If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

SciCafe: the story of the human body with Daniel Lieberman

As the human species evolved from Paleolithic to modern times, our bodies have changed to fit the world around us. But with the human landscape moving quickly from the Agricultural and Industrial Revolutions to the modern day of smartphones and junk food, are our bodies able to keep up? In this SciCafe, join evolutionary biologist Daniel Lieberman as he discusses how human bodies evolved from our ape ancestors, and how this evolution continues to affect our bodies and their ailments to this day. Created by American Museum of Natural History.

Want to join the conversation?

No posts yet.

Video transcript

People have been studying human evolution for a long time and ever since Darwin, of course, they've been studying it from a proper evolutionary perspective, and I've had the good fortune of being able to teach human evolution for a long time, and when I teach human evolution, and we, you go down to the wonderful exhibit downstairs, there's some basic questions that people in my field focus on. When and how and when and how and why we evolved bipedalism, big brains, language, all sorts of things that we really care about. What forces shaped our evolution? These are very important questions, and they're really the nuts and bolts of our field, and as I have been teaching these questions for many years, I find that there are always a few students who are very interested in those questions, and they're the ones who want to go on and become professors of human evolutionary biology and anthropology. But most of them, they learn it for the exam, but they're not really all that interested in those particular questions, they really want to know about their bodies, they want to know how and why is evolution relevant today, to people's lives. And that's what I want to try to address today in this lecture, and it's not that I don't care about those first questions, I do, but I also think we should address the second set of questions, for example, how are our bodies evolving today? What does it mean to have Stone Age bodies in a sort of Space Age world? And how can studying evolution help us craft a better future for our bodies today, as we deal with a lot of crises. So that's really going to be the focus of today's lecture. And so we'll start with the question of how our bodies are evolving today. So, quick review of evolution. By natural selection, so there's basically three ingredients that, by which evolution by natural selection occurs. First, you have to have variation. Then you have to have comparability, so those variations have to have some kind of genetic basis, and finally, there has to be differential reproductive success. So individuals who have variations that make them more likely to have offspring that survive and reproduce, will pass those on, and those two for example have variations that cause those problems with reproduction. Those variations get snuffed out in the next generation. So, natural selection produced the modern human body, but it ain't over, it's still going on today, and I'm going to use the Bush family as my example here, but we all know that the conditions for natural selection still exists, for example, there still is variation, we all know that Jeb Bush and his brother are very different, we know that there's inheritability, there's no question that #41 and #43 are related to each other. And there's also differential reproductive success, this Bush has more offspring than this Bush has more offspring. If, for example, one of these Bushes have traits that may cause benefit in terms of reproductive success, those will be more likely to be passed on to the next generation, and et cetera, et cetera. So, natural selection hasn't ended, it's still going on today. But, we also have to acknowledge that there's another important evolutionary force that's going on in our world today. And that's a different kind of evolution, it's cultural evolution. Cultural evolution also, culture is basically learned behavior. So culture has also evolved, changed over time, that's the basic definition of evolution but two appear to be really fundamentally important. The first is the origins of farming, the Agricultural Revolution, which by growing food rather than simply foraging and hunting it, we were able to get a lot more food and hence there was an increase in population size, but as people settled down in villages and started basically fouling their nests and creating more opportunities for infectious disease, also, there is an increase in disease burden. The second major revolution was the Industrial Revolution, where we started using machines to replace humans and animals to do our work and we also invented science, and medicine, and all kinds of other good things, and that led to an explosion of more food, and of course, more people, but, at the beginning of the Industrial Revolution, times were pretty bad and infectious diseases rose, but with pastor and sanitation, and various other transformations that have occurred with the Industrial Revolution, we've been able to shift that and go to a condition of very little disease compared to previous times, so we'll talk more about that in a sec. Now, cultural evolution has a number of important effects, and one of the important effects is that it can spur biological evolution, we know that since the origins of agriculture, for example, there's been selection for, in at least seven different