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7
Progress, Mismatch, and Dysevolution
The Consequences—Good and Bad—of Having Paleolithic Bodies in a Post-Paleolithic World
Though we are not so degenerate but that we might possibly live in a cave or a wigwam or wear skins today, it certainly is better to accept the advantages, though so dearly bought, which the invention and industry of mankind offer.
—HENRY DAVID THOREAU, Walden ave you ever wanted to abandon it all and seek a simpler life more in tune with your evolutionary legacy? In Walden, Henry David Thoreau describes the two years he spent in a hut in the woods by Walden Pond, detached from mid-nineteenth-century American culture, whose growing consumerist and materialist tendencies troubled him. People who have never read Walden sometimes mistakenly think that Thoreau spent these years as a hermit. In fact, he was seeking simplicity, self-sufficiency, a greater connection with nature, and only temporary solitude. Thoreau’s hut was a several-mile walk from the center of Concord, Massachusetts, which he visited every day or two to gossip and dine with friends, have his clothes laundered, and enjoy other comforts befitting a well-to-do man of letters. Even so, Walden has become a sort of bible for primitivists who decry the advances of civilization and yearn for a return to the good old days. According to this line of thinking, modern technology has led to the unfair development of social classes of
“haves” and “have-nots,” to widespread alienation and violence, and to an erosion of dignity. Some primitivists want to return the human species to an idealized agrarian way of life, and a few even think that the quality of human existence has been going downhill ever since we
ceased to be Paleolithic hunter-gatherers.
There is much to be said for a return to more of life’s simple pleasures, but a knee-jerk opposition to technology and progress is facile and futile (and was never advocated by Thoreau). By many measures, the human species has thrived since the end of the Paleolithic. The world’s population at the start of twenty-first century is at least a thousandfold greater than during the Stone Age. Despite ongoing poverty, war, hunger, and infectious disease in the poorest parts of the world, an unprecedented number of people around the globe not only have enough food but are also enjoying long and healthy lives. As an example, the typical Englishman today is 7 centimeters (nearly 3 inches) taller than his great-grandfather who lived one hundred years ago, his life expectancy is thirty years longer, and his children have about a tenfold greater chance of surviving infancy.1 In addition, capitalism has allowed average people such as me to take for granted opportunities unimagined by the richest aristocrats a few centuries ago. I have no desire to live permanently as a transcendentalist in the woods, let alone as a caveman without health care, education, and sanitation. I also enjoy the diversity of tasty foods I eat, I love my job, and I get a thrill out of living in a vibrant city full of interesting people, restaurants, museums, and shops. I also take pleasure in recent technologies like air travel, iPods, hot showers, air-conditioning, and 3D movies. Thoreau and others are correct in diagnosing modern life as increasingly consumerist and materialistic, but people’s desires haven’t changed so much as have their opportunities to satisfy them.
On the other hand, it is equally facile and foolish to ignore the many serious, novel challenges that human beings now confront. What followed the Paleolithic—farming, industrialization, and other forms of
“progress”—may have been a boon to the average person, but they promoted new diseases and other problems that were rare or absent during the Paleolithic. Almost every major infectious epidemic, such a s smallpox, polio, and the plague, happened after the Agricultural Revolution began. In addition, studies of recent hunter-gatherers show that although they don’t enjoy surpluses of food, they rarely suffer from famines or serious malnutrition. Modern lifestyles have
also fostered new noncommunicable but widespread illnesses such as heart disease, certain cancers, osteoporosis, type 2 diabetes, and Alzheimer’s, as well as scores of other lesser ailments, such as cavities and chronic constipation. There is good reason to believe that modern environments contribute to a sizeable percentage of mental illnesses, such as anxiety and depressive disorders.2
The story of progress achieved by the march of civilization since the end of the Stone Age has also been less gradual and continuous than many people suppose. As the next few chapters will show, farming created more food and allowed populations to grow, but for most of the last few thousand years, the average farmer had to work much harder than any hunter-gatherer, experienced worse health, and was more likely to die young. The majority of improvements in human health, such as greater longevity and decreased infant mortality, occurred over just the last hundred years. In fact, from the body’s perspective, many developed nations have recently made too much progress. For the first time in human history, a large number of people face excesses rather than shortages of food. Two out of three Americans are overweight or obese, and more than a third of their children weigh too much. In addition, a majority of adults in developed nations such as the United States and United Kingdom are physically unfit because our culture has made it easy, hence common, to spend the day without ever raising one’s heart rate. Thanks to
“progress,” I can wake up in my soft and comfortable bed, press a few buttons to get breakfast, drive to work, take an elevator to my office, and then pass the next eight hours sitting in a comfortable chair without breaking a sweat, getting hungry, or being too cold or too hot.
Machines now perform for me almost every task that once required physical effort: getting water, washing, acquiring and preparing food, traveling, even brushing my teeth.
In short, the human species has achieved considerable progress over the last few thousand years since we ceased to be hunter-gatherers, but how and why has some of this progress been bad for our bodies? The next few chapters will review how the human body has changed following the Paleolithic, but first let’s pause to consider the pros and cons of no longer living in ways for which our bodies
were adapted by millions of years of evolution. Are some forms of ill health a necessary consequence of civilization? And, more generally, how have biological and cultural evolution interacted after the Paleolithic in ways that affect the human body for the better and the worse?
