Narrator: Imagine hauling a treadmill, a symbol of modern fitness, for nearly a day over treacherous roads to a remote Kenyan village named Pemja. This is precisely what Harvard paleoanthropologist Daniel Lieberman did. His goal was to study the walking efficiency of local women, who are subsistence farmers and carry heavy loads daily. But when the women stepped onto the machine, they walked awkwardly, hesitantly, their natural gait lost on the moving belt. In that moment of disconnect, the treadmill became a perfect, absurd metaphor for a revolutionary idea: humans never evolved to exercise. This puzzling scene is the starting point for Lieberman’s book, Exercised: The Science of Physical Activity, Rest and Health, which dismantles our most common beliefs about fitness by taking us on a journey through our own evolutionary history.

Narrator: The book's central argument is that our modern concept of "exercise"—voluntary physical activity performed for the sake of health—is a bizarre and recent invention. For nearly all of human history, physical activity was not a choice; it was a necessary and inseparable part of survival. Our ancestors were hunter-gatherers and farmers who spent hours walking, digging, carrying, and running, not to burn calories or improve their cardiovascular health, but to find food, water, and shelter.

Lieberman contrasts the grueling, purposeful endurance of the Tarahumara runners in Mexico with the modern spectacle of the Ironman triathlon. When he met an elderly Tarahumara man named Ernesto, he tried to explain why Americans run for fun. Ernesto was baffled, asking a simple but profound question: "Why would anyone run when they didn’t have to?" His confusion highlights the evolutionary logic hardwired into our brains. For an ancestor living on the edge of starvation, spending precious calories on non-essential movement would have been foolish, even dangerous. Our bodies are therefore finely tuned to ask, "Is this activity worth the energy?" This instinct to avoid unnecessary exertion isn't a sign of laziness or a moral failing; it’s an ancient survival strategy.

Narrator: Challenging the modern mantra to be constantly active, Lieberman argues that it is perfectly normal, and indeed evolutionarily sensible, to be inactive whenever possible. Our closest living relatives, chimpanzees and gorillas, spend the majority of their days resting, eating, and conserving energy. Likewise, studies of hunter-gatherer groups like the Hadza of Tanzania show that while they are significantly more active than the average Western office worker, they also spend many hours a day sitting and resting.

The instinct to conserve energy is a powerful biological drive. A stark illustration of this is the Minnesota Starvation Experiment, conducted during World War II. In the study, healthy male volunteers had their caloric intake cut in half for six months. Their bodies responded dramatically. Not only did they lose weight, but their metabolisms slowed down, and they became profoundly lethargic, obsessed with food, and averse to any physical effort. Their bodies were intelligently shutting down non-essential energy expenditures to survive. This reveals a fundamental truth: our bodies are designed to budget energy carefully, making our innate disinclination for a five-mile jog on a treadmill a perfectly logical, inherited trait.

Narrator: While we may not be the strongest or fastest animals, humans are exceptional in one key area: endurance. Lieberman dismantles the myth of the brawny, "primal" ancestor. Compared to chimpanzees, humans are relatively weak. And when it comes to speed, even the world's fastest sprinter, Usain Bolt, would be outrun by a white-tailed deer or a grizzly bear. Our bipedalism, while efficient for walking, limits our power and stability compared to four-legged animals.

However, humans evolved a suite of remarkable adaptations for endurance running. Features like long legs with springy tendons, a large gluteus maximus, and the nuchal ligament at the back of the neck (which helps stabilize the head while running) point to a history of running long distances. Lieberman illustrates this with the "Man Against Horse Race" in Arizona, an event where runners compete against horses over a grueling 25-mile mountain course. While horses are much faster sprinters, many human runners can outlast them over long distances, especially in the heat. This capacity for endurance likely evolved to help our ancestors practice persistence hunting—chasing animals for hours under the hot sun until the prey collapsed from heat exhaustion.

Narrator: Many of the chronic illnesses that plague modern society—such as type 2 diabetes, heart disease, osteoporosis, and certain cancers—are "mismatch diseases." They arise from the mismatch between our ancient genes, which are adapted for a physically active world, and our modern, sedentary environment. Inactivity prevents our bodies from activating the essential repair and maintenance systems that physical stress is meant to trigger.

The Stanford Runners Study provides powerful evidence for this. Beginning in 1984, researchers followed a group of runners and a control group of non-runners for over two decades. The results were clear: the runners lived longer, but more importantly, they experienced a "compression of morbidity." This means the period of disability and illness toward the end of their lives was significantly shorter than in the non-running group. By their 70s and 80s, the runners were not just living longer; they were living healthier. Physical activity, Lieberman concludes, is the best medicine we have to slow the process of aging and extend our "healthspan," not just our lifespan.

Narrator: Given our evolved aversion to discretionary exercise, how can we make ourselves move in a world that no longer requires it? Lieberman argues that the "just do it" mentality is unhelpful because it ignores our basic biology. Instead, we need to be more strategic and compassionate with ourselves. The key is to make physical activity both necessary and rewarding.

This can be achieved by making it social—exercising with friends, joining a team, or taking a dance class. It can also be achieved by making it a necessity. Lieberman points to the Swedish company Björn Borg, which implemented a mandatory "Sports Hour" for all employees every Friday. While some employees quit, the policy has largely been a success, improving health and building camaraderie. For the rest of us, the goal is to find a sustainable routine. The standard recommendation of 150 minutes of moderate activity per week is an excellent starting point, as the biggest health gains come from moving from a sedentary lifestyle to a moderately active one. Ultimately, the best exercise is the one you enjoy and will continue to do, whether it's walking, weightlifting, dancing, or gardening.

Narrator: The single most important takeaway from Exercised is that our struggle with exercise is not a personal failure but a biological inheritance. We are not designed to crave a workout; we are designed to conserve energy. This evolutionary perspective frees us from the guilt and shame so often associated with inactivity and allows us to approach physical activity with a new, more compassionate understanding.

The challenge, then, is not to simply force ourselves to exercise through sheer willpower. It is to intelligently redesign our environments and our routines to make movement a natural, necessary, and even joyful part of our modern lives. The question the book leaves us with is not if we should move, but how we can creatively bridge the gap between our ancient bodies and the comfortable, sedentary world we have built.