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Introduction

What Are Humans Adapted For?

Have you ever heard of the “Mystery Monkey,” which provided a sideshow to the 2012 Republican National Convention in Tampa, Florida? The monkey in question, an escaped rhesus macaque, had been living for more than three years on the city’s streets scavenging food from Dumpsters and trash cans, dodging cars, and cleverly evading capture by frustrated wildlife officials. It became a local legend. Then, as hordes of politicians and journalists descended on the city for the convention, the Mystery Monkey gained sudden international fame. Politicians were quick to use the monkey’s story as an opportunity to promote their views. Libertarians and liberals hailed the monkey’s persistent evasion of capture as symbolic of the instinct to be free from unjust intrusions on people’s (and monkeys’) freedom. Conservatives interpreted the years of failed efforts to capture the monkey as symbolic of inept, wasteful government. Journalists could not resist telling the story of the Mystery Monkey and its would-be captors as a metaphor for the political circus going on elsewhere in town. Most folks simply wondered what a solitary macaque was doing in suburban Florida, where it obviously didn’t belong.

As a biologist and anthropologist, I viewed the Mystery Monkey along with the reactions it inspired through a different lens—as emblematic of the evolutionarily naïve and inconsistent way that humans view our place in nature. On the face of it, the monkey epitomizes how some animals survive superbly in conditions for which they were not originally adapted. Rhesus macaques evolved in southern Asia, where their ability to forage for many different foods enables them to inhabit grasslands, woodlands, and even mountainous regions. They also thrive in villages, towns, and cities, and they are commonly used in laboratories. In this regard, the Mystery Monkey’s talent for surviving off trash in Tampa is unsurprising. However, the general conviction that a free-range macaque didn’t belong in a Florida city reveals how poorly we apply the same line of reasoning to ourselves. When considered from an evolutionary perspective, the monkey’s presence in Tampa was no more incongruous than the presence of the vast majority of humans living in cities, suburbs, and other modern environments.

You and I exist about as far removed from our natural environment as the Mystery Monkey. More than six hundred generations ago, everybody everywhere was a hunter-gatherer. Until relatively recently—the blink of an eye in evolutionary time—your ancestors lived in small bands of fewer than fifty people. They moved regularly from one camp to the next, and they survived by foraging for plants as well as hunting and fishing. Even after agriculture was invented starting about 10,000 years ago, most farmers still lived in small villages, labored daily to produce enough food for themselves, and never imagined an existence now common in places like Tampa, Florida, where people take for granted cars, toilets, air-conditioning, cell phones, and an abundance of highly processed, calorie-rich food.

I am sorry to report that the Mystery Monkey was finally captured in October 2012, but how much should we be concerned that the vast majority of humans today still exist, as the Mystery Monkey once did, in novel conditions for which our bodies were not originally adapted? In many respects, the answer is “very little,” because life at the start of the twenty-first century is pretty good for the average human being, and, overall, our species is thriving, in large part due to social, medical, and technological progress made over the last few generations. There are more than seven billion people alive, a large percentage of whom expect their children and grandchildren to live, as they will, into their seventies or above. Even countries with widespread poverty have achieved great progress: average life expectancy in India was less than fifty in 1970, but today is more than sixty-five. ¹ Billions of people will live longer, grow taller, and enjoy more comfort than most kings and queens of the past.