How Are We Still Evolving?
I have been teaching human evolution to college students for more than twenty years, and for most of that time, I wound up my lectures more or less where chapter 6 just ended, with the origin of modern humans and the dispersal of people across the globe. My reasoning for finishing in the Paleolithic was the general consensus that little significant biological evolution has occurred in H. sapiens since then.
According to this view, ever since cultural evolution became a more powerful force than natural selection, the human body has barely altered, and whatever changes did take place over the last 10,000 years are more the province of historians and archaeologists than evolutionary biologists.
I now regret the way I used to teach human evolution. For one, it is simply not true that H. sapiens stopped evolving once the Paleolithic ended. In fact, the idea must be wrong because natural selection is the consequence of heritable genetic variation and differential reproductive success. People continue to pass on genes to their children, and today, as in the Stone Age, some people have more offspring than others. It follows that if there is any heritable basis to differences in people’s fertility then natural selection must still be chugging along. What’s more, accelerating rates of cultural evolution have rapidly and substantially changed what we eat, the ways we work, the diseases we encounter, and other environmental factors that have created new selective pressures. Evolutionary biologists and anthropologists have shown that cultural evolution hasn’t halted natural selection, and it has not only driven but sometimes even accelerated selection.3 As we will see, the Agricultural Revolution has been an especially powerful force for evolutionary change.
One of the reasons we don’t think of evolution as being much of a
force today is that natural selection is gradual, often requiring hundreds of generations to have a dramatic effect. Since a human generation is typically twenty or more years, one cannot easily detect evolutionary changes in humans of the magnitude that we can observe rapidly in bacteria, yeast, and fruit flies. However, it is possible to measure very recent natural selection in humans over just a few generations with enormous samples and much effort, and a few such studies have managed to find evidence for low levels of selection during the last few hundred years. In Finnish and American populations, for example, there has been selection on the age that women first give birth and the age women start menopause, as well as people’s weight, height, cholesterol, and blood sugar levels.4 If we look over longer time periods, we can detect even more evidence for recent selection. New technologies that rapidly and inexpensively sequence entire genomes have revealed hundreds of genes that have been under strong selection during the last few thousand years within particular populations.5 As you might expect, many of these genes regulate reproduction or the immune system and were strongly selected because they helped people have more offspring and survive infectious diseases.6 Others play a role in metabolism and helped certain farming populations adapt to foods such as dairy products and starchy staple crops. A few selected genes are involved in thermoregulation, presumably because they enabled far-flung populations to adapt to a wide range of climates. My colleagues and I, for example, found evidence of strong selection for one gene variant that evolved in Asia near the end of the Ice Age, causing East Asians and native Americans to have thicker hair and more sweat glands.7 One practical benefit of studying these and other recently evolved genes is to understand better how and why people vary in their susceptibility to certain diseases and how they respond to different medicines.
Although natural selection has not ceased since the Paleolithic, it is nonetheless true that relatively less natural selection has occurred in humans over the last few thousand years compared to the previous few million years. This difference is to be expected because it has been only six hundred generations since the first farmers began to till
the soil of the Middle East, and most people’s ancestors started farming more recently, probably within the last three hundred generations. For perspective, about the same number of generations o f mice have lived in my house over the last century. Although considerable selection can occur in three hundred generations, the strength of selection needs to be very great to cause a beneficial mutation to sweep through a population or a harmful mutation to be eliminated that quickly.8 In addition, during the last few hundred generations, selection has not always been operating in a consistent direction, which can obscure its traces. For example, as temperatures and food supplies have fluctuated, selection during some periods probably favored people who were bigger, but then during other periods it likely favored people who were smaller. Finally, and most important, there is no question that some cultural developments have buffered untold numbers of humans from natural selection that might have otherwise occurred. Consider how penicillin must have affected selection once the drug became widely available in the 1940s. Millions of people are alive today who would otherwise have been more likely to die from diseases like tuberculosis or pneumonia if they have genes that increase their susceptibility. Consequently, although natural selection has not ceased to act, we know that it has had only limited, regional effects on human biology over the last few thousand years. If you were to raise a Cro-Magnon girl from the Upper Paleolithic in a modern French household, she would still be a typical modern human girl except for some modest biological differences, probably mostly in h e r immune system and her metabolism. We know this is true because everyone from every corner of the planet shares a last common ancestor from less than 200,000 years ago, and yet different populations are for the most part genetically, anatomically, and physiologically the same.9
Regardless of just how much selection has occurred since the Paleolithic, there are other important ways in which humans have evolved over the last few thousands and hundreds of years. Not all evolution occurs through natural selection. An even more powerful and rapid force today is cultural evolution, which has altered many crucial interactions between genes and the environment by altering
environments, not genes. Every organ in your body—your muscles, bones, brain, kidneys, and skin—is the product of how your genes were affected by signals from the environment (such as forces, molecules, temperatures) during the period you developed, and their current functions continue to be influenced by aspects of your current environment. Although human genes have changed modestly over the last few thousand years, cultural changes have dramatically transformed our environments, often resulting in a very different, arguably more important kind of evolutionary change than natural selection. For example, toxins in tobacco, certain plastics, and other industrial products can cause cancer, often years after initial exposure. If you grow up chewing soft, highly processed food, your face will be smaller than if you grow up chewing hard, tough food.1 0 If you spend your first few years in a hot climate, you develop more working sweat glands than if you were born in a cool environment.1 1 These and other changes aren’t genetically heritable, but they are culturally heritable. Just as you pass on a last name to your children, you also pass on environmental conditions, such as the toxins they encounter, the foods they eat, the temperatures they experience. As cultural evolution is accelerating, environmental changes that affect how our bodies grow and function are also accelerating.
