PDF Summary:Exercised, by Daniel E. Lieberman

We know exercising is important, but going for a jog can feel like fighting our most basic instincts. Why? According to evolutionary biologist Daniel Lieberman, it’s because humans evolved to conserve energy. When our ancestors weren’t busy doing necessary physical activities such as finding food, caring for children, or avoiding danger, they rested. Rest ensured they had enough energy for what mattered most: reproducing.

Today, most of us no longer hunt, grow food, or walk several miles to find water and shelter. But in Exercised, Lieberman argues that our bodies still expect physical activity. So, it’s important for us to exercise—to engage in voluntary physical activities. Otherwise, we become sick.

This guide explores Lieberman’s explanations for why our ancestors didn’t exercise and why modern humans do need to exercise. We also explore his advice on what kind of exercise you need and why, and how to motivate yourself to go for that jog. Our guide also offers tips for putting Lieberman’s insights into practice and shares perspectives from other health experts.

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Exercise Prevents Disease

The absence of regular physical activity allows disease and faster aging because our bodies aren’t getting the signals they evolved to expect to activate crucial repair processes. Lieberman argues that many chronic diseases, including those commonly associated with aging, result from a lack of physical activity. He characterizes these diseases as the results of evolutionary mismatches: Our bodies are poorly adapted to aspects of modern life like smoking, obesity, and sedentary lifestyles.

Lieberman supports this hypothesis with statistics showing how rare these conditions are among the elderly in modern-day hunter-gatherer societies. Exercise helps prevent and treat many modern diseases by activating the maintenance and repair mechanisms that evolved to respond to physical activity.

Your Ancestors Called and Said You Need to Go for a Swim

In Your Inner Fish, paleontologist Neil Shubin agrees that we can trace many of the ills and ailments we suffer to how our evolutionary history disagrees with our sedentary lifestyles. However, Shubin also believes that part of the problem is how complex our bodies evolved to be. For example, he explains that our arteries and veins evolved to have convoluted paths, making them rely on leg muscles to pump blood back up from the feet. The convoluted design of our cardiovascular system, plus our sedentary lifestyle, results in cardiovascular issues, like blood flow problems. When we don’t use our leg muscles enough, blood pools in our leg veins or around the rectum, creating varicose veins and hemorrhoids.

While Lieberman points to hunter-gatherers for evidence of an evolutionary mismatch, Shubin goes even further back. By tracing our body’s evolution back to the first limbed animal—a fish that walked on land—he argues that our evolutionary history can help us understand some of our human weaknesses. Some of our current features were designed for other purposes, like swimming or crawling. When you start with the body of a fish and turn it into a human mammal that walks on two legs, you get knee problems because limbs in fish were not originally developed to support walking upright on two legs.

Below, we’ll examine several conditions that Lieberman argues are the result of evolutionary mismatches. They’re virtually nonexistent in modern-day hunter-gatherers, but their numbers have increased exponentially in our modern societies. We’ll explain how exercise can help prevent or manage each condition.

Infectious Diseases

Lieberman argues that lack of exercise weakens our immune function and makes us more vulnerable to pathogens. By contrast, physical activity boosts our immune system by deploying immune cells to vulnerable areas like the respiratory tract and improving vaccine response. Moderate cardio exercise appears most beneficial, with studies showing that 45 minutes of walking five times a week can reduce respiratory tract infections.

(Shortform note: To enhance the positive effect of exercise on preventing respiratory tract infections, breathe through your nose while exercising. In The Oxygen Advantage, Peter McKeown explains that breathing through your nose strips harmful germs out of the air before they enter your body. Mouth breathing, on the other hand, causes you to lose moisture every time you take a breath, increasing the likelihood of general dehydration and a dry mouth. With excessive mouth breathing, the bacteria-killing compounds in your saliva dry out, allowing bacteria to infect your mouth and airways. This can cause respiratory infections.)

Metabolic Disorders

Exercise is essential for preventing metabolic disorders such as obesity, type 2 diabetes, and cardiovascular disease. When we eat more calories than we expend, excess calories are stored as fat, leading to enlarged fat cells that disrupt our metabolism. Lieberman outlines how obesity, poor diet, stress, and inactivity can lead to insulin resistance, where cells fail to respond to insulin—the hormone that helps our body turn sugar into energy. This results in high blood sugar levels that, over time, can damage vital organs like the heart and kidneys.

