Our ancestors didn’t go to the gym, yet they were healthier than most of us today. The difference? Their daily survival demanded constant movement, while ours requires little more than a commute to a desk.
Exercise isn’t just about fitting into your jeans or running a faster mile. It triggers natural repair mechanisms your body desperately needs to prevent disease, maintain mental sharpness, and age gracefully. From strengthening your immune system to protecting your brain, physical activity addresses the fundamental disconnect between how people used to live and how we live now. Read on to discover why we need exercise in today’s world.
Editor’s note: This article is part of Shortform’s guide to habits. If you like what you read here, there’s plenty more to check out in the guide!
Table of Contents
Why Modern Humans Need Exercise
In Exercised, Daniel Lieberman argues that, although our ancestors didn’t exercise, modern humans should. That’s because our ancestors had to work much harder than we do for fewer calories as they foraged for, hunted, or grew their own food. The modern benefit of easy calories and easy living comes at a cost: exercise.
Surviving and reproducing was our ancestors’ full-time job. Their lifestyles included daily, moderate physical activity and securing just enough calories to survive and reproduce. However, as Lieberman notes, most modern humans lead lifestyles where daily moderate physical activity isn’t necessary for survival, and we have access to more calories than we need.
The Importance of Exercise for Disease Prevention
Although our lifestyles have changed dramatically, Lieberman says our bodies haven’t caught up. According to the Costly Repair Hypothesis, our bodies require physical activity to maintain health. When we exercise, our bodies release stress hormones, cells leak harmful chemicals that can damage DNA, and muscles develop microtears. The body’s response to this exercise-induced damage is comprehensive and beneficial. It includes lowering heart rate and stress hormones, mounting anti-inflammatory responses, and repairing DNA. These repair mechanisms often overcompensate for the damage, making the body healthier than before the stress of exercise.
The evolutionary logic behind the Costly Repair Hypothesis is that organisms with limited energy must carefully allocate resources to reproduction, movement, and body maintenance. Natural selection favored individuals who could efficiently match their repair capacity to the demands of physical activity—not invest too much or too little energy into repair. This system evolved to activate maintenance and repair mechanisms in response to physical activity, but not necessarily without it.
Lieberman writes that we can’t simply trigger these repair mechanisms without exercise. Our ancestors never experienced a lifestyle without regular physical activity, so we never evolved mechanisms to activate these repairs without the stress of exercise.
Lieberman contends that lack of physical activity causes chronic diseases because our bodies need exercise to trigger essential repair mechanisms developed through evolution. Modern sedentary lifestyles create “evolutionary mismatches”; our bodies aren’t adapted to inactivity.
According to Lieberman, regular exercise activates the body’s natural maintenance and repair systems, preventing and managing diseases that have become epidemic in modern sedentary societies. He identifies several categories of disease that were rare in modern hunter-gatherer societies but common in sedentary populations:
- Infectious diseases—Exercise strengthens immune function; 45 minutes of walking five times a week can reduce respiratory infections.
- Metabolic disorders (obesity, type 2 diabetes, cardiovascular disease)—Exercise maintains healthy weight, regulates blood sugar, and improves metabolism. Lieberman recommends 150 minutes of moderate exercise a week, combining cardio and strength training.
- Cancer—Physical activity helps prevent cancer by regulating hormones, reducing blood sugar available to cancer cells, and enhancing immune function.
- Musculoskeletal disorders (muscle loss, weak bones, joint degradation)—Resistance training maintains muscle mass and bone density, while exercise protects joints by preventing excess weight. In The Body, Bill Bryson elaborates on this point, noting that all your joints are lined with cartilage, which, unlike the rest of your bodily tissues, doesn’t have a blood supply. The only way your body can maintain the cartilage in your joints is by circulating the synovial fluid in your joints, and the only way you can make your synovial fluid circulate is by moving your joints. If the cartilage in your joints deteriorates too much, movement becomes painful and difficult, a condition known as osteoarthritis.
(Shortform note: Just as the cartilage in your joints needs movement to circulate synovial fluid, so does your lymph system. Lymph cells are part of your immune system. They identify and eliminate unwelcome cells, such as harmful bacteria and cells that have become cancerous. Instead of circulating through your body in your bloodstream, lymph cells are carried by lymph fluid through a separate circulatory system that relies on the movement of your body to move fluid through the system.)
