Really?

LIEBERMAN: So hunter-gatherers have very high infant mortality rates. You know, it varies from population to population and the data aren't great, but maybe between 30 and 50 percent rates of infant mortality. But once they survived childhood, they actually tended to live reasonably long and healthy lives. And it was - actually the origins of farming that caused health to really decline in that respect. So farmers also have high infant mortality rates. But farmers also have high mortality rates in general and people started dying younger and becoming shorter. So farming really was initially a very bad thing for the human body in terms of longevity and nutrition and health. Of course, it was great for increasing the number of offspring people were able to have.

But to return to the Paleo diet, the thing about the Paleo diet is that there was also some truth to what some of the Paleo diet proposes. I mean, it is true that much of what we eat and do in our modern life we're poorly adapted for and the Paleo diet does go some of the way toward correcting that, but I'm not sure I agree with everything they say.

GROSS: Well, because it's going too far?

LIEBERMAN: In some respects. For example, there's a big debate going on about fats and what kinds of fats are healthy, and we've been hearing for years now that saturated fats are evil and unsaturated fats are healthy. But some hunter-gatherers eat a lot of saturated fats. And so some Paleolithic diet proponents believe that you should just have as much saturated fat as you want. And there are various arguments for and against that but that's an experiment that I, for example, am not willing to undergo, is to eat as much saturated fat as I'd like. There are still scientific arguments about what ratios of fats are the best and what's the right fat. And in fact, I think even asking what's the best is often problematic because again, what's the output parameter you're interested in? You know, are you interested in living longest or being more vigorous or having more offspring? Remember that what natural selection cared about the most was how many offspring you had who then survived to then have offspring themselves. So just because hunter-gatherers may eat certain diets doesn't mean that they're going to - that that's the same kind of diet that's going to promote health in a modern context.

GROSS: You've said that no organism is primarily adapted to be healthy or to live long or to be happy or to achieve any other goals, except for the goal of reproducing. That evolution is all about evolving to the point where you can more effectively reproduce. I suppose in some ways that's a pretty obvious notion. But when you think about it, it's a really, really rich idea. And so just like, work off of that for us a little bit. What are the implications of that in terms of how we live our lives?

LIEBERMAN: Well, I think we have this idea that if we evolve to do something, it must be good for us. But, you know, we live in a world in which what we think is good for us, or what we value, may not be the same currency that evolution and natural selection values. And that's why I think we need to be thoughtful about how we apply evolution to our lives today.

GROSS: Well...

LIEBERMAN: I'll give you an interesting example...

GROSS: Sure. Yeah.

LIEBERMAN: ...from the public health literature. So often there are very well-meaning and well-intentioned efforts to try to alleviate poor health that's caused by malnutrition and undernutrition, people not having enough food. And a very famous example was in the Gambia. I think it was in the 1950s, I believe, when, you know, lots of women were giving birth to underweight babies and small - you know, babies who are very small when they're born have a much greater chance of a wide range of health problems as they mature, because they're, essentially their mothers haven't had enough energy to help the body grow properly in utero and so that sets up a body that's much more likely to succumb to many diseases as it grows. So these mothers were - for the best of intentions - given nutritional supplements.

What happened was that instead of giving birth to larger weight babies, what happened was that these mothers essentially started to more often just give birth to underweight babies but more often. They just shortened what we call their interbirth interval. So again, it goes to the point that they're doing exactly what their bodies evolved to do, which is that if you're in a energy-poor environment, what really matters is how many, you know, what really matters from an evolutionary perspective is reproduction. And so they're doing the evolutionarily rational thing to do. I mean they're not making decisions obviously; this is their bodies responding - which is to use that extra energy to improve their reproductive success. So it failed - the program failed because it didn't consider that evolutionary principle.

(BREAK)

GROSS: You know, if our bodies are primarily designed as reproduction machines, you know, the things that evolve so we could better reproduce, well, now we're living in an age where so many people use birth control and limit the number of children they have intentionally or have no children at all, intentionally or unintentionally. It seems like we're really mismatched in terms of evolutionary development with the lives that we're leading.

LIEBERMAN: Yeah. Yeah. And, of course, an important example of that is breast cancer. So it's, you know, breast cancer was originally known as the nun's disease because it was very rare until recently and it was a, you know, medieval doctors noticed that it was more prevalent - or Renaissance doctors noticed it was more prevalent in nuns who didn't have offspring. And contraception has many benefits, of course. But everything has costs, right? There is no such thing as a pure benefit without a cost in your body. All things have trade-offs. Every time you do one thing, you don't do something else. And so contraception has enabled, you know, brought much good for women, giving them control over reproduction, but it does raise the risk of breast cancer because it means that women have more reproductive cycles where they're producing high amounts of estrogen and progesterone, hormones that are very important in that cycle, and those hormones are important for reproduction, but they also elevate the risk of breast cancer, because they are mitogens. They cause cells to reproduce, they cause cells to divide and proliferate, and the more cells that divide and proliferate, the more chances there are for mutations to arise that can turn into cancer. So that's another, that's a perfect example of a trade-off between reproductive strategies and the way we use our bodies today.