parts of the world for people to be able to just milk, as milk sugars as an adult, we know that people were able to move into habitats that varied in their solar radiation, so there's variation in skin color that evolved, we know that there's been evolution, for example, in different kinds of hair and sweat glands, work that we just recently published of what's going on in Asia. There's variation in metabolic genes, there's variation in genes that are important in infectious disease response, so we know that evolution has been going on since the origins of modern humans, some of it driven by cultural changes that actually changed the environment of which we live, which create new selective conditions for natural selection to act. A lot of people think, "Oh, well, everything is worse today. Right, you know, it's all been downhill, and Jared Diamond wrote a very famous essay that the origins of agriculture were the worst mistakes in human history." But actually, today is probably a pretty good time to live. In fact, I probably would not want to live in any other era in human history, I'm pretty happy to be in a 21st century human. We've made an enormous progress in terms of the health of our bodies over the last few thousand years, especially the last few hundred years, thanks to medicine and sanitation and transportation and all sorts of good things. And I'm going to give you a little bit of data to make you feel good about being a 21st century human, before we get to the really depressing stuff that's supposed to come. So let's start with infant mortality. Of course, there's no exact record of infant mortality during the Paleolithic, but we can guess that, in most populations, it was somewhere between 30 and 50%. During the farming era, actually, infant mortality remained quite high, 30 to 40% is a reasonable estimate of infant mortality rates for most of the time in which humans have been farming, but today, infant mortality rates in developed countries like the U.S. are less than 1%. This is a really unusual circumstance to be a human being. As parents, we most expect our children to survive and grow up and give us grandchildren. It's kind of a normal expectation today, but that was never the case forever in human evolutionary history. This is a very new thing. Adult mortality, something we care about a lot, too, is also shifted, as well. So people today in developed countries like the U.S. can live up to 70 or 80, but during the era of farming, people actually died much younger for the most part. Expected mortality was about 40 to 50 in most farming populations for most of human history. People often assume that's true of hunter-gatherers, but actually, hunter-gatherers, if they survive childhood, actually live to be pretty long. Most of them lived between about 60 and 70, sometimes up into their 80s. So actually, we're actually back up to where we used to be, about a little bit higher, but we're much better than we were during the era of farming. And as a result, we have fewer children dying and people living longer, there's been of course an explosion in population. And it sort of starts with the Paleolithic, and we know that populations must have started rising after the origins of agriculture, but of course, with the Industrial Revolution, things took off exponentially, and now there are more than seven billion people on the planet, we expect more than nine billion people by the end of the century, so, from that perspective, we're doing really, pretty well as a species. How about stature? Stature is actually a pretty useful parameter to look at because it kind of looks at a reasonable measure of overall health. You have a certain genetic potential you can grow to, so your height has a high inheritability. Many, many genes determine your height, but what if many people don't reach their genetic height, because of problems they encounter during development. If you look at the height of the overall populations, it tells you something about overall health. And we know that male hunter-gatherers in Europe were about five foot eight inches tall and then the earliest farmers in Europe were pretty tall, and then people shrank. They shrank because farming was pretty miserable for most of these folks, they were getting all kinds of horrible, nasty diseases, they were getting malnutrition, they were basically living around their feces and other people's feces, so they're getting cholera, and all kinds of horrible diseases, and, being a farmer really was a pretty horrible thing for most of the Farming Era, and then, of course, by the end of that, stature really started rising with the beginning of the Industrial Revolution, and now of course, in France, people have actually attained or even slightly passed their Paleolithic ancestors. Despite all these good things, I think most of us admit, or agree, that we could be doing a lot better, and the reason for that, and one way to calculate that is to use what as the Epidemiological Transition. The Epidemiological Transition is really a phenomenon of the last few hundred years, and it's well documented that over the last few hundred years, there's been a decrease in mortality because of infectious diseases. So, how many of you in the room are really worried about dying from the Plague? Oh, one person raises their hand, and thank you. But, your'e probably a hypochondriac, you probably shouldn't be worried about dying of the Plague. Smallpox, polio, et cetera, these diseases we no longer really worry about, and the result is