Yet as good as things are, they could be much better, and there are plenty of reasons to worry about the human body’s future. Apart from potential threats posed by climate change, we are also confronting a massive population boom combined with an epidemiological transition. As more people are living longer and fewer are dying young from diseases caused by infections or insufficient food, exponentially more middle-aged and elderly people are suffering from chronic noninfectious diseases that used to be rare or unknown. ² Cosseted by an embarrassment of riches, a majority of adults in developed countries such as the United States and the United Kingdom are unfit and overweight, and the prevalence of childhood obesity is skyrocketing globally, presaging billions more unfit and obese people in the decades to come. Poor fitness and excess weight, in turn, are accompanied by heart disease, strokes, and various cancers, as well as a multitude of costly, chronic illnesses such as type 2 diabetes and osteoporosis. Patterns of disability are also changing in disturbing ways as more people around the globe suffer from allergies, asthma, myopia, insomnia, flat feet, and other problems. Stated succinctly, lower mortality is being replaced by higher morbidity (ill health). To some extent, this shift is occurring because fewer people are dying when they are young from communicable diseases, but we mustn’t confuse diseases that become more common in older people with diseases that are actually caused by normal aging. ³ Morbidity and mortality at every age are significantly affected by lifestyle. Men and women aged forty-five to seventy-nine who are physically active, eat plenty of fruits and vegetables, do not smoke, and consume alcohol moderately have on average one-fourth the risk of death during a given year than people with unhealthy habits. ⁴

The soaring incidence of so many people with chronic diseases not only portends an escalation of suffering but also gargantuan medical bills. More than eight thousand dollars is spent per person each year on health care in the United States, adding up to nearly 18 percent of the nation’s gross domestic product (GDP). ⁵ A large percentage of this money is spent on treating preventable illnesses like type 2 diabetes and heart disease. Other countries spend less on health care, but their costs are rising at worrying rates as chronic diseases mount ( France, for example, now spends about 12 percent of its GDP on health care). As China, India, and other developing countries become wealthier, how will they cope with these illnesses and their costs? Clearly, we need to lower the cost of health care and to develop new, inexpensive treatments for the billions of current and future sick people. Yet wouldn’t it be better to prevent these illnesses in the first place? But how?

Which brings us back to the story of the Mystery Monkey. If people deemed it necessary to remove the monkey from the suburbs of Tampa, where it doesn’t belong, then maybe we should also return its former human neighbors to a more biologically normal state of nature. Even though humans, like rhesus macaques, can survive and multiply in a wide range of environments (including suburbs and laboratories), wouldn’t we enjoy better health if we ate the foods we were adapted to consume and exercised as our ancestors used to? The logic that evolution primarily adapted humans to survive and reproduce as hunter-gatherers rather than as farmers, factory laborers, or white-collar workers is inspiring a growing movement of modern-day cavemen. Followers of this approach to health contend that you would be healthier and happier if you ate and exercised more like your Stone Age ancestors. You can start by adopting a “paleodiet.” Eat plenty of meat (grass-fed, of course), as well as nuts, fruits, seeds, and leafy plants, and shun all processed foods with sugar and simple starches. If you are really serious, supplement your diet with worms, and never eat grains, dairy products, or anything fried. You can also incorporate more Paleolithic activities into your daily routine. Walk or run 10 kilometers (6.2 miles) a day (barefoot, of course), climb a few trees, chase squirrels in the park, throw rocks, eschew chairs, and sleep on a board instead of a mattress. To be fair, advocates of primal lifestyles are not advocating that you quit your job, move to the Kalahari Desert, and abandon all the best conveniences of modern life such as toilets, cars, and the Internet (which is essential to blog about your Stone Age experiences to other similarly minded folks). They are suggesting that you rethink how you use your body, especially what you eat and how you exercise.

But are they right? If a more Paleolithic lifestyle is obviously healthier, why don’t more people live this way? What are the drawbacks? Which foods and activities should we drop or adopt? Although it is obvious that humans are poorly adapted to gorging on too much junk food and lounging about in chairs all day long, our forebears also did not evolve to eat domesticated plants and animals, read books, take antibiotics, drink coffee, and run barefoot on glass-strewn streets.

These and other issues beg the fundamental question at the heart of this book: What are human bodies adapted for?

