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)
Have 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-19th century American culture, whose growing consumerist and materialist tendencies troubled him. People who 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 some 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 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 in futile (and was never advocated by Thoreau). By many measures, The human species has thrived since the end of the Paleolithic. The worlds population at the start of the 21st-century is at least 1000 fold greater than during the Stone Age. Despite ongoing poverty, war, hunger, and infectious disease in the poorest parts of the world, and 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 cm (nearly 3 inches) taller than his great grandfather who lived 100 years ago, his life expectancy is 30 years longer, and his children have about a 10 fold greater chance of surviving infancy. In addition, capitalism is 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 is a caveman without healthcare, 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, iPhones, hot showers, air-conditioning, and 3-D 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 as 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 two diabetes, and Alzheimer’s, as well as scores of other lesser ailment, such as cavities and chronic constipation. There is good reason to believe that modern environments contribute to a sizable percentage of mental illnesses, such as anxiety and depressive disorders.
The story of progress achieved by the march of civilization since the end of the Stone Age has also been less gradual and continuous then many people suppose. Farming created more food and allowed populations 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 way 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 cal 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 in 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? 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 in 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?
It is simply not true that Homo 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 the 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 biologist and anthropologists have shown that cultural evolution hasn’t halted natural selection, and it has not only driven but sometimes even accelerated selection.
One of the reasons we don’t think of evolution as being much of the force today is that natural selection is gradual, often requiring hundreds of generations to have a dramatic effect. Since a human generation is typically 20 or more years, one cannot easily detected evolutionary changes in humans of the magnitude that we can observe rapidly and 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. If we look over longer periods of time, we can detect even more evidence of 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. As you might expect, many of these genes regulate reproduction or the immune system and were strongly selected because they help people have more offspring and survive infectious disease. Others play a role in metabolism and helped certain farming populations adapt 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.
Although natural selection has not stopped 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 only been 600 generations since the first farmers began to till the soil of the Middle East, and most peoples ancestors started forming more recently, probably within the last 300 generations.
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. 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 the 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. 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. 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 the 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 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. There is also more reason to believe that people sleep less, experience higher levels of stress, anxiety, and depression, and are more likely 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 changed is not so much the genes that play a role in thee diseases as the environments 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 two diabetes, which helps explain why the disease is rapidly becoming more prevalent in places like China and India than in Europe and America. However, type two diabetes is not blooming 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 two diabetes, but the distant ancestors from whom you inherited the very same genes likely did not suffer 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.