How cultural evolution is changing interactions between the genes we inherited and the environments in which we live is of great consequence. Over the last few hundred generations, the human body has changed in various respects because of cultural change. We mature faster, our teeth have gotten smaller, our jaws are shorter, our bones are thinner, our feet are often flatter, and many of us have more cavities.1 2 As future chapters will examine, there is also good reason to believe that today more people sleep less, experience higher levels of stress, anxiety, and depression, and are more likely to be shortsighted. In addition, human bodies these days have to contend with numerous infectious diseases that used to be rare or nonexistent. Each of these changes to the human body has some genetic basis, but what has changed is not so much the genes that play a role in these diseases as the environments with which these genes interact.
Consider type 2 diabetes, a metabolic disease that used to be rare but is now becoming common all across the globe. Some people are genetically more susceptible to type 2 diabetes, which helps explain why the disease is rapidly becoming more prevalent in places like China and India than in Europe and America.1 3 However, type 2 diabetes is not booming faster in Asia than in America because of novel genes that are now spreading in the East. Instead, new Western lifestyles are sweeping across the globe and interacting with ancient genes that previously did not have negative effects.
Put differently, not all evolution occurs through natural selection, and interactions between genes and the environment have been changing rapidly, sometimes radically, primarily because of changes in our bodies’ environments caused by rapid cultural evolution. You may have genes that predispose you to having flat feet, myopia, and type 2 diabetes, but the distant ancestors from whom you inherited the very same genes likely did not suffer from these problems. We therefore have much to gain by using the lens of evolution to consider shifting gene-environment interactions that have occurred since the Paleolithic ended. How well do the genes and bodies we inherited from our early modern human ancestors fare in the novel environments to which we subject them? And how can an evolutionary perspective on these changes be of practical use?
Why Medicine Needs a Dose of Evolution
Few words cause more terror in a doctor’s office, and are less likely to make you think about evolution, than “cancer.” If I were to receive a diagnosis of cancer tomorrow, my first concern would be to figure out how to rid myself of the disease. I’d want to know what kind of cells were cancerous, what mutations were causing them to divide out of control, and what medical interventions such as surgery, radiation, and chemotherapy would have the best chance of killing those cells without killing me. Even though I study human evolution, the theory of natural selection would be far from my mind as I confronted the disease. The same would be true if I had a heart attack, a painful tooth cavity, or a torn hamstring. When sick, I see a doctor, not an
evolutionary biologist. By the same token, my doctors have studied little if any evolutionary biology as part of their training. And why should they? Evolution, after all, is something that mostly occurred in the past, and today’s patients are not hunter-gatherers, let alone Neanderthals. Someone with heart disease needs surgery, drugs, or other medical procedures that require a thorough understanding of fields such as genetics, physiology, anatomy, and biochemistry.
Doctors and nurses are therefore not required to take courses on evolutionary biology, and I doubt if they, insurance companies, and others in the health-care industry ever give Darwin or Lucy much thought at their jobs. Just as knowing the history of the Industrial Revolution will not help a mechanic fix your car, why would knowing the Paleolithic history of the human body help a doctor treat your disease?
Considering evolution to be irrelevant to medicine may seem logical at first, but this way of thinking is deeply flawed and shortsighted.
Your body was not engineered like a car, but instead evolved through descent with modification. It therefore follows that knowing your body’s evolutionary history helps to evaluate why your body looks and works as it does, hence why you get sick. Although scientific fields such as physiology and biochemistry can help us understand the proximate mechanisms that underlie a disease, the burgeoning field of evolutionary medicine helps us make sense of why the disease occurs in the first place.1 4 Cancer, for example, is actually an aberrant evolutionary process going on within a body. Every time a cell divides, its genes have a certain chance of mutating, so cells that divide more frequently (examples include blood and skin cells) or that are more often exposed to chemicals that cause mutations (for ex ample , lung and stomach cells) have a greater chance of accidentally acquiring mutations that cause them to divide out of control, forming tumors. Most tumors, however, are not cancers. To become cancerous, the tumor cells need to gain further mutations that allow them to outcompete other, healthy cells by taking their nutrients and interfering with normal function. In essence, cancerous cells are nothing more than abnormal cells with mutations that enable them to survive and reproduce better than other cells. If we hadn’t