This is a profoundly challenging question to answer and it demands multiple approaches, one of which is to explore the evolutionary story of the human body. How and why did our bodies evolve to be the way they are? What foods did we evolve to eat? What activities did we evolve to do? Why do we have big brains, no fur, arched feet, and other distinctive features? As we shall see, the answers to these questions are fascinating, often hypothetical, and sometimes counterintuitive. A first order of business, however, is to consider the deeper, thornier question of what “adaptation” means. In truth, the concept of adaptation is notoriously tricky to define and apply. Just because we evolved to eat certain foods or do certain activities doesn’t mean they are good for us, or that other foods and activities aren’t better. Thus, before we tackle the story of the human body, let’s consider how the concept of adaptation derives from the theory of natural selection, what the term really means, and how it might be relevant to our bodies today.

How Natural Selection Works

Like sex, evolution elicits equally strong opinions from those who study it professionally and those who consider it so wrong and dangerous that they believe the subject shouldn’t be taught to children. Yet, despite much controversy and passionate ignorance, the idea that evolution occurs should not be contentious. Evolution is simply change over time. Even die-hard creationists recognize that the earth and its species have not always been the same. When Darwin published On the Origin of Species in 1859, scientists were already aware that former portions of the ocean floor, replete with shells and marine fossils, somehow had been thrust up into mountainous highlands. Discoveries of fossil mammoths and other extinct creatures testified that the world had altered profoundly. What was radical about Darwin’s theory was its breathtakingly comprehensive explanation for how evolution occurs through natural selection without any agency. ⁶

Natural selection is a remarkably simple process that is essentially the outcome of three common phenomena. The first is variation: every organism differs from other members of its species. Your family, your neighbors, and other humans vary widely in weight, leg length, nose shape, personality, and so on. The second phenomenon is genetic heritability: some of the variations present in every population are inherited because parents pass their genes on to their offspring. Your height is much more heritable than your personality, and which language you speak has no genetically heritable basis at all. The third and final phenomenon is differential reproductive success: all organisms, including humans, differ in how many offspring they produce who, themselves, survive to reproduce. Often, differences in reproductive success seem small and inconsequential (my brother has one more child than I do), but these differences can be dramatic and significant when individuals have to struggle or compete to survive and reproduce. Every winter, about 30 to 40 percent of the squirrels in my neighborhood perish, as did similar proportions of humans during great famines and plagues. The Black Death wiped out at least a third of Europe’s population between 1348 and 1350.

If you agree that variation, heritability, and differential reproductive success occur, then you must accept that natural selection occurs, because the inevitable outcome of these combined phenomena is natural selection. Like it or not, natural selection just happens. Stated formally, natural selection occurs whenever individuals with heritable variations differ in the number of surviving offspring they have compared to other individuals in the population (in other words, they differ in their relative fitness ) ⁷ . Natural selection occurs most commonly and strongly when organisms inherit rare, harmful variations, like hemophilia (the inability to form blood clots), that impair an individual’s ability to survive and reproduce. Such traits are less likely to be passed on to the next generation, thus reducing or eliminating them from the population. This sort of filter is called negative selection and often leads to a lack of change over time within a population, maintaining the status quo. Occasionally, however, positive selection occurs when an organism inherits by chance an adaptation , a new, heritable feature that helps it survive and reproduce better than its competitors. Adaptive features, by their very nature, tend to increase in frequency from generation to generation, causing change over time.

On the face of it, adaptation appears to be a straightforward concept that should be similarly straightforward to apply to humans, Mystery Monkeys, and other living beings. If a species evolved—and hence is presumably “adapted” to a particular diet or habitat—then members of that species should be most successful eating those foods and living in those circumstances. We have little difficulty accepting that lions, for example, are adapted for the African savanna rather than temperate forests, desert islands, or zoos. By the same logic, if lions are adapted, hence best suited, to the Serengeti, aren’t humans adapted, hence optimally suited, to living as hunter-gatherers? For many reasons, the answer is “not necessarily,” and considering how and why this is the case has profound ramifications for thinking about how the evolutionary story of the human body is relevant to its present and future.