(Shortform note: Obesity is a modern problem, but it’s deeply intertwined with our evolutionary history. In 1962, anthropologist James Neel theorized that early humans had “thrifty genotypes” that stored food in the body as fat when food was plentiful, so it could be burned for energy later when food was scarce. Although most of us don’t face periods of famine today, our genes still tell our bodies to save fat—which leads to obesity. Neel’s theory might explain why we crave fatty foods: Eating these high-energy foods would have been an advantage in hunter-gatherer days.)

Lieberman explains that physical activity is crucial for maintaining a healthy weight and preventing the negative effects of obesity. In addition, regular exercise prevents diabetes and lowers blood pressure by improving your metabolism, including reducing organ fat and regulating blood sugar levels.

(Shortform note: While exercise helps maintain a healthy weight and prevent metabolic disorders, Lieberman doesn’t suggest it’s the only way to achieve these goals. That’s because a healthy diet is still a prerequisite for metabolic health. In The Obesity Code, Jason Fung explains that our metabolism falls out of balance when we eat the wrong foods and when we eat too frequently. Both of these activities increase insulin levels. To avoid metabolic disorders, follow Fung’s advice to avoid processed foods containing wheat and added sugars, industrial meat and dairy, trans fats, and refined vegetable oils. In addition, he argues you should avoid snacking and space your meals to allow your insulin to return to normal levels.)

Lieberman suggests spending at least 150 minutes a week on moderate-intensity exercise, emphasizing that a combination of cardiovascular and strength training is best. This approach not only helps prevent and manage diabetes but also lowers triglycerides and raises HDL (good cholesterol), significantly decreasing the risk of cardiovascular diseases.

(Shortform note: While moderate-intensity exercise improves cardiovascular health markers, it can be difficult to know what moderate-intensity means for you. One exercise expert suggests finding an activity that raises your heart rate and makes it challenging but still possible to engage in a conversation. According to a case study conducted by the same expert, an overweight patient was able to lower triglycerides and bad cholesterol, and raise good cholesterol, by walking briskly for 150 minutes per week for 11 weeks. The patient didn’t engage in strength training during this time. This suggests that if you have to choose between cardio and strength training, cardio should be your first choice for cardiovascular and, as Lieberman mentions, respiratory health.)

Musculoskeletal Disorders

Lieberman explains that exercise can prevent three major musculoskeletal conditions: sarcopenia (muscle loss), osteoporosis (weak bones), and osteoarthritis (degraded joints). Our muscles consume a lot of energy, so our body eliminates muscle cells if it suspects we don’t need them. Similarly, our bodies naturally deplete bone cells as we age. The best way to keep healthy muscle mass and bone density is to exercise our bones and muscles often with resistance training. Finally, exercise can also protect your joints by preventing excess body weight, which can inflame and degrade the cartilage.

Humans Evolved to Be Smart and Productive—Not Strong

Evolution can help explain why it’s so easy for the quality of our bones, joints, and muscles to degrade over time.

In the case of bones and joints, researchers traced the drop in tissue density to the reduction in physical activity that occurred when humans transitioned from hunting-gathering to agriculture. One study found that modern human skeletons have 50-75% less dense spongy trabecular bone in their joints compared to other primates and ancient human ancestors. Another study examined archaeological remains and found that forager groups had thicker, higher-volume trabecular bone in the hip joint compared to farmer groups.

In the case of muscles, we might have to go further back to understand why our muscles are prone to degrading. Some scientists speculate that human muscles evolved to be weaker to save energy for our brains. This allowed us to have bigger brains than our primate ancestors, but it made our muscles more susceptible to lifestyle changes. Primates are stronger than humans, even after weeks of little physical activity. However, humans need regular exercise to maintain their muscles because our body prioritizes sending energy to other parts of the body.

Oncological Disorders

Cancer involves cells that mutate and divide uncontrollably, competing for resources with healthy cells. Lieberman argues that physical activity helps prevent cancer through multiple mechanisms, such as regulating reproductive hormones that trigger mutations, reducing blood sugar availability to cancer cells, and enhancing immune function to suppress cancer cells. In addition, higher levels of exercise generally show a stronger impact.

