Chapter 1

The Persistence High

The runner's high is often held up as a lure for reluctant exercisers, described in terms that strain credulity. In 1855, Scottish philosopher Alexander Bain described the pleasure of a fast walk or run as "a species of mechanical intoxication" that produces an exhilaration akin to the ancient ecstatic worship of Bacchus, the Roman god of wine. In his memoir Footnotes, cultural historian Vybarr Cregan-Reid also likens his highs to inebriation. "They are as strong as bootleg whisky. They make you want to stop everyone that you pass and tell them how beautiful they are, what a wonderful world this is, isn't it great to be alive?"Trail runner and triathlete Scott Dunlap sums up his running high this way: "I would equate it to two Red Bulls and vodka, three ibuprofen, plus a $50 winning Lotto ticket in your pocket."

While many runners favor comparisons to intoxicants, others liken the high to a spiritual experience. In The Runner's High, Dan Sturn describes tears streaming down his face during mile seven of his morning jog. "I flew closer and closer to the place mystics and shamans and acidheads all try to describe. Each moment became precious. I felt simultaneously all alone and completely connected." Still others draw parallels not to alcohol or religion, but to love. On a Reddit Forum dedicated to explaining what the runner's high feels like, one user posted, "I love what I'm doing and love everyone I see." Another offered, "It's like when you fancy someone and they tell you that they like you too."Ultrarunner Stephanie Case describes her midrun glow this way: "I feel connected to the people around me, the loved ones in my life, and I'm infinitely positive about the future."

While runners have a reputation for praising the exercise high, the side effect is not exclusive to running. A similar bliss can be found in any sustained physical activity, whether that's hiking, swimming, cycling, dancing, or yoga. However, the high emerges only after a significant effort. It seems to be the brain's way of rewarding you for working hard. Why does such a reward exist? And more important, why would it make you feel loving?

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.

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 the 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.