The Thorny Concept of Adaptation

Your body has many thousands of obvious adaptations. Your sweat glands help you stay cool, your brain helps you think, and your gut’s enzymes help you digest. These attributes are adaptations because they are useful, inherited features that were shaped by natural selection and that promote survival and reproduction. You normally take these adaptations for granted, and their adaptive value often becomes evident only when they fail to function properly. For example, you might consider earwax a useless annoyance, but these secretions are actually beneficial because they help prevent ear infections. However, not all our bodies’ features are adaptations (I can think of nothing useful about my dimples, nostril hair, or tendency to yawn), and many adaptations function in counterintuitive or unpredictable ways. Appreciating what we are adapted for requires us to identify true adaptations and interpret their relevance. This, however, is easier said than done.

A first problem is to identify which features are adaptations and why. Consider your genome, which is a sequence of about three billion pairs of molecules (known as base pairs) that code for slightly more than twenty thousand genes. Every instant of your life, thousands of your body’s cells are replicating these billions of base pairs, each time with nearly perfect accuracy. It would be logical to infer that these billions of lines of code are all vital adaptations, but it turns out that nearly a third of your genome has no apparent function but exists because it somehow got added or lost its function over eons. ⁸ Your phenotype (your observable traits, such as the color of your eyes or the size of your appendix) is also replete with features that perhaps once had a useful role but no longer do, or which are simply the by-products of the way you developed. ⁹ Your wisdom teeth (if you still have them) exist because you inherited them, and they affect your ability to survive and reproduce no more than many other features you may have, such as a double-jointed thumb, an ear whose lower lobe is attached to the skin of the cheek, or nipples if you are a male. It is therefore erroneous to assume that all features are adaptations. Further, while it is easy to make up “just-so” stories about each feature’s adaptive value (an absurd example being that noses evolved to hold eyeglasses), careful science requires testing whether particular features are actually adaptations. ¹⁰

Although adaptations are not as widespread and easy to identify as you might assume, your body is nonetheless loaded with them. However, what makes an adaptation truly adaptive (that is, it improves an individual’s ability to survive and reproduce) is often dependent on context. This realization was, in fact, one of the key insights that Darwin gained from his celebrated trip around the world on the Beagle. Darwin inferred (after he returned to London) that variations in beak shape among the finches of the Galápagos Islands are adaptations for eating different foods. During the wet season, longer and thinner beaks help finches eat preferred foods such as cactus fruits and ticks, but during dry periods, shorter and thicker beaks help finches eat less desirable foods such as seeds, which are harder and less nutritious. ¹¹ Beak shapes, which are genetically heritable and vary within populations, are thus subject to natural selection among the Galápagos finches. As rainfall patterns fluctuate seasonally and annually, finches with longer beaks have relatively fewer offspring during dry spells, and finches with shorter beaks have relatively fewer offspring during wet spells, causing the percentage of short and long beaks to change. The same processes apply to other species, including humans. Many human variations such as height, nose shape, and the ability to digest foods like milk are heritable and evolved among certain populations because of specific environmental circumstances. Fair skin, for example, does not protect against sunburns but is an adaptation to help cells below the skin’s surface synthesize enough vitamin D in temperate habitats with low levels of ultraviolet radiation during the winter. ¹²

If adaptations are context dependent, what contexts matter the most? Here things can get tricky in consequential ways. Since adaptations are, by definition, features that help you have more offspring than others in your population, it follows that selection for adaptations will be most potent when the number of surviving descendants you have is most likely to vary. Put crudely, adaptations evolve most strongly when the going gets tough. As an example, your ancestors from about 6 million years ago mostly consumed fruit, but that doesn’t mean their teeth were just adapted to chew figs and grapes. If rare but serious droughts made fruit scarce, then individuals with bigger, thicker molars that helped them chew other, less preferred foods such as tough leaves, stems, and roots would have had a strong selective advantage. Along the same lines, the nearly universal tendency to crave rich food like cake and cheeseburgers and store the excess calories as fat is maladaptive under today’s conditions of relentless abundance, but it must have been highly advantageous in the past when food was scarcer and less calorific.