we die less from them. But there's been a incomitant increase in infectious diseases, and these have been going up and we have good data from all around the world, if any one of you are interested, there's an entire issue of the Lancet devoted to data, worldwide data on this issue. I highly recommend the issue, it's a really fascinating issue. And so, diseases like cancer, which are not a new disease, but there's no question that cancer is more prevalent now than it used to be. Heart disease is more prevalent than it used to be. But those are just the tip of the iceberg. There's also other diseases which may not kill you, but still create problems. Like, myopia, which is much more prevalent than it used to be. I would argue that there's not great data, but there's some data that lower back-pain is more prevalent than it used to be, cavities are certainly more common than they used to be during the Paleolithic, flat feet, thirty percent of Americans have flat feet, but people who are barefoot, almost never do, and arguably, there's a number of mental health issues that are also more prevalent, like Alzheimer's,and insomnia, and depression and we have good data for some of these. And so, a lot of folks in the public health sector think, "Well, this is actually good news, right? If you got a, after all we all have to die of something, right? So if you're not going to die of turbeculosis, or smallpox, or polio, aren't we lucky to get cancer or heart disease when we're older, right? I mean, that's really the argument that's often made, right? That essentially, there's a trade-off, it's an inevitable trade-off that as more people live longer, more of us are going to get these diseases. And to some extent, that's actually true. Cancer, for example, is a disease that becomes more prevalent as you age because mutations accrue, so you're more likely, as populations age, to see some of those diseases. But, we have to understand also that it's also partly true, and people are often confusing diseases that are caused by old age. With diseases that become more prevalent as you age. And there are plenty of people who are able to live to be old and long and healthy lives without getting these diseases, so they're not inevitable outcomes of aging. So, there's an evolutionary medicine hypothesis. So, there's evolutionary medicine, if you've not heard of this field, you will, because I think it's one of the most, I would argue the most important branch of evolutionary biology today, and it's the transfer of biology that applies evolutionary theory to issues of health and disease, and there's that wonderful book, if any of you have read it, it's still worth reading, it's called "Why We Get Sick" by Nesse and Williams, and it actually sort of helps jump-start this field, now a large literature that's growing, there's actually a journal devoted to evolutionary medicine. The argument, though, is that many of the diseases we're encountering today, many of these chronic, non-infectious diseases are what we call Mismatch Diseases, now I did not come up with the term, it's been around for a while, but Mismatch Diseases are defined as diseases that are more common today, more prevalent, more severe because our bodies are poorly or inadequately adapted to the modern environmental conditions that we have created. What's the relationship between culture that we're creating and the biology that we're inheriting? And I think the way to think about it, one way to think about it is that, not only does culture drive natural selection, for sure, it does, but culture creates its own dynamic by interacting with the bodies that we evolved over the millennia. So what are the characteristics of diseases that cause this kind of evolution? Well, the first thing is, all diseases are caused by interactions between our genes that we inherited and the environments that we live in, and these are diseases that, in which it's not the genes that are so important, but more of the environmental shifts cause the problems. For example, we know that there's actually a genetic component to flat feet, there's actually good data on that, but it hasn't been, you know, sudden sweep of gene through the American generations that caused this epidemic of flat feet in America. Secondly, most of the causes of these diseases are incremental. They're hard to perceive, they're difficult to even relate cause to effect, right? Every time you wear a shoe, you don't think, "Oh, my God, my feet are going to become flat,", right? Every time you have a teaspoon of sugar you don't feel the pounds coming on, right? These are difficult diseases and difficult stimuli to perceive and hard to understand the relationship between cause and effect. Many of these diseases have very little effect on reproductive success, they occur when you're, heart disease, for example, tends not to crop up in most people, but until they're already grandparents, right, so it's not really directly affecting the reproductive success or flat feet, for example, how many of you with flat feet had trouble mating? Right? Probably not many of you, right? You know, they're not that serious, so, it's not going to be a strong selective effect on them. And then finally, all of these things have trade-offs. I'm not opposed to shoes, there's lots of good things about shoes, they can be sexy, they can be comfortable, they can be good, they protect you from needles and dog crap on the street, there's all kinds of good reasons to wear shoes, so there's trade-offs for everything, and