Born to Rest
Daniel Lieberman has spent much of his professional career exploring how natural selection shaped humans into one of the best endurance athletes on the planet. Now he is investigating how evolution simultaneously honed a propensity to laziness. “It is natural and normal to be physically lazy,” he writes in a recent paper, “Is Exercise Really Medicine? An Evolutionary Perspective.” Lieberman reconciles the apparent juxtaposition of evolutionary forces, showing how competing mandates to run or to rest affect human behavior, physiology, and health. He explains the deep origins of the obesity epidemic—40 percent of women, 35 percent of men, and 17 percent of children in the United States are obese—and by pinpointing the causes, points to a solution. In an interview, Lieberman even suggests how Harvard could make changes to support student well-being.
He has shown that human evolution has been a story of adaptations that enhance running ability: shorter toes and heel bones, and the ability to cool off through sweating, for example. He’s drawn attention to human prowess in persistence hunting, rarely practiced today, in which small bands of hunters chase animals until they collapse of heat exhaustion.
But Lieberman, Lerner professor of biological sciences, says that humans have also been selected to exercise only as much as they must to survive. The ancestors of modern humans lived as hunter-gatherers. In this subsistence lifestyle, food was often scarce, so resting was key to conserving energy for survival and reproduction. In other words, humans were born to run— but as little as possible.
“No hunter-gatherer goes out for a jog, just for the sake of it, I can tell you from personal experience,” says Lieberman. “They go out to forage, they go out to work, but anything else would be unwise, not to mention maladaptive” in calorie-restricted environments.
This tension between activity and rest, he says, plays out in human physiological and anatomical systems that “evolved to require stimuli from physical activity to adjust capacity to demand.” Muscles become bigger and more powerful with use, for example. With disuse, they atrophy. Bone deposition and repair mechanisms likewise require the presence of mechanical stimulation, such as running. The absence of such stimuli can eventually lead to a risk of osteoporosis. “In the circulatory system,” Lieberman continues, “vigorous activity stimulates expansion of peripheral circulation,” improves the heart’s ability to pump blood, “and increases arterial elasticity.” Without exercise, arteries stiffen, the heart pumps less blood, and metabolism slows.
All of this “downregulation” of biological systems evolved to conserve energy whenever possible. Muscles use about 25 percent of daily calories, so they are costly to maintain. Muscle wasting evolved as an adaptive mechanism that lowers energy consumption whenever physical activity is not required. Muscle wasting thus evolved as one among a range of adaptive mechanisms that lowered energy consumption whenever physical activity was not required. But at no prior point in human history was it feasible to lead an existence devoid of activity; exercise was literally part of the environment. The result is that mechanisms for reducing energy expenditure in the absence of physical activity now manifest as diseases. Heart disease, diabetes, osteoporosis, and other maladies of modern life are the consequence of adaptations that evolved as a means of trimming energy demand, and modern medicine is stuck with treating the symptoms. There is thus “no silver bullet,” says Lieberman, that will cure them all. And exercise itself stands against two million years of human evolution screaming, “Don’t do it!” That is why getting people to exercise is often so difficult.
Lieberman returns to the hunter-gatherer model in dissecting the motivations for activity, which he describes in terms of carrots and sticks. For a Kalahari bushman, who travels five to 10 miles a day over the course of four to six hours, the reward is food. But humans also “evolved with a very large stick: if you didn’t exercise, you had nothing to eat.” Exercise was mandatory. For many humans today, he points out, there are very few incentives and no penalties.
One way to fix that as a society, he reasons, is “to figure out ways to make activity more fun for more people, and the way to make it fun is to make it social.” Lieberman encourages investment in community sports, ranging from soccer teams to fun runs. He decries the fact that “we spend less than 5 percent of our national healthcare budget on prevention” when “more than 70 percent of all diseases are preventable.”
But are carrots enough? Within the University, Lieberman has an idea inspired by evolution, and best described as compelling, that “might start a firestorm.” Given its long history of research demonstrating massive reductions in rates of morbidity and mortality among alumni who exercise, he says, Harvard “needs to reopen a discussion about…a physical-education requirement.” Surveys of undergraduates by Harvard University Health Services (HUHS) have shown that very few students who are not athletes on a team get sufficient exercise, and that a quarter are sedentary. Furthermore, says HUHS director Paul Barreira, the same surveys show that students’ own sense of health and well-being tracks the amount of exercise they report getting. Those with the most depression and anxiety also get the least exercise. The happiest students get the most.
The Faculty of Arts and Sciences voted Harvard’s physical-education requirement “out of existence in the 1970s,” says Barreira. Any new requirement would have to respect students’ differing backgrounds, and those who have disabilities, Lieberman says. But “if ever there was a coercive environment it is a university,” he continues. “Faculty spend hours dreaming up new ways to compel students. We have all kinds of requirements in language, writing, math, and various mandatory courses within departments because we think they are beneficial for our students. Given the correlation between the mind and the body, how is a physical-education requirement any different?”
The most common critique Lieberman hears for this idea is that students don’t have the time to exercise. “But study after study shows that there is actually no trade-off in time because people who get more physical activity have better concentration, their memories are better, they focus better,” he adds. “So the time spent exercising is not time lost, but returned in spades. And not only in the short term, but also in the long term. Shouldn’t we care about the long-term mental and physical health of our students? Just giving people information is not enough. As a University—as a community—we need to ask ourselves whether or not we should help each other do what most of us want to do already: be more physically active.”