Learn more about the ways physical activity prevents disease.
| Our Modern Bodies Need a Challenge It’s possible that what our bodies need to maintain health isn’t just physical exercise, but physical challenges. However, our modern lifestyles have largely removed those challenges from our lives by making our daily tasks, such as securing food, easier. According to Wim Hof, author of The Wim Hof Method, we’ve also made it too easy to stay warm, and this comes with health consequences, too. Hof explains that climate-control technology and modern clothing have made it so that you’re rarely cold anymore. This makes your circulatory system—which regulates your core body temperature—less efficient and more prone to illness. You can introduce some health-inducing challenge back into your life in the form of cold exposure. According to Hof, by repeatedly exposing yourself to cold, you strengthen your circulatory system. Your circulatory system muscles get stronger and better at protecting your core body temperature and delivering nutrition to your cells. And since all the muscles in your circulatory system are working well, your heart relaxes and pumps at a lower rate, making you less susceptible to heart disease. In addition, Hof argues that repeated cold exposure improves your ability to handle stress. The cold causes a physiological stress response that raises your heart rate and triggers the production of stress hormones such as cortisol. However, Lieberman believes you can’t trigger the body’s repair mechanisms without exercise. So, consider implementing the cold exposure Hof recommends alongside a workout—this could possibly enhance the body’s repair mechanisms. For example, a post-workout recovery routine that incorporates cold plunges—immersing yourself in cold water (50-60°F) for around 10 minutes—can reduce inflammation, stimulate DNA maintenance, and promote muscle and cell repair. This practice could mirror the evolutionary principle of the Costly Repair Hypothesis—using mild stress to make the body stronger than before. |
The Importance of Exercise for Staying Young
Lieberman argues that exercise is crucial for healthy aging because humans evolved to stay physically active throughout life. The Active Grandparent Hypothesis suggests evolution favored human longevity because active grandparents could help feed grandchildren, ensuring family survival.
Hunter-gatherer societies provide evidence for this hypothesis, as elderly members remain highly active well into their later years. For example, Hadza grandmothers often forage more than mothers, and grandfathers hunt and travel similar distances as younger men.
However, modern industrialized societies promote sedentary aging, which contradicts our evolutionary design. Humans evolved to stay healthy while physically active, not during inactive retirement. Lieberman writes that this lifestyle difference creates distinct patterns of morbidity between societies. Hunter-gatherers typically have shorter lifespans but remain healthy until shortly before death, experiencing compressed morbidity. In contrast, people in industrialized societies live longer but endure extended periods of disability and poor quality of life before dying, resulting in extended morbidity.
The good news is that the body’s repair mechanisms respond to physical activity even in old age, meaning you can still benefit from exercise regardless of your current health status.
(Shortform note: In Ikigai, Héctor García agrees that the sedentary lifestyle of modern societies hinders healthy and graceful aging. García cites numerous health reasons for avoiding too much inactivity, adding that sitting even for five minutes can drop good cholesterol levels. Thus, he encourages you to find simple ways to stay active and specifically recommends you incorporate gentle physical exercises that also promote mindful breathing into your daily routine. This includes exercises like radio taiso, yoga, tai chi, and qigong.)
The Importance of Exercise for Mental Wellness
Exercise offers a powerful alternative to traditional psychiatric treatment approaches. While medications have dominated mental health care since the 1980s, Harvard psychiatrist John Ratey argues in Spark that physical activity can be prescribed as an effective intervention for many psychiatric conditions—without the adverse effects associated with pharmaceutical approaches. This represents a significant shift in how we think about treating mental health disorders.
The key to exercise’s effectiveness lies in how it balances neurotransmitters in the brain. Physical activity stimulates the optimal release of serotonin (which regulates mood), norepinephrine (which influences attention), and dopamine (which drives motivation)—the same three chemicals targeted by most psychiatric medications. Exercise also acts as a mild stressor that builds resilience, strengthening the brain’s neural networks and helping it better manage future stressors. This inoculation effect comes from proteins like BDNF that grow and fortify brain connections, as well as from increased levels of galanin, a neuropeptide linked to stress resistance.
