The latest theory about the runner’s high makes a bold claim: Our ability to experience exercise-induced euphoria is linked to our earliest ancestors’ lives as hunters, scavengers, and foragers. As biologist Dennis Bramble and paleoanthropologist Daniel Lieberman write, “Today, endurance running is primarily a form of exercise and recreation, but its roots may be as ancient as the origin of the human genus.” The neurochemical state that makes running gratifying may have originally served as a reward to keep early humans hunting and gathering. What we call the runner’s high may even have encouraged our ancestors to cooperate and share the spoils of a hunt.
In our evolutionary past, humans may have survived in part because physical activity was pleasurable. In our modern landscape, that same high—whether you achieve it through running or some other physical activity—can elevate your mood and make social connection easier. Understanding the science behind the runner’s high can help you capitalize on these effects, whether your goal is to feel more connected to your community or to find a form of exercise that leaves you love-drunk and glad to be alive.
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In 2010, anthropologist Herman Pontzer was startled awake in his nylon tent by the sound of lions roaring. Pontzer, who is now a professor at Duke University, was camped near Lake Eyasi in northern Tanzania. The campsite was not far from the Olduvai Gorge, where one of the first hominid species to use tools, Homo habilis, lived two million years ago. Ponzter was in Tanzania to observe the physical activity habits of the Hadza, one of the last hunter-gatherer tribes in Africa. He and his team had only been at the Hadza campsite for a couple of days, and Pontzer was still getting used to the environment. He estimated that the roaring lions were no more than half a mile away. Pontzer tried to push the sounds out of mind and went back to sleep.
The next morning, he woke at six and joined his research team around a fire. As they boiled water for instant coffee and oatmeal, a group of Hadza men walked into camp carrying huge pieces of a hooved animal over their shoulders. These men had heard the same lions that had woken Pontzer, but instead of going back to sleep, they had left camp in the dark, tracked the lions, and taken their prey, a practice known as meat pirating. “Nothing makes you feel less adequate as a man,” Pontzer recalls, “than sitting there eating your bowl of instant oatmeal while five Hadza guys come back with a freshly killed antelope that they stole from a pride of lions.”
This stark difference between Hadza and Western lifestyles was exactly what Pontzer and his colleagues were in Tanzania to study. The Hadza live in an environment close to the one in which modern humans evolved, and analyses of their DNA reveal that they are one of the oldest human lineages on earth. The Hadza are by no means walking fossils. They are as evolved as any human being you’d find anywhere on the planet. However, their culture has not changed at the same rapid rate as those of other societies. For the three hundred or so Hadza who still follow a hunter-gatherer lifestyle, their survival depends on strategies similar to those that early humans relied on. As one of Pontzer’s colleagues told me, if you want to understand what human life was like in the distant past, “This is as close as you can get.” And if you want to understand the type of physical activity that the human body and brain are adapted for, this is your best chance to see it in action.
The Hadza spend most of the day hunting and foraging. Men head out in the early morning, carrying handmade bows and poison-tipped arrows to stalk everything from small birds to baboons. (The first time Pontzer went on a hunt with two Hadza men, they tracked the blood trail of a single wounded warthog for hours.) Women spend the morning collecting berries and baobab fruit and digging starchy tubers out of the ground. They carry up to twenty pounds of food back to camp, then go out again in the afternoon. As part of Pontzer’s research project, his team gave nineteen Hadza men and twenty-seven Hadza women activity trackers and heart rate monitors, then recorded their dawn-to-dusk activity. On a typical day, the Hadza engage in two hours of moderate to vigorous activity, like running, and several more hours of light activity, like walking. There is no difference in activity level between men and women or between young and old. If anything, the Hadza become more active as they age. Contrast this to the United States, where the average adult engages in less than ten minutes of moderate to vigorous activity a day, and physical activity peaks at age six. If the Hadza lifestyle reflects what human bodies are adapted for, something has gone seriously awry for the rest of us.
