No one knows why we walk. Out of some 250 species of primates, we are the only ones that have elected to get up and move around exclusively on two legs. Some authorities think bipedalism is at least as important a defining characteristic of what it is to be human as our high-functioning brain.

Many theories have been proposed as to why our distant ancestors dropped out of trees and adopted an upright posture—to free their hands to carry babies and other objects; to gain a better line of sight across open ground; to be better able to throw projectiles—but the one certainty is that walking on two legs came at a price. Moving about in the open made our ancient forebears exceedingly vulnerable, for they were not formidable creatures, to say the least. The young and gracile protohuman famously known as Lucy, who lived in what is now Ethiopia some 3.2 million years ago and is often used as a model for early bipedalism, was only about three and a half feet tall and weighed just 60 pounds—hardly the sort of presence to intimidate a lion or cheetah.

It’s likely Lucy and her tribal kin had little choice but to take the risk of stepping out into the open. As climate change made their forest habitats shrink, they very probably needed to forage over larger and larger areas to survive, but they almost certainly scampered back to trees when they could. Even Lucy appears to have been only a partial convert to life at ground level. In 2016, anthropologists at the University of Texas concluded that Lucy died after falling out of a tree (or suffered a “vertical deceleration event,” as they put it, just a touch drily), the implication being that she spent a great deal of time in the canopy of trees and was probably as much at home up there as on the ground. Or at least she was until the last three or four seconds of her life.

Walking is a more skillful undertaking than we generally appreciate. By balancing on just two supports, we exist in permanent defiance of gravity. As toddlers amusingly demonstrate, walking is essentially a matter of hurling the body forward and letting the legs run to catch up. A pedestrian in motion has one foot or the other off the ground for as much as 90 percent of the time, and thus engages in constant unconscious adjustments of balance. In addition, our center of gravity is high—just above our waists—which adds to our innate tippiness.

In order to proceed from arboreal ape to upright modern human, we had to undertake some pretty profound changes to our anatomy. As noted earlier, our necks became longer and straighter and joined the skull more or less centrally rather than toward the rear as in other apes. We have a supple back that bends, outsized knees, and ingeniously angled thigh bones. You may think your legs drop straight down from your waist—they do in apes—but in fact the femur angles inward as it descends from pelvis to knee. This has the effect of moving our lower legs closer together, giving us a much smoother, more graceful gait. No ape can be trained to walk like a human. They are compelled by their bone structure to waddle, and to do so in a most inefficient way. A chimpanzee uses four times as much energy to move around at ground level as does a human.

First, we became walkers and climbers, but not runners. Then, gradually, we became walkers and runners, but no longer climbers.

To power our forward motion, we have a distinctively gigantic muscle in our buttocks, the gluteus maximus, and an Achilles tendon, something no ape has. We have arches in our feet (for springiness), a sinuous spine (to redistribute weight), and reconfigured pathways for our nerves and blood vessels—all made necessary, or at least advisable, by the evolutionary imperative of putting our head way above our feet. To keep from overheating when we exert ourselves, we became relatively hairless and developed abundant sweat glands.

Above all, we evolved a very different head from other primates. Our faces are flat and conspicuously snoutless. We have a high forehead to accommodate our more impressive brain. Cooking has left us with smaller teeth and a more delicate jaw. Inside, we have a short oral cavity and thus a shorter, more rounded tongue, and a larynx that sits lower in the throat. The changes to our upper anatomy left us by happy accident with vocal tracts uniquely able to make articulate speech. Walking and talking probably went hand in hand. If you are a little creature that hunts big creatures, being able to communicate is obviously an advantage.

At the back of your head is a modest ligament, not found on other apes, that instantly betrays what it is about us that allowed us to thrive as a species. It is the nuchal ligament, and it has just one job: to hold the head steady when running. And running—serious, dogged, long-distance running—is the one thing we do superlatively well.

We are not the speediest of creatures, as anyone who has ever chased a dog or cat or even an escaped hamster will know. The very fastest humans can run about 20 miles an hour, though only for short bursts. But put us up against an antelope or wildebeest on a hot day and allow us to trot after it, and we can run it into the ground. We perspire to keep cool, but quadrupedal mammals lose heat by respiration—by panting. If they can’t stop to collect themselves, they overheat and become helpless. Most large animals can’t run for more than about nine miles before they drop. That our ancestors could also organize themselves into hunting parties, to harry quarry from different sides or drive prey into confined spaces, made us all the more effective.

These anatomical changes were so monumental that they spawned an entirely new genus (the biological rank above species but below family) called Homo. Daniel Lieberman, of Harvard, stresses that the transformation was a two-stage process. First, we became walkers and climbers, but not runners. Then, gradually, we became walkers and runners, but no longer climbers. Running is not just a faster form of locomotion than walking but mechanically quite different. “Walking is a stilt-like gait and involves very different adaptations from running,” he says. Lucy was a walker and climber but lacked the physique for running. That came much later, after climate change turned much of Africa into open woodlands and grassy savanna, impelling our vegetarian ancestors to adjust their diets and become carnivores (or really omnivores).

All these changes, in lifestyle and anatomy, happened with exceeding slowness. Fossil evidence suggests that early hominins were walking by about six million years ago, but needed an additional four million years to acquire the capabilities for endurance running and, with it, persistence hunting. Then a further million and a half years had to pass before they gathered enough cerebral momentum to manufacture tipped spears. That’s a long time to wait for a full set of survival capabilities in a hostile, hungry world. Despite these deficiencies, our ancient forebears were successfully hunting large animals 1.9 million years ago.

