This highly engaging landmark work, a natural history of exerciseby the author of the best seller The Story of the Human Bodyseeks to answer a fundamental question: were you born to run or rest?
The first three parts of Exercised roughly follow the evolutionary story of human physical activity and inactivity, even as each chapter shatters a particular myth about exercise. Because we cannot understand physical activity without understanding its absence, Part One begins with physical inactivity. What are our bodies doing when we take it easy, including when we sit or sleep? Part Two explores physical activities that require speed, strength, and power, such as sprinting, lifting, and fighting. Part Three surveys physical activities that involve endurance, such as walking, running, or dancing, as well as their effect on aging. Part Four considers how anthropological and evolutionary approaches can help us exercise better in the modern world. How can we more effectively manage to exercise, and in what ways? To what extent, how, and why do different types and durations of exercise help prevent or treat the major diseases that are likely to make us sick and kill us?
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About the Author
DANIEL E. LIEBERMAN is a paleoanthropologist at Harvard University, where he is the Edwin M. Lerner II Professor of Biological Sciences, and chair of the Department of Human Evolutionary Biology. He is best known for his research on the evolution of the human head and the evolution of the human body.
Read an Excerpt
From Chapter One: Are We Born to Rest or Run?
Are “Normal” Humans Couch Potatoes?
Imagine you have been asked to conduct a scientific study on how much, when, and why “normal” people exercise. Because we tend to think of ourselves and our societies as normal, you’d probably collect data on the exercise habits of people like you and me. This approach is the norm in many fields of inquiry. For example, because most psychologists live and work in the United States and Europe, about 96 percent of the subjects in psychological studies are also from the United States and Europe.8 Such a narrow perspective is appropriate if we are interested only in contemporary Westerners, but people in Western industrialized countries aren’t necessarily representative of the other 88 percent of the world’s population. Moreover, today’s world is profoundly different from that of the past, calling into question who among us is “normal” by historical or evolutionary standards. Imagine trying to explain cell phones and Facebook to your great-great- great- grandparents. If we really want to know what ordinary humans do and think about exercise, we need to sample everyday people from a variety of cultures instead of focusing solely on contemporary Americans and Europeans who are, comparatively speaking, WEIRD (Western, educated, industrialized, rich, and democratic).9
To go a step further, until a few hundred generations ago, all human beings were hunter-gatherers, and until about eighty thousand years ago everyone’s ancestors lived in Africa. So if we genuinely want to know about the exercise habits of evolutionarily “normal” humans, it behooves us to learn about hunter-gatherers, especially those who live in arid, tropical Africa.
Studying hunter-gatherers, however, is easier said than done because their way of life has almost entirely vanished. Only a handful of hunter-gatherer tribes persist in some of the most remote corners of the globe. Further, none are isolated from civilization and none subsist solely on the wild foods they hunt and gather. All of these tribes trade with neighboring farmers, they smoke tobacco, and their way of life is changing so rapidly that in a few decades they will cease to be hunter-gatherers. 10 Anthropologists and other scientists are therefore scrambling to learn as much as possible from these few tribes before their way of life irrevocably disappears.
Of all of them, the most intensely studied is the Hadza, who live in a dry, hot woodland region of Tanzania in Africa, the continent where humans evolved. In fact, doing research on the Hadza has become something of a cottage industry for anthropologists. In the last decade, researchers have studied almost everything you can imagine about the Hadza. You can read books and articles about how the Hadza eat, hunt, sleep, digest, collect honey, make friends, squat, walk, run, evaluate each other’s attractiveness, and more.11 You can even read about their poop.12 In turn, the Hadza have become so used to visiting scientists that hosting the researchers who observe them has become a way to supplement their income. Sadly, visiting scientists who want to emphasize how much they are studying bona fide hunter-gatherers sometimes turn a blind eye to the degree to which the Hadza’s way of life is changing as a result of contact with the outside world. These papers rarely mention how many Hadza children now go to government schools, and how the Hadza’s territory is almost entirely shared with neighboring tribes of farmers and pastoralists, with whom they trade and whose cows tramp all over the region. As I write this, the Hadza don’t yet have cell phones, but they are not isolated as they once were.
Despite these limitations, there is still much to learn from the Hadza, and I am fortunate to have visited them on a couple of occasions. But to get to the Hadza is not easy. They live in a ring of inhospitable hills surrounding a seasonal, salty lake in northwestern Tanzania—a hot, arid, sunbaked region that is almost impossible to farm.13 The area has some of the worst roads on the planet. Of the roughly twelve hundred Hadza, only about four hundred still predominantly hunt and gather, and to find these few, more traditional Hadza, you need sturdy jeeps, an experienced guide, and a lot of skill to travel over treacherous terrain. After a rainstorm, driving twenty miles can take most of the day.