Adaptations also have costs that balance their benefits. Every time you do something, you can’t do something else. Further, as conditions inevitably change, the relative costs and benefits of variations inevitably change too, depending on context. Among the Galápagos finches, thick beaks are less effective for eating cactuses, thin beaks are less effective for eating hard seeds, and intermediate beaks are less effective for eating both kinds of foods. Among humans, having short legs is advantageous for conserving heat in cold climates but disadvantageous for walking or running long distances efficiently. One consequence of these and other compromises is that natural selection rarely, if ever, achieves perfection because environments are always changing. As rainfall, temperatures, foods, predators, prey, and other factors shift and vary seasonally, annually, and over longer time spans, the adaptive value of every feature also changes. Each individual’s adaptations are thus the imperfect product of an endless series of constantly altering compromises. Natural selection constantly pushes organisms toward optimality, but optimality is almost always impossible to achieve.

Perfection may be unattainable, but bodies function remarkably well under a wide range of circumstances because of the way evolution accumulates adaptations in bodies much like the way you probably keep accumulating new kitchen utensils, books, or items of clothing. Your body is a jumble of adaptations that accrued over millions of years. An analogy for this hodgepodge effect is a palimpsest, an ancient manuscript page that was written on more than once and thus contains multiple layers of texts that begin to mix up over time as the more superficial texts rub away. Like a palimpsest, a body has multiple related adaptations that sometimes conflict with one another, but at other times work in combination to help you function effectively in a broad range of conditions. Consider your diet. Human teeth are superbly adapted for chewing fruit because we evolved from apes that mostly ate fruit, but our teeth are extremely ineffective for chewing raw meat, especially tough game. Later, we evolved other adaptations such as the ability to make stones into tools and cook that now allow us to chew meat, coconuts, nettles, and just about everything else that isn’t poisonous. Multiple interacting adaptations, however, sometimes lead to compromises. As later chapters will explore, humans evolved adaptations to walk and run upright, but these limited our ability to sprint fast or climb with great agility.

The final and most important point about adaptation is really a crucial caveat: no organism is primarily adapted to be healthy, long-lived, happy, or to achieve many other goals for which people strive. As a reminder, adaptations are features shaped by natural selection that promote relative reproductive success (fitness). Consequently, adaptations evolve to promote health, longevity, and happiness only insofar as these qualities benefit an individual’s ability to have more surviving offspring . To return to an earlier topic, humans evolved to be prone to obesity not because excess fat makes us healthy, but because it increases fertility. Along the same lines, our species’ proclivities to be worried, anxious, and stressed cause much misery and unhappiness, but they are ancient adaptations to avoid or cope with danger. And we not only evolved to cooperate, innovate, communicate, and nurture, but also to cheat, steal, lie, and murder. The bottom line is that many human adaptations did not necessarily evolve to promote physical or mental well-being.

All in all, trying to answer the question “What are humans adapted for?” is paradoxically both simple and quixotic. On the one hand, the most fundamental answer is that humans are adapted to have as many children, grandchildren, and great-grandchildren as possible! On the other hand, how our bodies actually manage to pass themselves on to the next generation is anything but straightforward. Because of your complex evolutionary history, you are not adapted for any single diet, habitat, social environment, or exercise regime. From an evolutionary perspective, there is no such thing as optimal health. As a result, humans—like our friend the Mystery Monkey—not only survive but sometimes also thrive in novel conditions for which we did not evolve (like the suburbs of Florida).

If evolution provides no easy-to-follow guidelines for optimizing health or preventing illness, then why should anyone interested in his or her well-being think about what happened in human evolution? How are apes, Neanderthals, and early Neolithic farmers relevant to your body? I can think of two very important answers, one involving the evolutionary past, and the other involving the evolutionary present and future.