It’s worth noting that the Hadza show no signs of the cardiovascular disease so prevalent in industrialized societies. Compared to age-matched Americans, the Hadza have lower blood pressure and healthier levels of cholesterol, triglycerides, and C-reactive protein, a measure of inflammation in the bloodstream that predicts future heart attacks. These signs of heart health are exactly what you’d expect to see in a population with high levels of physical activity. But Pontzer told me that he was even more personally struck by the apparent absence of two other modern epidemics among the Hadza: anxiety and depression. Whether this has anything to do with their active lifestyle is impossible to say, but hard not to speculate about. In the United States, daily physical activity—as captured by an accelerometer—is correlated with a sense of purpose in life. Real-time tracking also shows that people are happier during moments when they are physically active than when they are sedentary. And on days when people are more active than their usual, they report greater satisfaction with their lives.
Other experiments in the U.S. and the UK have forced moderately active adults to become sedentary for a period of time, only to watch their well-being wither. Regular exercisers who replace physical activity with a sedentary activity for two weeks become more anxious, tired, and hostile. When adults are randomly assigned to reduce their daily step count, 88 percent become more depressed. Within one week of becoming more sedentary, they report a 31 percent decline in life satisfaction. The average daily step count required to induce feelings of anxiety and depression and decrease satisfaction with life is 5,649. The typical American takes 4,774 steps per day. Across the globe, the average is 4,961.
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Humans weren’t always hunters and foragers. Two million years ago, a major climactic event cooled the Earth and changed the landscape of East Africa. Forested areas became more patchy and transformed into open woodlands and grasslands. As the habitat changed, so did the food supply, forcing early humans to travel far and wide to chase animals, scavenge for carcasses, and gather plants. Anthropologists believe this was a turning point in the evolution of our species—the moment natural selection began to favor physical traits that helped our ancestors run. The humans who survived were the ones whose bodies could endure the hunt.
Running doesn’t fossilize, but skeletons do, and the human fossil record clearly shows the appearance, over the past two million years, of anatomical adaptations that make running possible. Predecessors to modern humans were walking upright over four million years ago, but those hominins—who spent some of their time in trees—didn’t have the right feet for running. Theirs were flexible and curved, with long toes suited for clasping branches. Feet more like ours, stiffer and non-grasping, and better able to push off the earth, first show up in fossils dated to between one and two million years ago. This is around the time period you also start to see Homo erectus skeletons with thighbones 50 percent longer than earlier hominids, as well as wider shoulders and smaller forearms—all changes to the human form that support a more efficient running stride.
Leave the fossil record aside and you can observe many features in your own physique that help you run. Large gluteal muscles and longer Achilles tendons propel us forward. Compared to other primates, humans have more slow-twitch muscle fibers, which resist fatigue, and more mitochondria in running muscles, allowing them to consume more oxygen as fuel. We are also the only primate to have a nuchal ligament, the strip of connective tissue that fixes th
e base of the skull to the spine. This ligament—shared by other running species, such as wolves and horses—keeps your head from bobbing when you run. All of these adaptations suggest that we evolved as endurance athletes. Because the survival of early humans depended on traveling far and fast, you were born with bones, muscles, and joints that help you go the distance.
David Raichlen, an anthropologist at the University of Arizona, was familiar with the idea that natural selection favored traits that allowed humans to run. His own work in graduate school helped establish the theory, including a 2005 academic paper titled, “Why Is the Human Gluteus So Maximus?” But he was stymied by the problem of motivation. Nature can build a skeleton that makes running easier, but that alone is not enough to create an endurance athlete. What would make early humans willing to exert so much effort? If anything, humans seem predisposed to conserve energy. It’s a caloric risk to travel all day, using up your energy reserves in the hopes of catching something big. As Herman Pontzer puts it, hunting and gathering is “a high-stakes game in which the currency is calories and going bust means death.” Hunting and gathering all day can also be painful, tiring, and boring. Was an empty stomach sufficient to make a person persist on an all-day hunt or put up with the demands of foraging from dawn to dusk?
