What role does exercise play in shaping the health of our brains and bodies? For Lerner professor of biological sciences Daniel Lieberman, the answer lies in our distant past.
In his scholarship, Lieberman has explored how the arc of human natural history (see “Head to Toe” January-February 2011, page 24)—from our origins as hunter-gatherers to our current sedentary lifestyles—can explain the profound impact of physical activity on quality of life.
The 40-year-old banker in the checkout aisle at Trader Joe’s and the average man from the Mesolithic may share the same basic genetic blueprint, but live vastly different lives: not just in their experiences, but in their susceptibilities to certain types of disease. Although the banker is less likely to die of a simple infection, he may fall prey to heart disease—and other degenerative and chronic illnesses of which our hunter-gatherer ancestors remained largely free.
Lieberman argues that this history has left humans confronting a fundamental mismatch: we are creatures designed for a world that no longer exists. Modernity, with its Uber Eats and Amazon Fresh, ergonomic desk chairs and ubiquitous screens in rooms with perfect climate control, yields a degree of physical inactivity that our ancestors would not have known.
The Harvard Alumni Study: A Case for Movement
The notion that exercise extends life and improves its quality isn’t new. But how deeply is this connection rooted in human biology? One landmark investigation that Lieberman frequently cites is the Harvard Alumni Study, spearheaded by epidemiologist Ralph S. Paffenbarger in the mid-twentieth century. For two decades, Paffenbarger and his colleagues tracked more than 20,000 alumni, dissecting and analyzing their lifestyles while meticulously controlling for variables like smoking and chronic conditions.
The results were striking. Alumni who expended approximately 2,000 calories weekly during exercise experienced a 21 percent lower mortality rate in their early adult years than their sedentary counterparts. As the decades passed, this gap widened; by the time they were in their fifties, alumni who exercised a reasonable amount died at a 36 percent lower rate than those who were sedentary. By the time they were in their seventies and eighties, the death rates of the active cohort were more than 50 percent lower—an unmistakable, and statistically significant, effect (see “Exercise, a Changing Prescription,” March-April 2004, page 43).
The results also demonstrate that while aging is inevitable, how we age remains, to some extent, within our control.
Tracing the Evolutionary Roots of Physical Activity
To understand these large effects of exercise, says Lieberman, we must look into the origins of human physicality. The word “exercise” might conjure up images of weight rooms and bike paths, but for most of human history, physical exertion meant survival.
Hunting, foraging, and the sheer demands of staying alive ensured that our ancestors were in near-constant motion. But modern conveniences have created a culture and environment in which our bodies, shaped by millennia of toil, are now offered a lifestyle of relative ease.
The differences between humans and other primates underscore this shift. Humans diverged from the chimpanzees around seven million years ago, setting the former on a path toward a life of greater physical demand than their primarily arboreal cousins. “It’s important to understand that chimpanzees, like other apes and most primates, are basically couch potatoes,” says Lieberman (see “Born to Rest,” September-October 2016, page 9). “If you get a chance to watch chimpanzees for a few hours, you might want to bring a book—because they mostly spend their days sitting on their butts, eating and digesting.”
Thus, for the most part, chimpanzees don’t move like humans. This shift, he says, probably started with the origins of bipedalism, when the ancestors of Homo sapiens, like the famous australopithecine Lucy, began walking around trying to find food. As hunting and gathering developed, the fossil record demonstrates, early humans became bound by the need to run, search, and explore in order to survive.
Today, Lieberman says, even within a typical hunter-gatherer society with much more technology than humans had a few hundred (much less a million) years ago, individuals spend “over two hours a day in moderate to vigorous physical activity”—far more than the average office worker in any major city.
The Mismatch Hypothesis: Evolutionary Design in a Modern World
Lieberman suggests that the consequences extend beyond aesthetics or weight management. Although new medications like Ozempic and other GLP-1s have become heralded as panaceas to help reverse America’s obesity epidemic, these drugs can’t provide all the health-related benefits of exercise. There is no on- or off-brand corrective to compensate fully for what evolution designed.
Without regular physical strain, the body’s natural repair mechanisms lie dormant. Muscles that aren’t challenged grow weaker, and bones that aren’t stressed become less dense. Physical inactivity even impairs people’s ability to regulate oxidative stress, leading to increased cellular damage over time.
Exercise also plays a critical role in brain health. Physical activity induces the production of brain-derived neurotrophic factor (BDNF), a protein often referred to as “Miracle-Gro for the brain,” which promotes the growth and repair of neurons. Lieberman argues that the cognitive benefits of exercise may be an evolutionary vestige, reflecting the demands placed on the brain by complex tasks such as tracking prey or navigating landscapes. Today, we see its impact in mitigating age-related cognitive decline and in potentially delaying the onset of conditions like Alzheimer’s.
The evolutionary case for exercise becomes even more compelling given that humans are among the few species that live long after reproduction ceases. Unlike most animals, humans evolved to persist as grandparents: a role that, in hunter-gatherer societies, still entails significant physical exertion (see “Active Grandparenting, Costly Repair,” September-October 2020, page 28). In essence, being able to survive to become an actively involved grandparent ensures that an individual’s genes are protected.
Lieberman further discusses a study from Cowles professor of anthropology emeritus Peter Ellison, who examined women running about 20 kilometers around the Charles River weekly. He showed that during the second half of their menstrual cycle, the women in the exercise group had 50 percent lower levels of estrogen and progesterone than the sedentary women. Intuitively, it might seem as though running causes reproductive hormone levels to decrease. But, explains Lieberman, it’s the reverse: women in the study who weren’t exercising moderately have extra energy available, elevating their reproductive performance by half: an evolutionary adaptation that would have increased fecundity within a Paleolithic population where energy sources were limited.
This has especially important implications for modern humans: high levels of reproductive hormones are tied to elevated rates of breast cancer. According to Lieberman, women who get just moderate amounts of exercise lower their lifetime risks of breast cancer by 30 to 50 percent, because they have more evolutionarily regularized levels of progesterone and estrogen.
How Much Exercise Did Evolution Order?
The good news, says Lieberman, is that people don’t have to run marathons to benefit. Just 150 minutes of exercise a week can the risk of mortality by about 30 percent.
The human body, in this sense, is an ancestral tool that requires use, stimulation, and a little bit of strain to function optimally. By embracing the intrinsic need to move, people are not only adding years to their lives—they’re adding health to those years, enriching the quality of their time and recapturing an essential part of what it means to be human.