Why the Human Evolutionary Past Matters

Everybody and every body has a story. Your body in fact has several stories. One is the story of your life, your biography: who your parents are and how they met, where you grew up, and how your body was molded by life’s vicissitudes. The other story is evolutionary: the long chain of events that transformed your ancestors’ bodies from one generation to the next over millions of years, and which made your body different from that of a Homo erectus , a fish, and a fruit fly. ¹³ Both stories are worth knowing, and they share certain common elements: characters (including putative heroes and villains), settings, chance events, triumphs, and tribulations. ¹⁴ Both stories can also be approached using the scientific method by framing them as hypotheses whose facts and assumptions can be questioned and rejected.

The evolutionary history of the human body is an interesting yarn. One of its most valuable lessons is that we are not an inevitable species: had circumstances been different, even slightly so, we would be very different creatures (or in all probability we wouldn’t exist at all). For many people, however, the chief reason to tell (and test) the story of the human body is to shed light on why we are the way we are. Why do we have big brains, long legs, especially visible belly buttons, and other peculiarities? Why do we walk on just two legs and communicate with language? Why do we cooperate so much and cook our food? A related, urgent, and practical reason to consider how the human body evolved is to help evaluate what we are and are not adapted for, hence why we get sick. In turn, evaluating why we get sick is essential for preventing and treating diseases.

To appreciate this logic, consider the example of type 2 diabetes, an almost entirely preventable disease whose incidence is soaring throughout the world. This disease arises when cells throughout your body cease to respond to insulin, a hormone that shuttles sugar out of the bloodstream and stores it as fat. When the inability to respond to insulin sets in, the body starts acting like a broken heating system that fails to deliver heat from the furnace to the rest of the house, causing the furnace to overheat while the house freezes. With diabetes, blood sugar levels keep rising, which in turn stimulates the pancreas to produce even more insulin, but with futile results. After several years, the fatigued pancreas cannot produce enough insulin, and blood sugar levels stay persistently high. Too much blood sugar is toxic and causes horrid health problems and eventually death. Fortunately, medical science has become adept at recognizing and treating the symptoms of diabetes early on, enabling millions of diabetics to survive for decades.

On the face of it, the evolutionary history of the human body seems irrelevant to treating patients with type 2 diabetes. Because these patients need urgent, costly care, thousands of scientists now study the disease’s causal mechanisms, such as how obesity makes certain cells resistant to insulin, how overworked insulin-producing cells in the pancreas stop functioning, and how certain genes predispose some but not others to the disease. Such research is essential for better treatment. But what about preventing the disease in the first place? To prevent a disease or any other complex problem one not only needs to know about its proximate, causal mechanisms, but also its deeper underlying roots. Why does it occur? In the case of type 2 diabetes, why are humans so susceptible to this disease? Why do our bodies sometimes cope poorly with modern lifestyles in ways that lead to type 2 diabetes? Why are some people more at risk? Why aren’t we better at encouraging people to eat healthier food and be more physically active in order to prevent the disease?

Efforts to answer these and other why questions impel us to consider the evolutionary history of the human body. No one has ever expressed this imperative better than the pioneering geneticist Theodosius Dobzhansky, who famously wrote, “Nothing in biology makes sense except in the light of evolution.” ¹⁵ Why? Because life is most essentially the process by which living things use energy to make more living things. Therefore if you want to know why you look, function, and get sick differently from your grandparents, your neighbor, or the Mystery Monkey, you need to know the biological history—the long chain of processes—by which you, your neighbor, and the monkey came into being differently. The important details of this story, morever, go back many, many generations. Your body’s various adaptations were selected to help your ancestors survive and reproduce in an untold number of distant incarnations, not just as hunter-gatherers, but also as fish, monkeys, apes, australopiths, and more recently as farmers. These adaptations explain and constrain how your body normally functions in terms of how you digest, think, reproduce, sleep, walk, run, and more. It follows that considering the body’s long evolutionary history helps explain why you and others get sick or injured when you behave in ways for which you are poorly or insufficiently adapted.