there are some reasons to use these things, even if they have negative effects, as well. So, to kind of explore this, I'd like to take three different kinds of Mismatch Cycles, three kinds of dysevolution, and think about how they might be contributing to this vicious circle, and we'll talk about environmental changes that are resultant in too much of a stimulus, too little of a stimulus or stimuli that are very novel for which evolution didn't prepare us. And of course, we'll start with too much. We evolved to be sort of the gas guzzlers of the primate world. We're a very, very interesting species from a perspective of energetics, we really use energy in a really interesting and impressive way, and one of the ways in which we use it is that we have really large brains. And I'm happy to see that Liz Ahealiia is here, because she's published ehliali on this problem, but brains are very expensive tissues. Those of you sitting in the audience paying attention to me are using about twenty percent of your resting metabolism to pay for your brain, those of you who are not listening to me are also spending about twenty percent of their metabolism paying for your brain. Brains are very, very expensive tissues, and it took a lot of energy, availability of energy for us to grow big brains, which started some time around two million years ago, and really accelerated and sort of, hit modern size in the last five hundred thousand years or so. So, brains are big and costly, and in fact, in an infant, for example, about eighty percent of its metabolism goes towards spending, paying for its big brain. But we also have very costly life histories. So, our ancestors probably reproduced pretty slowly, so apes, for example, chimpanzees, have offspring, chimpanzee mothers have offspring every six years or so. But human hunter-gatherers are able to have them at about every three years, so they double the rate of which they are able to produce offspring. But at the same time, we take a lot longer growing them. So how did we get that energy? And the answer appears to have been the origins of hunting and gathering, it's a really important, complex, way of which people work together to get energy. In hunting and gathering there is a system, it's a behavioral system that involves tool-making, it involves food preparation, so cooking, and food processing, it involves hunting, it involves running, it involves running, it involves running, I think. It involves throwing, it involves the division of labor and lots of cooperation, and all kinds of things work together to make hunting and gathering a system, and it helps people also acquire lots of high energy foods, foods that are high in sugar, for example, like honey, or meat, and various other things like that turn into carbohydrates and fat, which are the basic key kinds of food in which you eat to get energy, to pay for all the things you do today, right? But most hunter-gatherers don't have huge energy surpluses. They don't have to work terribly hard, but they don't have huge amounts of energy available to them. But that change for the origin of agriculture, when we started growing food, and farmers have to work pretty hard, but boy, can they get in a lot of energy. So, plowing a field and growing food and domesticating animals gave people access to a lot of carbohydrates and a lot of fat, but that only started about six hundred generations ago, but it's completely transformed our planet. I'm pretty sure that everything, that everybody ate in this room today was domesticated, in fact, most of the calories that almost all of us ate came from just four kinds of foods. It came from cereals, tubers, maize, or rice, right? Those are all domesticated foods. We, in the Paleolithic, can make estimates about how much sugar people got in the Paleolithic by looking at hunter-gatherer diets, and it's a reasonable estimate that people got four to eight pounds of sugar a year, in honey and stuff like that. But today, the average American consumes over a hundred pounds of sugar a year. That's a pretty astonishing figure. But at the same time, we've removed fiber from our food, so, typical American can only get about twelve pounds of fiber a year, which is why so many people are constipated, but, hunter-gatherers, of course, get a lot more, they get, this is a crude estimate, but of course fiber is of course very crude in its own nature, so maybe fiber is about eighty pounds a year, okay? So a big shift, and that shift is pretty important because when you eat lots of sugar in the absence of fiber, it slams into your body at a really high rate, so you get lots of sugar at a high dose rate. So, lots of glucose and lots of fructose, fructose is the really sweet stuff, in the absence of fiber, fiber slows the rate at which you digest it, causes you to have a high insulin level, the fructose goes straight to your liver where it gets turned into mostly into fat because your liver can't burn that fructose fast enough, and it causes metabolic syndromes, lots of central adiposity, lots of belly fat, high levels of cholesterol, high levels of blood sugars and all kinds of other problems, blood pressure, et cetera. And that leads to a number of diseases, probably the cynical non-disease of metabolic syndrome is diabetes. So what do we do? People who have Type 2 Diabetes, well, there's two major ways of treating it. One is good old fashioned exercise and diet, which we know works very well. As for example, look what it did for for Bill Clinton, right? And the