(Shortform note: Not only does exercise help prevent cancer, but it also plays a significant role in recovery. Studies involving survivors of breast, colon, and prostate cancer show that patients who exercised after their diagnosis had lower mortality rates than those who stopped (or never started) exercising. Besides improving chances of survival, exercise can also alleviate the side effects of cancer treatments and help patients recover following chemotherapy, radiation, and surgery.)

Exercise Prevents Senescence

In addition to preventing illness, exercise significantly improves the probability of aging well—avoiding the health deterioration that often happens as we age. Lieberman cites evidence from modern-day hunter-gatherer societies to demonstrate that humans evolved to be physically active into old age, and avoiding a sedentary lifestyle helps us age well.

(Shortform note: In Lifespan, David Sinclair argues that exercise can greatly extend our lifespans, not just improve how we age. He says researchers have found that exercise lengthens and protects telomeres: small complexes of DNA and proteins at the ends of our chromosomes that get shorter each time a cell divides. When the telomere runs out, the cell stops dividing, leading to many of the problems of old age. Therefore, lengthening telomeres can literally keep our bodies younger for longer.)

We Evolved to Be Active Into Old Age

According to Lieberman, the Active Grandparent Hypothesis helps explain why physical activity is so effective at fighting senescence.

(Shortform note: In Lifespan, Sinclair expands on why senescence is problematic. Senescent cells aren’t able to perform their normal functions, but they also don’t die. Instead, they’re in a zombie-like state, clogging up previously healthy tissue with cells that can’t do their jobs. Furthermore, senescent cells can cause other cells to enter senescence, so the process only accelerates once it’s begun. Senescent cells send out chemicals that cause inflammation in surrounding tissue, which is associated with symptoms of aging.)

Compared to our primate relatives, we live much longer, way past our reproductive prime. According to the Active Grandparent Hypothesis, evolution favored longevity in humans because grandparents who were still around and active could help feed their grandchildren, ensuring the survival of their line.

Modern-day hunter-gatherer societies offer evidence to support this hypothesis because elderly members remain physically active well into their later years, continuing to contribute to their community’s survival. For example, among the Hadza of Tanzania, grandmothers often spend more time foraging than mothers do, and grandfathers travel similar distances as younger men to hunt and collect honey.

(Shortform note: In hunter-gatherer societies, elderly members’ active engagement doesn’t just benefit their physical health. This engagement also has social and cultural advantages. Older adults can pass on their wisdom, skills, and cultural traditions to younger generations, ensuring the continuity of cultural practices. Their engagement also fosters a sense of community and social cohesion, as all members are valued for their contributions and provided for as a result. For instance, researchers have found that Hadza foragers and hunters share their food with the rest of the community, including disabled or elderly people who are unable to help secure food.)

Unlike elderly members of hunter-gatherer societies, elderly people in industrialized societies typically become increasingly sedentary with age. This is problematic because the Active Grandparent Hypothesis means that grandparents—or elderly individuals—need to be active to benefit from the ways our bodies evolved to resist senescence.

Why is this? While humans were selected to live longer than most other species to help younger generations, this selection was only effective if the elderly remained physically active and productive. There was never evolutionary pressure to maintain health during a sedentary retirement. Instead, the optimal evolutionary strategy appears to be living long and actively, then dying relatively quickly when activity becomes impossible.

(Shortform note: Evolutionary pressure is the reason active grandparents were needed in the first place. In Sapiens, Yuval Noah Harari explains that, as humans started walking on two legs, women’s hips narrowed. The women who survived childbirth (and continued to pass on their genes) were the ones who gave birth early in the fetus’s gestation, when the baby’s head was smaller and undeveloped. The need to care for these vulnerable infants created unique social situations for humans, like relying on the community to help raise our children and shaping them through socialization to be whatever we want them to be. These situations and the resulting societal bonds may have contributed to the rise of human dominance in the animal kingdom.)