Returning to the problem of why humans get type 2 diabetes: the answer does not lie solely in the cellular and genetic mechanisms that precipitate the disease. More deeply, diabetes is a growing problem because human bodies, like those of captive primates, were adapted primarily for very different conditions that render us inadequately adapted to cope with modern diets and physical inactivity. ¹⁶ Millions of years of evolution favored ancestors who craved energy-rich foods, including simple carbohydrates like sugar that used to be rare, and who efficiently stored excess calories as fat. In addition, few if any of your distant ancestors had the opportunity to become diabetic by being physically inactive and by eating lots of soda and donuts. Apparently, our ancestors also did not experience strong selection to adapt to the causes of other recent diseases and disabilities like hardening of the arteries, osteoporosis, and myopia. The fundamental answer to why so many humans are now getting sick from previously rare illnesses is that many of the body’s features were adaptive in the environments for which we evolved but have become maladaptive in the modern environments we have now created. This idea, known as the mismatch hypothesis, is the core of the new, emerging field of evolutionary medicine, which applies evolutionary biology to health and disease. ¹⁷

The mismatch hypothesis is the focus of the second part of this book, but to figure out which diseases are or are not caused by evolutionary mismatches requires more than a superficial consideration of human evolution. Some simplistic applications of the mismatch hypothesis propose that since humans evolved to be hunter-gatherers we are therefore optimally adapted to a hunter-gatherer way of life. This kind of thinking can lead to naïve prescriptions based on what Bushmen of the Kalahari or the Inuit of Alaska have been observed to eat and do. One problem is that hunter-gatherers themselves are not always healthy, and they are highly variable, in large part because they inhabit a wide range of environments including deserts, rain forests, woodlands, and the arctic tundra. There is no one ideal, quintessential hunter-gatherer way of life. More important, as discussed above, natural selection did not necessarily adapt hunter-gatherers (or any creature) to be healthy, but instead to have as many babies as possible who then survived to breed as well. It also bears repeating that human bodies (including those of hunter-gatherers) are palimpsest-like compilations of adaptations that were accumulated and modified over countless generations. Before our ancestors were hunter-gatherers they were apelike bipeds, and before then they were monkeys, small mammals, and so on. And since then, some populations have evolved new adaptations to being farmers. Consequently, there was no single environment for which the human body evolved, and hence is adapted. Thus, to answer the question “What are we adapted for?” requires that we consider not just hunter-gatherers realistically, but also look at the long chain of events that led to the evolution of hunting and gathering, as well as what happened since we started to farm our food. As an analogy, trying to understand what the human body is adapted for by focusing on just hunter-gatherers is like trying to understand the result of a football game from watching just part of the fourth quarter.

The bottom line is that we have much to gain from considering in more than superficial depth the story of how and why the human body evolved if we wish to comprehend what humans are (and are not) adapted for. Like every family story, our species’ evolutionary history is rewarding to learn but confusingly messy and full of gaps. Trying to figure out the family tree of human ancestors can make keeping track of the characters in War and Peace seem like child’s play. However, more than a century of intense research has yielded a coherent and generally accepted understanding of how our lineage evolved from being apes in an African forest into modern humans who inhabit most of the globe. Leaving aside the precise details of the family tree (essentially, who begat whom), the story of the human body can be boiled down to five major transformations. None of them were inevitable, but each altered our ancestors’ bodies in different ways by adding new adaptations and by removing others.