other, of course, is medication. There are a lot of important medications out there which help deal with many parts of this metabolic pathway, but let's think about the difference between these two different pathways. Pharmaceuticals are out there don't cure you of diabetes, but they mitigate the symptoms, which is, fine, we should do that, we should give people drugs to pharmaceuticals to mitigate the symptoms but everybody agrees that exercise and diet are more important because they cannot only prevent disease from occurring in the first place, actually, it turns out that very high levels of exercise and very serious diets can actually, in some cases, reverse the disease. Many people don't actually know that. So I'd make the case that Type 2 Diabetes is an example of dysevolution that I was talking about earlier, right? It obeys all the criteria, there's a gene--environment interaction, I didn't talk about the genes, but there are certain genes that give people a greater proclivity towards getting the disease, but what's really shifted is the environment. For example, the reason that India is undergoing an explosion in Type 2 Diabetes is not because genes are shifting, but it's because the environment is shifting. We also know the causes of diabetes are incremental and not obvious every time you drink a glass of orange juice, you don't necessarily, you know, become suddenly diabetic or feel the diabetes coming on. In most cases, little or no effect on reproduction, because most people who get the disease tend to be older in life, so they're no longer having their kids, they're tending to be grandparents at that point. And there's no question that there's a trade-off in the cost and the benefits of the factors that lead to Type 2 Diabetes. I mean, there's, we can't, for example, get rid of carbohydrates and stop feeding the world grains, we have seven billion people to fill, we can't just suddenly have them no longer eat bread and rice and imagine a world without bagels, it would be terrible, right? So there are trade-offs, and we all accept those trade-offs. Okay, so that's an example, I'm sure you can think of a lot of other examples of too much, but let's think of an example of too little. Here's, there are many to pick from and one, of course that I'm very interested in is the muscular skeletal system. We all know that stress is very important for most every system in your body. And by stress, I don't mean like emotional stress, like, when you tell a joke and nobody laughs, which is very stressful, but I mean is, a stress is when you make a system of the body work harder, that's sort of the physiological definition of stress, and we all know that, for example, if you want your muscles to grow, you have to have pain, you have to have strain, for in order for you to have gains. We also know that if you don't continue to use your muscles, you lose it, as is very much obvious in the former governor of California. And there's a reason for that, right? The reason for both of these is that you don't know how much capacity you want, so you want to have a labile response to stress so you can have capacity matched to men, muscles are expensive. I told you earlier that your brain consumes about twenty percent of your body's metabolism, but most of us spend about forty percent of our metabolism on muscles, so you don't want any more muscle than you need, because it's going to cost you a lot, particularly if your'e at a margin of energy balance, if you're a hunter-gatherer, and you don't want to lose it, right? So that if you're the governor of California, you have to eat a lot, just to, public embarrassment to eat as much as Arnold Schwarzzenager when he had to do at the height of his body-building days. So, an important example of this is physical activity and everybody knows that I'm not going to belabor the evidence that physical activity is important for inducing stresses in many systems of your body, from your brain to your immune system, to your muscles, to your bones, et cetera, it's very, very important for many systems of the body. So apes, for example, chimpanzees walk about two to three kilometers a day, I think your average gorilla walks less than one kilometer a day, your average chimpanzee may be climbs about a hundred meters of tree, if you're a chimpanzee, you spend most of your day doing: eating. Eating, chimpanzees spend fifty percent of their day just putting food in their mouth and then they have to wait to the food to digest before they can fill their belly again, and they basically do that all day long and occasionally copulate and do other exciting things, but mostly, they just simply feed. That's what they do. But humans have evolved a very different system. We are very active, your average hunter-gatherer female walks about nine kilometers a day, your average hunter-gatherer male walks about fifteen kilometers a day. There are many ways to measure physical activity, but a very simple, very simplistic measure called the physical activity level, or the PAL. Your PAL is basically how much energy you're spending divided by your basic metabolic rate, how much energy it takes just to lie in bed and do nothing but watch sitcoms all day, right? And, so, it turns out that data show that hunter-gatherers, their physical activity levels average about 1.9, there's a lot of variation around these numbers, these are just means, right, it's not all variation here. So subsistence famers have to work a little bit harder than hunter-gatherers, but today, most of