These mechanisms translate into practical benefits for specific mental health conditions. Exercise calms the overactive stress response underlying anxiety disorders, rebuilds the broken neural connections characteristic of depression, and retrains the dysregulated attention systems found in ADHD. For addiction, physical activity helps wean the brain off unnatural dopamine spikes by providing a balanced neurochemical reward. Ratey emphasizes that, while exercise shouldn’t replace medication in all cases, it offers a side-effect-free treatment option that increasingly more clinicians are recommending.
Learn more about the ways exercise benefits mental wellness.
The Importance of Exercise for Cognitive Acuity
In The Joy of Movement, Kelly McGonigal explains that, when you exercise, your muscles produce hormones called myokines, which circulate through your bloodstream and stimulate your brain (as well as other parts of your body). She asserts that myokines increase your cognitive performance and alleviate both physical pain and emotional depression.
John Ratey (Spark) was inspired to study the connection between exercise and the brain after reading about Chicago’s Naperville public high school. Their novel fitness-first approach to gym class focused on getting students moving through regular exercise. A direct result of this improved fitness was significant gains in student performance. In a 1999 standardized test comparing Naperville students’ science and math knowledge with that of students from different countries, Naperville finished first in the world in science and sixth in math. Ratey concluded that the school’s focus on physical health was in large part the reason for their academic success.
| Correlations Between Physical Activity and Academic Performance Ratey’s conclusion that Naperville’s gym class model improved academic performance is supported by recent evidence—indicating that Naperville wasn’t a fluke. In the time since the publication of Spark, national surveys by the Centers for Disease Control (CDC) have found significant correlations between physical activity and academic performance. A collection of ten intervention studies showed significant improvements in academic performance as well as other markers of healthy cognition such as attention and focus. In addition, students who regularly participated in physical activity reported higher self-esteem and confidence in their intellectual capabilities, factors that can lead directly to stronger academic achievement. In Brain Rules, John Medina explains why exercise boosts your memory and ability to focus and problem-solve: it increases blood flow to your brain. |
Ratey contends that exercise has survival value. When our ancestors ran down their prey in prehistory, their bodies and brains worked in concert. They not only had to sustain high levels of physical exertion, but they also had to read and respond to their environment quickly and accurately to survive.
Today, our brains still operate as our ancestors’ did. Ratey explains that the same neurological systems they used to hunt and gather are the ones we use to program computers, and our minds still function best in a mode of persistent activity. However, in our modern era we no longer face the same challenges our ancestors did. We may not share our ancestors’ reasons for moving, but we do share their need to move on a fundamental, biological level.
| The Supposed Evolutionary Origins of the Brain Ratey never outlines a complete theory of the evolutionary origins of the human brain. He instead gives a brief sketch of our ancestors’ presumed lifestyles to draw a correlation between exercise and the brain. In the years since the publication of Spark, a more complete theory of the causal link between exercise and brain function has emerged in neuroscience. According to the adaptive-capacity model (ACM) of the human brain, our brain adapted specifically to the complex foraging behavior of our ancestors. Searching for edible fruits, plants, fungi, and so on was cognitively demanding: It required complex physical coordination, decision-making skills, and memory. All this complicated behavior had to be done while running long distances, climbing, or otherwise being aerobically active. The evolving brain rose to the occasion by directing resources to areas of the brain that could support this behavior. The ACM holds that, if a brain is not using the complex structures it evolved to forage, it will adapt to these reduced demands and cut back on the resources it sends to those parts of the brain, leading to cognitive decline. Proponents of the ACM believe this explains why we see more cognitive decline in the brains of people who have a lower educational attainment, fewer social contacts, or live more sedentary lifestyles: These are aspects of a foraging lifestyle that would have impacted our ancestors’ survival, in addition to exercise. The ACM thus offers a more complete account of the relationship between the brain and exercise than Ratey’s brief sketch of our ancestors’ lifestyle does, and it gives researchers a framework for studying how our present lifestyles affect our brains. |
Explore Further
To learn more about why we need exercise, read Shortform’s full guides to these books:
- Exercised by Daniel Lieberman
- The Joy of Movement by Kelly McGonigal
- Spark by John Ratey