Raichlen is a recreational runner, and he began to think about the runner’s high. No one had ever come up with a good explanation for why it exists. What if the high wasn’t some random physiological by-product of running long distances, but nature’s reward for persisting? Was it possible that evolution had found a way to harness the brain’s feel-good chemicals to make endurance exercise rewarding? Maybe, Raichlen mused, early humans got high when they ran so that they wouldn’t starve. He reasoned that such a neuro-reward would have to do two things: relieve pain and induce pleasure. Scientists have long speculated that endorphins are behind the runner’s high, and studies show that high-intensity exercise causes an endorphin rush. But Raichlen had in mind another candidate, a class of brain chemicals called endocannabinoids. These are the same chemicals mimicked by cannabis, or marijuana. Endocannabinoids alleviate pain and boost mood, which fit Raichlen’s requirements for rewarding physical labor. And many of the effects of cannabis are consistent with descriptions of exercise-induced highs, including the sudden disappearance of worries or stress, a reduction in pain, the slowing of time, and a heightening of the senses.
Earlier research had hinted that exercise might trigger a release of these brain chemicals, but no one had ever documented it during running. So Raichlen put regular runners through treadmill workouts of differing intensities. Before and after each run, he drew blood to measure endocannabinoid levels. Walking slowly for thirty minutes had no effect. Nor did the most intense workout, running at maximum effort. Jogging, however, tripled the runners’ levels of endocannabinoids. Moreover, the elevation in endocannabinoids correlated with the runners’ self-reported high. Raichlen’s hunch was correct. The runner’s high is a buzz.
Why did jogging increase endocannabinoids, but walking slowly and running at an exhausting pace did not? Raichlen speculates that our brains reward us for exercising at intensities similar to those successfully used for hunting and foraging two million years ago. If that is true, then natural selection should also have rewarded other animals who hunt or scavenge in similar ways. Canines, for example, evolved to chase prey over large distances. Raichlen decided to put pet dogs on his treadmill, too, to see if they got a high. (Wolves would have made even better candidates for the study, but it’s easier to get dogs to cooperate.) As a comparison group, Raichlen recruited pet ferrets. Wild ferrets are nocturnal, hunting small mammals asleep in their burrows. They also forage for toads, bird eggs, and other food sources unlikely or unable to lead the ferrets in a wearying chase. Natural selection had no reason to reward ferrets for physical endurance—and apparently it didn’t. After thirty minutes of jogging, the dogs showed increased blood levels of endocannabinoids. The ferrets, despite trotting on the treadmill at an impressive speed of 1.9 miles per hour, did not.
What does all this mean for today’s recreational exerciser? For one thing, it suggests that the key to unlocking the runner’s high is not the physical action of running itself, but its continuous moderate intensity. And in fact scientists have documented a similar increase in endocannabinoids from cycling, walking on a treadmill at an incline, and outdoor hiking. If you want the high, you just have to put in the time and effort. Consider Julia, who was diagnosed twenty-two years ago with a rare genetic form of spinocerebellar ataxia, a progressive disease with symptoms that include balance problems, tremors, and muscle spasms. Julia is retired and lives alone, and one of the most important things in her life is maintaining the mobility she needs to babysit her grandchildren. So every morning she walks 500 meters (about a third of a mile) and climbs 140 stairs in her apartment building. Her family helped her calculate the distance and put together a playlist for her to listen to when she trains. The other residents in her building support Julia when they see her out; they refer warmly to her as being “on patrol.” These daily sessions challenge Julia enough to give her a high. As she explains it, “I must be getting a kick from it because I really enjoy it. . . . Is it adrenaline you get when you—the walkers, the marathon runners—I think I might be getting a bit of, is it heroin?”