sedentary Americans have physical activity levels of 1.5 or 1.6. That's a pretty significant reduction. That's a fifteen to twenty percent reduction. And now what is a fifteen to twenty percent reduction in physical activity level mean? Let me give you an example of something that just happened over the last few hundred years. So my grandmother, who lived in Brooklyn, she used to have an old pedal sewing machine, she loved it, and I remember as a kid going there and she would be pedaling away on her machine. And there are actually people who have measured, they put oxygen masks on people, and they measure how much it costs you to use a sewing machine with a pedal, then they measure how much it costs for the mayor of London here to use an electronic machine without a pedal, and it turns out to be a difference of fifteen calories an hour. About a fifteen percent reduction. You think, "Okay, big deal. Fifteen calories an hour." but imagine your job, imagine Boris here had a union job, right, he's only working five days a week for eight hours a week, for fifty weeks a year, do you know how many calories that is in a year? Well, you can whip out your iPhone and do the calculation, I'll do it for you, it's fifty-two thousand calories in a year, which is enough to run eighteen New York marathons. That's a pretty substantial reduction and that's just from putting a machine, you're an engine, not a sewing machine. Imagine all the elevators and the escalators and shopping carts and you could even buy an electric toothbrush and all kinds of great stuff out there. We've reduced how much energy we spend in many dimensions of our life, and the estimates are that today, Americans spend on average, three hundred to five hundred calories less per day being physically active. That's an average estimate, alright? So that's a, so if you could keep your diet the same, you know, you don't have to invoke any complex biological arguments to understand how we're getting bigger. But another insidious effect of loss of physical activity is osteoporosis, which is a disease that affects more than thirty percent of women in the United States over the age of fifty, and becoming more prevalent in men, now at ten percent of men, and it's a disease that apparently, as far as we can tell, is rare or completely unknown among hunter-gatherers of subsistence populations. There's no evidence that I know of that, it exists, it comes from applying forces to your body, and there's a reason for this response to loading, because bones are costly, you don't want to have any more than you need, and you don't want to have any less than you need, which is why when you become more active, you add bone and when you become very less active, or if you go to like, space where's there's no gravity, you start losing bone. So there's this trade-off between growing it and losing it. That makes sense. But the problem is that there's a pernicious and ancient constraint, which is we grow our skeletons when we're young, right? So most people grow their bones and get peak bone mass at the age of about twenty or thirty, and there's a lot of evidence that shows that people who are more active attain higher peak bone mass, so if you're inactive when you're growing, you attain less bone by the time you're twenty or thirty. And after that point, we're all screwed, we're all going to lose bone for the rest of our life, and unfortunately, if you're a female, you're going to lose bone faster once you go through menopause, because estrogen has a protective effect on bone health, and it turns out that women, people who are less active actually lose bone at a faster rate, and the end result is that you're less active when you're young, you achieve less bone mass, if you're less active as you continue to age, you lose bone faster, and you're just more likely to fall below that threshold that causes osteoporosis. So, I would argue that osteoporosis is also an example of dysevolution. And now since we're all depressed, let's talk about something that might be a little bit less depressing, let's talk about things that are too new. Things that are novel that we never encountered before in our evolutionary history like DDT. Right? Cavemen never had to worry about DDT or car crashes, or smoking, or bungee-jumping, we can think of all these things as being novel, we're obviously not adapted to them, but that's not really scientifically very interesting. But how about this guy over here? He looks like a comfortable gent, right, he's sitting there with his shoes and his newspaper and he's reading in a chair, et cetera, and what could be more normal, right? But everything this guy is doing is killers, he's going to die from them all. In fact, well, maybe that's a little bit of a hyperbole, but let's just talk about one of these things, let's talk about reading, right, because we think reading as being about as normal and natural, we encourage our children to do it, but of course, it's only very recently, in fact, it wasn't until about 3000 B.C. that anybody read anything at all, basically, and it was up until the Industrial Revolution that universal literacy started occurring in developed countries like England. And it turns out that that's when myopia started occurring. But various studies of hunter-gatherers in subsistence populations have found repeatedly that myopia is extremely rare, right? It's very rare among the Inuit, it's very rare among the subsistence farmers in Polynesia, et cetera, there are many, many studies all around the world