Many things surprised me when I first walked into a Hadza camp mid-morning on a torrid, sunny day in 2013, but I remember being especially struck by how everyone was apparently doing nothing. Hadza camps consist of a few temporary grass huts that blend in with the surrounding bushes. I didn’t realize I had walked into a camp until I found myself amid about fifteen Hadza men, women, and children who were sitting on the ground as shown in figure 2. The women and children were relaxing on one side, and the men on another. One fellow was straightening some arrows, and a few children were toddling about, but no one was engaged in any hard work. To be sure, the Hadza weren’t lounging on sofas, watching TV, munching potato chips, and sipping soda, but they were doing what so many health experts warn us to avoid: sitting.
My observations since that day along with published studies of their activity levels confirm my initial impression: when Hadza men and women are in camp, they are almost always doing light chores while sitting on the ground, gossiping, looking after children, and otherwise just hanging around. Of course, Hadza men and women head out almost every day to the bush to hunt or gather food. The women typically leave camp in the morning and walk several miles to somewhere they can dig for tubers. Digging is a relaxing and social task that usually involves sitting in a group under the bushes in the shade and using sticks to excavate edible tubers and roots. As Hadza women dig, they eat some of what they extract while chatting and minding their infants and toddlers. On the way there and back, women often stop to collect berries, nuts, or other foods. On the few occasions when I have accompanied Hadza men on hunts, we walked between seven and ten miles. When they are tracking animals, the pace is varied but never so fast that I wasn’t able to keep up, and often the hunters stop to rest and look around. Whenever they encounter a honeybee hive, they stop, make a fire, smoke out the bees, and gorge themselves on fresh honey.
Among the many studies of the Hadza, one asked forty-six Hadza adults to wear lightweight heart rate monitors for several days.14 According to these sensors, the average adult Hadza spends a grand total of three hours and forty minutes a day doing light activities and two hours and fourteen minutes a day doing moderate or vigorous activities. Although these few hours of hustling and bustling per day make them about twelve times more active than the average American or European, by no stretch of the imagination could one characterize their workloads as backbreaking. On average, the women walk five miles a day and dig for several hours, whereas the men walk between seven and ten miles a day.15 And when they aren’t being very active, they typically rest or do light work.
The Hadza, moreover, are typical of other hunter-gatherer groups whose physical activity levels have been studied. The anthropologist Richard B. Lee astonished the world in 1979 by documenting that San hunter-gatherers in the Kalahari spend only two to three hours a day foraging for food.16 Lee might have underestimated how much work the San do, but more recent studies of other foraging populations report similarly modest physical activity levels as the Hadza.17 One especially well-studied group is the Tsimane, who fish, hunt, and grow a few crops in the Amazon rain forest. Overall, Tsimane adults are physically active for four to seven hours per day, with men engaging in vigorous tasks like hunting for only about seventy-two minutes a day and women engaging in almost no vigorous activity at all but instead doing mostly light to moderate tasks such as child care and food processing.18
All in all, assuming that what hunter-gatherers do is evolutionarily “normal,” then comprehensive studies of contemporary foraging populations from Africa, Asia, and the Americas indicate that a typical human workday used to be about seven hours, with much of that time spent on light activities and at most an hour of vigorous activity.19 To be sure, there is variation from group to group and from season to season, and there is no such thing as a vacation or retirement, but most hunter-gatherers engage in modest levels of physical effort, much of it accomplished while sitting. How different, then, are such “normal” humans from postindustrial people like me (and perhaps you), not to mention farmers like the Tarahumara, factory workers, and others whose lives have been transformed by civilization?
Table of ContentsPrologue
1. Are We Born to Rest or Run?
PART I: INACTIVITY
2. Inactivity: The Importance of Being Lazy
3. Sitting: Is It the New Smoking?
4. Sleep: Why Stress Thwarts Rest
PART II: SPEED, STRENGTH, AND POWER
5. Speed: Neither Tortoise nor Hare
6. Strength: From Brawny to Scrawny
7. Fighting and Sports: From Fangs to Football
PART III: ENDURANCE
8. Walking: All in a Day’s Walk
9. Running and Dancing: Jumping from One Leg to the Other
10. Endurance and Aging: The Active Grandparent and Costly Repair Hypotheses
PART IV: EXERCISE IN THE MODERN WORLD
11. To Move or Not to Move: How to Make Exercise Happen
12. How Much and What Type?
13. Exercise and Disease