They were able to do this because of an additional trick in the Homo armamentarium: throwing. Throwing required us to change our bodies in three crucial ways. We needed a high and mobile waist (to create a lot of torsion), loose and maneuverable shoulders, and an upper arm capable of flinging in a whiplike fashion. The shoulder joint in humans is not a snug ball and socket, as in our hips, but a more loose and open arrangement. This allows the shoulder to be limber and to rotate freely—exactly what’s needed for forceful throwing—but it also means that we dislocate our shoulders easily.

What can be said about exercise is that most of us are not getting nearly enough.

We throw with our whole bodies. Try throwing an object forcefully while standing still and you can hardly do it. A good throw involves a forward step, a brisk rotation of waist and torso, a long backward stretch of the arm at the shoulder, and a powerful hurl. When executed well, a human can throw an object with considerable accuracy at speeds easily in excess of 90 miles an hour, as professional baseball pitchers repeatedly demonstrate. The ability to wound and torment exhausted prey with rocks from a relatively safe distance must have been a highly useful skill among early hunters.

Bipedalism had consequences, too—consequences that we all live with today, as anyone with chronic back pain or knee problems can attest. Above all, the adoption of a narrower pelvis to accommodate our new gait brought a huge amount of pain and danger to women in childbirth. Until recent times, no other animal on Earth was more likely to die in childbirth than a human, and perhaps none even now suffers as much.

For the longest time the crucial importance to health of just moving around was hardly appreciated. But in the late 1940s a doctor at Britain’s Medical Research Council, Jeremy Morris, became convinced that the increasing occurrence of heart attacks and coronary disease was related to levels of activity, and not just to age or chronic stress, as was almost universally thought at the time. Because Britain was still recovering from the war, research funding was tight, so Morris had to think of a low-cost way to conduct an effective large-scale study. While traveling to work one day, it occurred to him that every double-decker bus in London was a perfect laboratory for his purposes because each had a driver who spent his entire working life sitting and a conductor who was on his feet constantly. In addition to moving about laterally, conductors climbed an average of 600 steps per shift. Morris could hardly have invented two more ideal groups to compare. He followed 35,000 drivers and conductors for two years and found that after he adjusted for all other variables, the drivers—no matter how healthy—were twice as likely to have a heart attack as the conductors. It was the first time that anyone had demonstrated a direct and measurable link between exercise and health.

Study after study since then has shown that exercise produces extraordinary benefits. Going for regular walks reduces the risk of heart attack or stroke by 31 percent. An analysis of 655,000 people in 2012 found that being active for just eleven minutes a day after the age of 40 yielded 1.8 years of added life expectancy. Being active for an hour or more a day improved life expectancy by 4.2 years.

As well as strengthening bones, exercise boosts your immune system, nurtures hormones, lessens the risk of getting diabetes and a number of cancers (including breast and colorectal), improves mood, and even staves off senility. As has been noted many times, there is probably not a single organ or system in the body that does not benefit from exercise. If someone invented a pill that could do for us all that a moderate amount of exercise achieves, it would instantly become the most successful drug in history.

And how much exercise should we get? That’s not easy to say. The more or less universal belief that we should all walk ten thousand steps a day—that’s about five miles—is not a bad idea, but it has no special basis in science. Clearly, any ambulation is likely to be beneficial, but the notion that there is a universal magic number of steps that will give us health and longevity is a myth. The 10,000-step idea is often attributed to a single study done in Japan in the 1960s, though it appears that also may be a myth. In the same way, the Centers for Disease Control’s recommendations on exercise, namely 150 minutes per week of moderate activity, are based not on the optimal amount needed for health, because no one can say what that is, but on what the CDC’s advisers think people will perceive as realistic goals.

What can be said about exercise is that most of us are not getting nearly enough. Only about 20 percent of people manage even a moderate level of regular activity. Many get almost none at all. Today the average American walks only about a third of a mile a day—and that’s walking of all types, including around the house and workplace. Even in an indolent society, it would seem almost impossible to do less. According to The Economist, some American companies have begun offering rewards to employees who log a million steps a year on an activity tracker such as a Fitbit. That seems a pretty ambitious number but actually works out to just 2,740 steps a day, or a little over a mile. Even that, however, seems to be beyond many. “Some workers have reportedly strapped their Fitbits to their dogs to boost their activity scores,” The Economist noted. Modern hunter-gatherers, by contrast, average about 19 miles of walking and trotting to secure a day’s food, and it is reasonable to assume that our ancient forebears would have done about the same.

In short, they worked hard for what they ate and consequently ended up with bodies designed to do two somewhat contradictory things: to be active much of the time, but never to be more active than absolutely necessary. As Daniel Lieberman explains, “If you want to understand the human body, you have to understand that we evolved to be hunter-gatherers. That means being prepared to expend a lot of energy to acquire food, but not wasting energy when you don’t need to.” So exercise is important, but rest is vital, too. “For one thing,” Lieberman says, “you can’t digest food while you are exercising because the body shunts blood away from the digestive system in order to meet the increased demand to supply oxygen to the muscles. So you have to rest sometimes just for metabolic purposes and to recover from the exertions of exercise.”

Because our ancient ancestors had to survive lean times as well as good, they evolved a tendency to store fat as a fuel reserve—a survival reflex that is now, all too often, killing us. The upshot is that millions of us spend our lives struggling to maintain a balance between paleolithically designed bodies and modern gustatory excess. It’s a battle too many of us are losing.

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From The Body: A Guide for Occupants by Bill Bryson. Used with the permission of the publisher, Doubleday, a division of Penguin Random House LLC. Copyright © 2019 by Bill Bryson. Audio excerpted courtesy Penguin Random House Audio from The Body by Bill Bryson, read by the author.