that show that it's less than three percent in these populations. Less than three percent. And myopia is caused by having an eyeball that's too long. So basically, everything that is far, looks blurry. And it turns out, that there are two major causes for this: and there is a huge debate that's going on, actually, in opthamology, so I'm not going to pick-saw it here, I suspect that both are true. But one is closer, so I'm spending a lot of time looking at things up close, like reading, or sewing, or microscopy, any microspists here? You have bad eyes, I can see that. It's actually microscopists are famous for having particularly, there's actually a big cottage industry for bad eyes, but anyway, what happens is that when you're looking at things that are up close, you're constantly stretching, you're constantly firing the ciliary muscles that attach onto the filaments that hold the lens in place, those filaments are called the zonules of zinn, I just had to say that. And, the stretching, the firing of the ciliary muscles actually puts tension on the eyeball, and it actually raises the pressures inside this major acquiesce chamber of the eyeball, and that's thought to cause elongation and stretching, which then leads to the eyeball being longer, but it turns out that if you sew up the eyelids of little kittie-cats, which people did, I'm sorry to say, it turns out those little kittie-cats, they didn't do this for myopia studies, it was for other research, but they found out that those eyeballs were too long, now cats, obviously have not been reading, right? And so, further experiments where they have cats and chickens and weird things that wear glasses and fur that split, the blurred vision, et cetera and reduced visual stimulus, have shown that an absence of complex visual stimulus can also cause your eyeballs to become too long. And so, it turns out that there's good data that would show, that children, for example, who spend a more time outdoors, independent to how much they read, are much less likely to develop myopia. So probably both factors are going on in terms of this problem. So, I think we can agree that myopia, now for another example, a little less scary than osteoporosis, or diabetes, another example of dysevolution, right? It's caused by a gene-environment interaction, there are genes that again, do predispose you, but again, these genes haven't been sweeping through populations in the last few generations, rather, it's the environments that we live in, reading lots and spending a lot of time indoors, doing microscopy, whatever, which cause people to get myopia. Each time you read a book or spend an afternoon inside, you don't feel your eyeballs growing longer, you can't really feel the stimulus, there's certainly no effect on reproduction, in fact, I think people who wear eyeglasses are rather attractive. And then, finally, none of us would basically have our children stop reading, or spend all their time outdoors, there's no question about the benefits outweigh the costs of having to go and buy eyeglasses. So finally, now that I've depressed you all about how the interaction between our Paleolithic bodies and our modern worlds can lead to this epidemiological transition, I'd like to conclude by asking how can evolutionary theory, and thinking about evolution, help us get out of this mess? How can we do better by thinking about evolution? So, I would repeat, again, the famous statement by Theodosius Dobzhansky, that nothing in biology makes sense except in light of evolution. I would include, medicine in that statement, right? And I would argue that this Epidemiological Transition can only really be interpreted and only be countered by using an evolutionary framework. Let's think about our options. Well, our first option is to do nothing, let's just, you know, let nature sort the problem out, let's let natural selection act and get rid of all those myopics and get rid of all those flat-footed people and get rid of all those diabetics, et cetera. Well, there's obviously two flaws to this problem: first, it's cruel and inhumane, and it's un-appropriate, secondly, natural selection, even if it were to operate on these things, it would take a long time, it wouldn't help us, or our children or our grandchildren, and third, it wouldn't necessarily wouldn't happen, because you need to have the inheritable variations to exist in the first place, plus the selective conditions for them to act, for natural selection to really solve the problems. I think we can agree that we can cross this one off for many, many reasons. Number Two and Number Three is really what we're doing now. We can invest more in treatment and we can help educate more people to understand their bodies. We can send more money of your tax dollars to NIH so scientists in white lab coats can come up with solutions to Diabetes and cancer, et cetera, et cetera, andI'm all for that. We should all increase our budget on research for treating diseases because that's the right thing to do, but I think it's science fiction to think that we're going to solve these diseases easily, they're complex diseases that are caused by many, many genes, most of these genes have very small effects, most of the genes turn out to be very rare, most of the pathways are difficult to treat, there's still going to be pastor for these chronic, non-infectious diseases, there's no microbe you can identify and kill and thereby solve the problem of smallpox or whatever. Most of these diseases are very difficult to treat, we