GROSS: Speaking of cancer, you describe cancer in a way as evolution gone wrong.

LIEBERMAN: It really is, because cancer is a kind of natural - it's almost a kind of selection that occurs in your body, because cancer cells are cells that have acquired a series of mutations, usually it's more than one mutation, that enable them to essentially outcompete other cells. So it's a kind of selection within the body and cancer cells essentially start taking over the cells and other organs, taking their nutrients, which is what causes us to die from cancers. And so in a sense we wouldn't have evolved cancer if we hadn't evolved multicellular life and if natural selection didn't occur.

GROSS: You write a little bit about exercise. Some people love to exercise, some people hate it. But one way or another it's important. And it relates to the fact with muscles it's a kind of use-it-or-lose-it condition, which is again an evolutionary issue. Would you explain that?

LIEBERMAN: Well, many systems of our bodies evolved to require some form of stress. And I don't mean stress in a bad way, like something that makes your cortisol levels go up. But stress in a good way; they evolve to have some, require some circumstances, some stimuli in which capacity might not match demand. And our bodies respond to those stresses. So when you lift something very heavy, you know, your muscles get tired but that soreness in your muscle actually is partly related to the response of your muscle. That makes your muscle stronger. And so our bones, our muscles, our brains, many aspects of our body require stimuli to grow and develop properly. And some aspects of our body even require those stimuli to maintain those tissues because, for example, muscle is very expensive. If you're not using it, you want to get rid of it, because it's costly just to maintain.

GROSS: By costly you mean it takes a lot of energy, a lot of...

LIEBERMAN: Calories.

GROSS: Calories. Right.

LIEBERMAN: Yeah.

GROSS: And in a calorie-poor environment, that's expensive.

LIEBERMAN: Yeah. About 40 percent of a typical person's body is spent just paying for your muscles, just to keep those muscles functioning. So if you're not using them and you're energy poor and you want to spend as much energy on reproduction, as opposed to sitting around, you're better off not having those extra muscles, which is why our muscles waste away when we don't use them.

The problem is that we live in a world for which we've kind of taken that to an extreme, right? So I think osteoporosis is really for me the most extreme example or the most, one of the most profound examples, which is that our skeletal system is designed to grow under conditions of stress. It never, we never evolved to hang around all day long and get rides to school and not exercise very much, and as a result, many people today are developing skeletons that just don't have that much mass, that's just, there's not that much bone. But our skeletons also evolve to lose mass as we age and that mass loss, that loss is exacerbated by lack of exercise. So when astronauts, for example, go to space in microgravity environments, they lose bone at a really rapid rate.

And so what happens is that we tend to grow skeletons that don't have enough mass in the first place and then as we age, we accelerate that bone loss process. And the result is that more than 30 percent of women today have osteoporosis, and those rates are also climbing.

And that's preventable. It's preventable by having more physical activity when we're young and it's preventable by having more physical activity as we age to slow that rate of bone loss that inevitably occurs. So yes, we do need to use our bodies in order to maintain health, and that use-it-or-lose-it phenomenon really has an evolutionary origin.

GROSS: You write that we often confuse comfort with well-being. Explain what you mean.

LIEBERMAN: Yeah. Well, we all love comfort. Comfort is, well, comfort. Right? I'm right now, for example, sitting in a very lovely, comfortable chair that's cushioned and it's just at the right height and angle. And we like that, right? And we like comfortable shoes and we like comfortable beds. But just because something is comfortable, it doesn't mean it's good for us.

And in fact, many things that are comfortable we actually know are bad for us. I mean there's increasing awareness that chairs, for example, cause a host of problems if overused. For example, when I'm sitting in this chair now, I'm not using my back muscles to hold my upper body up, and that means that my back is not as strong as it could or should be.

My hips are flexed and, you know, just as you, you know, people who wear high-heeled shoes end up getting shorter calves and Achilles tendons, that's causing my hip flexors to be short and that can result in poor posture. So there are many costs to comfort that we don't perceive because those costs tend to be very small and incremental, in tiny doses, and they add up over decades. And we're unaware of those costs until it's too late.

GROSS: Have we stopped evolving?

LIEBERMAN: Definitely not. Evolution is just always churning along. Evolution, after all, is just change over time. And natural selection, which is the kind of evolution we tend to be thinking about the most, is caused by just a few phenomena that are always there. So if you have variation, you know, if you go in a room of 10 people, they'll all look a little bit different. And if some of that variation is genetic, has a genetic basis, then - and if some of those individuals have more offspring than others, which of course is going to be the case, then those individuals who have genetic variations that are more likely to help them have more offspring or more likely to help them have fewer offspring or prevent them from having as many offspring, that will change the frequency of those genes from one population to the next.

So whether we like it or not, evolution is going on, but it's going on at a very slow pace. And really what's a more dominant form of evolution today is cultural evolution. It's how we learn and use our bodies and interact with each other based on learned information. And that's also a kind of evolution. It's not Darwinian evolution. It's not biological evolution, but it affects our bodies. So we're evolving. We're evolving slowly through natural selection and rapidly through cultural evolution.