If you got sick, you probably wouldn't go to an evolutionary biologist to get treated. But Daniel Lieberman, professor of evolutionary biology at Harvard University, says that his field can help you understand why you got sick, and make you more aware of healthy and harmful behaviors.
He joins Fresh Air's Terry Gross to talk about some of the noninfectious diseases that may result from our Paleolithic bodies not being well-adapted to modern conditions. It's a list of ailments big and small, including acid reflux, acne, anxiety, asthma, certain cancers, depression, Type 2 diabetes, flat feet, high blood pressure, irritable bowel syndrome, lower back pain and osteoporosis. In his new book, The Story of the Human Body: Evolution, Health and Disease, Lieberman traces these troubles back to their origins
Interview Highlights
On "mismatched diseases"
Many of the illnesses that we confront today are what evolutionary biologists called "mismatched diseases": ... Diseases that occur because our bodies are poorly or inadequately adapted to environments in which we now live. 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 mismatched diseases are diseases that are more 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.
On our relatively "new," unfettered access to sweet foods
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'd be surprised at how unsweet 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 sweet was honey, and honey of course was a special treat — honey was pretty much the only form of dessert in the Paleolithic. But now we have access to abundant quantities of sugar and 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.
On why stress leads us to crave comfort foods
The hormone that gets elicited when you get stressed is cortisol. ... If you jump out of an airplane or something really bad happens — a loved one dies — our body goes into a fight or fright [sic] 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 exacts a toll. Cortisol makes us want to bring in more energy to cope again, with 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.
On the way the stress we experience today is different from Paleolithic stress
Stress creates this vicious cycle, this positive feedback loop. 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, the stimulus that causes the stress should be a short-lived one; that's what evolution predicted. ... So a lion chases you, that's a very stressful event, obviously, hopefully you managed to run away from the lion ... life goes on. But much of the stress we create today results from social conditions. If you think about the most stressful things we experience, they're often our lives — they're our jobs, our commutes, not having enough money, the list goes on. Those, of course, elicit chronic levels of stress. And when stress becomes chronic, then it helps feed a variety of mismatched diseases that make us ill, that make us depressed, that make us anxious, that make us overweight, which causes more stress and then keeps the cycle going.
On the evolutionary explanation for the rise in allergies and autoimmune diseases
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 of those germs in the outside world. We've now created environments that are very sterile, that are extremely clean; we have very few pathogens that we have to deal with. And if we do get them, we nuke them with antibiotics. 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 germs and worms that used to make us sick, but now those pathogens are absent, so it sometimes by chance finds the wrong targets. So that's the hypothesis for why so many allergies and 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.
On cancer as "evolution gone wrong"
Cancer is a kind of natural 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 out-compete 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 cancer. In a sense, we wouldn't have evolved cancer if we hadn't evolved multicellular life and if natural selection didn't occur.
On how we are still evolving
Evolution is always churning along. Evolution, after all, is just change over time and natural selection. ... 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. We're evolving — we're evolving slowly through natural selection and rapidly through cultural evolution, and we need to think about those interactions as we plan to use our bodies better.
Excerpt: The Story Of The Human Body
PrefaceLike most people, I am fascinated by the human body, but unlike most folks, who sensibly relegate their interest in people's bodies to evenings and weekends, I have made the human body the focus of my career. In fact, I am extremely lucky to be a professor at Harvard University, where I teach and study how and why the human body is the way it is. My job and my interests allow me to be a jack-of-all trades. In addition to working with students, I study fossils, I travel to interesting corners of the earth to see how people use their bodies, and I do experiments in the lab on how human and animal bodies work.
Like most professors, I also love to talk, and I enjoy people's questions. But of all the questions I am commonly asked, the one I used to dread the most was "What will human beings look like in the future?" I hated this question! I am a professor of human evolutionary biology, which means I study the past, not what lies ahead. I am not a soothsayer, and the question made me think of tawdry science fiction movies that depict humans of the distant future as having enormous brains, pale and tiny bodies, and shiny clothing. My reflexive answer was always something along the lines of: "Human beings aren't evolving very much because of culture." This response is a variant of the standard answer that many of my colleagues give when asked the same question.
I have since changed my mind about this question and now consider the human body's future to be one of the most important issues we can think about. We live in paradoxical times for our bodies. On the one hand, this era is probably the healthiest in human history. If you live in a developed country, you can reasonably expect all your offspring to survive childhood, to live to their dotage, and to become parents and grandparents. We have conquered or quelled many diseases that used to kill people in droves: smallpox, measles, polio, and the plague. People are taller, and formerly life-threatening conditions like appendicitis, dysentery, a broken leg, or anemia are easily remedied. To be sure, there is still too much malnutrition and disease in some countries, but these evils are often the result of bad government and social inequality, not a lack of food or medical know-how.