can only expect incramental progress, it's going to take a long time, lots and lots of research, many, many Ph.Ds sacrificing their careers, slowly, slowly, we can get some progress. Education is also important, but education goes so far, I mean, if you tell people to eat a healthy diet, will they eat a healthy diet? The answer is hell, sadly, no, it's only so effective, and we need to do it, of course, but it only has so much effect, but the final solution is to change our environments, and I think that's where evolutionary perspective makes the most sense, because like it or not, we evolve to be bipeds, sweaty, slightly fat bipeds that are, you know, furless and big-brained, and dependent on using tools, and we evolved to the diet that's high in fiber and low in carbohydrates, but we also evolve to crave sugar, we evolved to crave starch, to crave fat, we evolved to be very physically active, but we also evolved to be lazy, because, you know, if there are escalators in the Kalahari desert, I promise the hunter-gatherers there would be using them, it make sense to save energy, if you are on the margin of energy balance. It makes sense. It's clearly an evolutionary adaptation, even though I'm not a big sociobologist, but even I will agree with that one. So, we evolved to be physically active, and enjoy rest and comfort, and more, so, we need to confront these ideas, because, it's clear that we evolved, we never evolved to make the kinds of choices that we make today. So if you go back, you know, a few hundred generations, a few thousand generations ago and listen in on a Homo Erectus mother from the Indonesian Museum, you know, talking to her daughter about, you know, what you're going to do today, and she's probably not going to tell her daughter, "Don't forget to eat the healthy food and exercise." Right? That kid had no option but to eat the healthy food and exercise, that's what you did every day, but now, we have to exert our children to make choices for which we are really not prepared from an evolutionary perspective, right? And so we have this mismatch between old genes and old aspects of our biology which really can't change very easily, and novel environments, which we really actually can change, we've done it in many respects, why can't we do it for some other things? That's going to require either coercion or nudging, right? So we can do it, you know, you've done here in New York, and ban trans fats, which in fact, was a good thing. How many people here approve of the trans fat ban in New York? Well, we have a really nice liberal crowd here, that's pretty exciting, right? Maybe we require physical education in schools, I think it's not required enough, or it's often pretty meesly, we have many schools now have junk food bans, where you can't buy junk food in school, so that's coercion, we're basically shape-shifting our environments without asking people's permission, or the other approach is to do nudging. To kind of help us help ourselves. To help us act in our best interest without taking away our rights. So that occur through tax, right? We could, for example, have a tax on soda, we've done this for cigarettes, right? Cigarettes and alcohol, we've actually done, it used to be 50% of Americans who used to smoke is now down to twenty percent, through pretty modest means, actually. Taxes, they were fought over, tooth and nail, but actually, cigarette taxes and bans on advertising, et cetera, are not that all radical, right? Why can't we do that, for example, sugar, which is poisoning many of our children or for junk food, I mean, how many of you in the room have thought that we're in favor of Mayor Bloomberg's Big Gulp ban, which I would consider a nudge, right? Oh, it's not so popular in this room, maybe I have fifty percent. He's not preventing you from drinking soda, you just have to use two hands to get 32 ounces instead of one hand. I would consider that a nudge, it's not taking away our right, but we're all going to have a debate about that later on. Now, I think when you apply that to adults, it gets very controversial, but who disagrees about that for children? We already coerce our children. We require them to wear seat belts, we require them to go to school, most days require inoculations, how would that be any different than actually requiring physical education in schools, children need about an hour of strenuous activity every day to grow a healthy body, and very few children in the United States get that, and we're really, we're doing our children a great disservice by not making them run around more. We also agree that you know, you shouldn't have children smoke, you shouldn't have children drink, those are coercion, so why not have similar laws to restrict the amount of junk food that they get. I mean, how different really is it when you consider that thirty percent of American children are overweight, and they can't make rational decisions on their own, so we have to it for them so they can grow healthy bodies. So I hope that I convinced you that evolution not only explains why are bodies are the way they are, but evolution still matters, right? It helps explain not only why we are the way we are, but how and why we get sick, but I also think it provides us with clues, pathways, ways to think about how we can make the world a better place and how we can do better with ourselves. So, for that, I'd like to thank you, especially thank the Leakey Foundation and I'd like to thank the AMNH and I'm sorry I went on for so long. Thank you.