Anything that keeps you moving and increases your heart rate is enough to trigger nature’s reward for not giving up. There’s no objective measure of performance you must achieve, no pace or distance you need to reach, that determines whether you experience an exercise-induced euphoria. You just have to do something that is moderately difficult for you and stick with it for at least twenty minutes. That’s because the runner’s high isn’t a running high. It’s a persistence high.
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If you were to see Jody Bender, a thirty-year-old human resources manager, on one of her frequent runs through her neighborhood park in Austin, Texas, one of the first things you might notice is her right leg. Unlike her left leg, it’s covered in tattoos. Across the front of her thigh, a black and white Pegasus stretches its wings. From her ankle to her knee, a muscular blue goat with curved horns and a golden mane stands in a field of red poppies. A lucky rabbit’s foot is inked near her right foot. The lopsided distribution of tattoos is not a coincidence. When Bender was twenty-three, a stroke left her unable to feel her right leg. She was at home, trying to relieve a sore neck with a heating pad, when she was overcome by the oddest sensation—like a snake was wriggling through the left side of her skull. When she stood up, she realized she couldn’t walk straight. It felt like she was on a sinking ship. She made it to the bathroom, became violently ill, crawled back to bed, and passed out.
Bender now knows that the snakelike sensation in her skull was blood seeping through her brain. She has a genetic condition, fibromuscular dysplasia, that leads to abnormally weak and easily damaged blood vessels. When she was stretching her neck, she ruptured an artery, causing a hemorrhagic stroke. In an MRI image taken one week later, you can see a white spot the size of a golf ball on the left side of her brain where blood had pooled. After the stroke, Bender was unable to feel her right leg and foot—it was if they had permanently fallen asleep. Her doctors were not sure if she would ever regain sensation. A year later, she was able to walk, but she would often trip and fall. She was on blood thinners to reduce the risk of a future stroke, and these drugs made any accident more risky. If she injured herself, her body would be unable to control the blood loss. She remembers tripping and falling outside her apartment one day while walking her dog. Lying on the sidewalk, her palm and knee bleeding from where she had broken her fall, Bender became determined to increase her steadiness and strength.
She started more intensive physical therapy, even though her doctors weren’t sure it would help. In her first session, the physical therapist put her on a balance machine painted to look like a mountain range
. As the platform she stood on rotated, Bender immediately fell off. Her physical therapist, a marathon runner, thought running on a treadmill would be good for developing her equilibrium. “I was like, ‘Are you crazy? I’m going to fall on my face,’” Bender recalls. But her physical therapist stood by her side so Bender couldn’t fall, and encouraged her to alternate walking and running for thirty seconds at a time. “It was hardly running, it was like a quick walking.” It took a month of physical therapy sessions to build up to running a mile. After two months, her physical therapist challenged her to run a 5K on the treadmill. In a photo from that session, Bender is smiling, her gaze straight ahead, her therapist cheering her on. “I was so surprised I could do it,” she told me. “I didn’t think I would ever get to that point.”
Before her stroke, Bender was decidedly not a runner. “I hated running. I don’t know if I had ever run a mile in my life. If I had to run for my life, I was probably in trouble.” Now she runs almost every day. She often takes her dog, Cujo, with her. (“He’s the sweetest dog,” Bender, who is a big horror movie fan, told me when I did at double take at the name. “He’s an excellent runner. He pushes me to run faster.”) She has a closet full of running shoes, and when she gets ready to go for a run, she always puts her left sock and shoe on first. She slides the left sock on by feel, then carefully tugs her right sock on by sight, trying to mimic how the left sock fits. She repeats this with her sneakers. The routine takes several minutes. It’s the only way she can tell if her right sock and shoe are on correctly. “The lack of feeling led me to get more blisters because I simply didn’t notice that side. I’ve run for miles with rocks in my right shoe, only to notice when I see blood on my feet afterwards.”
The Joy of Movement Page 2