Why Evolution Matters for the Present and Future, Too

Do you think that evolution is just the study of the past? I used to, and so does my dictionary, which defines evolution as “the process by which different kinds of living organisms are thought to have developed and diversified from earlier forms during the history of the earth.” I am dissatisfied with this definition because evolution (which I prefer to define as change over time) is also a dynamic process that is still occurring today. Contrary to what some people assume, the human body didn’t stop evolving once the Paleolithic ended. Instead, natural selection is still relentlessly chugging along, and will carry on as long as people inherit variations that influence, even slightly, how many offspring they have who survive and then breed again. As a result, our bodies are not entirely the same as our ancestors’ bodies were a few hundred generations ago. Along the same lines, our descendants hundreds of generations from now will also differ from us.

Evolution, in addition, isn’t just about biological evolution. How genes and bodies change over time is incredibly important, but another momentous dynamic to grapple with is cultural evolution , now the most powerful force of change on the planet and one that is radically transforming our bodies. Culture is essentially what people learn, and so cultures evolve. Yet a crucial difference between cultural and biological evolution is that culture doesn’t change solely through chance but also through intention, and the source of this change can come from anyone, not just your parents. Culture can therefore evolve with breathtaking rapidity and degree. Human cultural evolution got its start millions of years ago, but it accelerated dramatically after modern humans first evolved around 200,000 years ago, and it has now reached dizzying speeds. Looking back on the last few hundred generations, two cultural transformations have been of vital importance to the human body and need to be added to the list of evolutionary transformations above:

Although these last two transformations did not generate new species, it is difficult to exaggerate their importance for the story of the human body because they radically altered what we eat and how we work, sleep, regulate our body temperature, interact, and even defecate. Although these and other shifts in our bodies’ environments have spurred some natural selection, they have mostly interacted with the bodies we inherited in ways we have yet to fathom. Some of these interactions have been beneficial, especially allowing us to have more children. Others, however, have been deleterious, including a host of novel mismatch diseases caused by contagion, malnutrition, and physical inactivity. Over the last few generations we have learned how to conquer or curb many of these diseases, but other chronic, noninfectious mismatch diseases—many linked to obesity—are now increasing rapidly in prevalence and intensity. By any standard, the human body’s evolution is far from over thanks to rapid cultural change.

I would therefore argue that, when applied to humans, Dobzhansky’s brilliant statement that “nothing in biology makes sense except in the light of evolution” applies not just to evolution by natural selection but also to cultural evolution . To go a step further, since cultural evolution is now the dominant force of evolutionary change acting on the human body, it follows that we can better understand why more people are getting chronic noninfectious mismatch diseases and how to prevent these illnesses by considering interactions between cultural evolution and our inherited, still-evolving bodies. These interactions sometimes set in motion an unfortunate dynamic that typically works as follows: First, we get sick from noninfectious mismatch diseases caused by our bodies being poorly or inadequately adapted to the novel environments we have created through culture. Then, for various reasons, we sometimes fail to prevent these mismatch diseases. In some cases, we don’t understand a disease’s causes well enough to prevent it. Often, efforts at prevention fail because it is difficult or impossible to change the novel environmental factors responsible for the mismatch. Occasionally, we even promote mismatch diseases by treating their symptoms so effectively that we unwittingly perpetuate their causes. In all cases, however, by not addressing the novel environmental causes of mismatch diseases we permit a vicious circle to occur that allows the disease to remain prevalent or sometimes to become more common or severe. This feedback loop is not a form of biological evolution because we don’t pass on mismatch diseases directly to our children. Instead it is a form of cultural evolution because we pass on the environments and behaviors that cause them.

But I am getting ahead of myself, and the human body’s story. Before we think about how biological and cultural evolution interact, we first need to consider the long trajectory of evolutionary history, how we evolved the capacity for culture, and what the human body is really adapted for. This exploration requires turning back the clock about 6 million years or so to a forest somewhere in Africa… 9s0fW8/ei66340fnSZBGujY8RHrUSSBPWikR2h4TMOx1S6ffIYlBjosatkxiZptK