On the other hand, we could be doing better, much better. A wave of obesity and chronic, preventable illnesses and disabilities is sweeping across the globe. These preventable diseases include certain cancers, type 2 diabetes, osteoporosis, heart disease, strokes, kidney disease, some allergies, dementia, depression, anxiety, insomnia, and other illnesses. Billions of people are also suffering from ailments like lower back pain, fallen arches, plantar fasciitis, myopia, arthritis, constipation, acid reflux, and irritable bowel syndrome. Some of these troubles are ancient, but many are novel or have recently exploded in prevalence and intensity. To some extent, these diseases are on the rise because people are living longer, but most of them are showing up in middle-aged people. This epidemiological transition is causing not just misery but also economic woe. As baby boomers retire, their chronic illnesses are straining health-care systems and stifling economies. Moreover, the image in the crystal ball looks bad because these diseases are also growing in prevalence as development spreads across the planet.
The health challenges we face are causing an intense worldwide conversation among parents, doctors, patients, politicians, journalists, researchers, and others. Much of the focus has been on obesity. Why are people getting fatter? How do we lose weight and change our diets? How do we prevent our children from becoming overweight? How can we encourage them to exercise? Because of the urgent necessity to help people who are sick, there is also an intense focus on devising new cures for increasingly common noninfectious diseases. How do we treat and cure cancer, heart disease, diabetes, osteoporosis, and the other illnesses most likely to kill us and the people we love?
As doctors, patients, researchers, and parents debate and investigate these questions, I suspect that few of them cast their thoughts back to the ancient forests of Africa, where our ancestors diverged from the apes and stood upright. They rarely think about Lucy or Neanderthals, and if they do consider evolution it is usually to acknowledge the obvious fact that we used to be cavemen (whatever that means), which perhaps implies that our bodies are not well adapted to modern lifestyles. A patient with a heart attack needs immediate medical care, not a lesson in human evolution.
If I ever suffer a heart attack, I too want my doctor to focus on the exigencies of my care rather than on human evolution. This book, however, argues that our society's general failure to think about human evolution is a major reason we fail to prevent preventable diseases. Our bodies have a story—an evolutionary story—that matters intensely. For one, evolution explains why our bodies are the way they are, and thus yields clues on how to avoid getting sick. Why are we so liable to become fat? Why do we sometimes choke on our food? Why do we have arches in our feet that flatten? Why do we have backs that ache? A related reason to consider the human body's evolutionary story is to help understand what our bodies are and are not adapted for. The answers to this question are tricky and unintuitive but have profound implications for making sense of what promotes health and disease and for comprehending why our bodies sometimes naturally make us sick. Finally, I think the most pressing reason to study the human body's story is that it isn't over. We are still evolving. Right now, however, the most potent form of evolution is not biological evolution of the sort described by Darwin, but cultural evolution, in which we develop and pass on new ideas and behaviors to our children, friends, and others. Some of these novel behaviors, especially the foods we eat and the activities we do (or don't do), make us sick.
Human evolution is fun, interesting, and illuminating, and much of this book explores the amazing journey that created our bodies. I also try to highlight the progress achieved by farming, industrialization, medical science, and other professions that have made this era the best of all times so far to be a human. But I am no Pangloss, and since our challenge is to do better, the last few chapters focus on how and why we get sick. If Tolstoy were writing this book, perhaps he might write that "all healthy bodies are alike; each unhealthy body is unhealthy in its own way."
The core subjects of this book—human evolution, health, and disease—are enormous and complex. I have done my best to try to keep the facts, explanations, and arguments simple and clear without dumbing them down or avoiding essential issues, especially for serious diseases such as breast cancer and diabetes. I have also included many references, including websites, where you can investigate further. Another struggle was to find the right balance between breadth and depth. Why our bodies are the way they are is simply too large a topic to cover because bodies are so complex. I have therefore focused on just a few aspects of our bodies' evolution that relate to diet and physical activity, and for every topic I cover, there are at least ten I don't. The same caveat applies to the final chapters, which focus on just a few diseases that I chose as exemplars of larger problems. Moreover, research in these fields is changing fast. Inevitably some of what I include will become out of date. I apologize.
Finally, I have rashly concluded the book with my thoughts about how to apply the lessons of the human body's past story to its future. I'll spill the beans right now and summarize the core of my argument. We didn't evolve to be healthy, but instead we were selected to have as many offspring as possible under diverse, challenging conditions. As a consequence, we never evolved to make rational choices about what to eat or how to exercise in conditions of abundance and comfort. What's more, interactions between the bodies we inherited, the environments we create, and the decisions we sometimes make have set in motion an insidious feedback loop. We get sick from chronic diseases by doing what we evolved to do but under conditions for which our bodies are poorly adapted, and we then pass on those same conditions to our children, who also then get sick. If we wish to halt this vicious circle then we need to figure out how to respectfully and sensibly nudge, push, and sometimes oblige ourselves to eat foods that promote health and to be more physically active. That, too, is what we evolved to do.
Excerpted from The Story of the Human Body by Daniel E. Lieberman.
Copyright © 2013 by Daniel Lieberman. Excerpted by permission of
Pantheon, a division of Random House LLC. All rights reserved. No part of this
excerpt may be reproduced or reprinted without permission in writing from
the publisher.
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