While elderly people in modern-day hunter-gatherer societies are more physically active than their peers in industrialized societies, hunter-gatherers still tend to have shorter life expectancies. Lieberman believes this suggests that physical activity compresses morbidity, allowing people to stay healthy for the time they’re alive. This is why hunter-gatherers typically remain healthy until shortly before their death. In contrast, elderly people in industrialized societies have an extension of morbidity: They live longer but experience long periods of disability and low quality of life before dying. According to Lieberman, the extension of morbidity is a consequence of lifestyle factors, such as a lack of exercise.

(Shortform note: Our modern approach to medicine might also be behind the extension of morbidity Lieberman describes. Sinclair, author of Lifespan, says that if we could prevent or reverse the effects of aging, then average life expectancy and quality of life would both skyrocket. But instead of preventing or reversing the effects of aging, modern medicine treats one symptom of aging at a time, then sends the patient away until the next problem arises. As we age, those problems become more frequent and more severe until the treatments can no longer keep up, the body fails, and we die.)

Lieberman notes that it’s never too late to benefit from increased physical activity and that the body’s repair mechanisms continue to respond to exercise even in old age. So, even if your body is already showing signs of senescence, starting to exercise today can make a difference.

(Shortform note: For older adults who haven’t exercised much, the notion that exercise is beneficial no matter how old you are is great news. That said, seniors may find it difficult to exercise regularly without injuring themselves, thereby doing more harm than good. To help avoid that problem, there are many exercise programs geared toward older adults, such as aquatic yoga classes.)

Evolutionary Answers for Modern Fitness Questions

We’ve discussed why exercise is healthy despite humans not having evolved to exercise. Now, we’ll explore Lieberman’s advice for your well-being based on his insights from evolutionary science and anthropology. This section will discuss his insights on motivating yourself to exercise, what kind of exercise to do, and how much to exercise.

How Can You Motivate Yourself to Exercise?

Lieberman explains that evolution made us resistant to unnecessary physical activity. He advocates making exercise fun, social, and necessary to overcome this resistance.

Make Exercise Fun and Social

Fortunately, exercise triggers reward systems in the body that help us find physical activity enjoyable. It releases chemicals like dopamine, serotonin, and endorphins that give us a rush of excitement and pleasure, and even endocannabinoids that trigger a meditative state—a “runner’s high.” However, these chemicals only start working once you’ve started working out, so they don’t offer motivation to get started. In addition, Lieberman points out that these reward systems work best in people who are already physically active, making it harder for sedentary individuals to find exercise rewarding during their first workout sessions.

(Shortform note: While a “runner’s high” might be tricky to achieve if you’re only starting to work out regularly, there are other emotional benefits you can achieve as an exercise novice. In Emotional Intelligence, Daniel Goleman says that exercise can help you fend off sadness, work through anger, and even lift mild depression. However, he cites research suggesting that the antidepressive benefit is heightened for those who don't work out regularly.)

To get through the initial workouts where the reward systems aren’t yet kicking in, make exercise more social. Lieberman notes that, throughout human evolution, physical activity like hunting and foraging was typically done in groups. He suggests that making exercise social can help overcome natural resistance to physical activity, as social support can make exercise feel more enjoyable. Making exercise social can look like signing up for a group dance class or joining a walkers’ club at your local park.

Make Exercise Necessary

If you need more than social support and the promise of endorphins, Lieberman suggests making exercise necessary through prods and pushes. Prods are gentle encouragements that guide you in the right direction. For example, you can sign up for the gym you walk by every morning on your way to class so you have a daily reminder to go in, or put your running shoes in the driver’s seat so they’re the first thing you see when you leave work.

You can also leverage the social aspect of exercise to make exercise necessary by designing pushes—forceful commitments with a social component. Examples of pushes include signing up for an office-wide weight loss challenge or asking your spouse to remind you to put a dollar in a jar every time you miss a workout.

To Motivate Yourself, Get to Know Yourself First

To know what kind of push or a prod to implement, you need to understand your personality. In Atomic Habits, James Clear argues that five key personality traits indicate which behaviors you’ll gravitate toward and be successful with.

1. Extroversion levels—from sociable to reclusive. If you’re extroverted, making exercise social, as Lieberman suggests, will be especially effective. However, if you’re introverted, it might make you less excited to exercise. If that’s you, find a prod that leverages other personality traits.

2. Openness to experience—from curious to cautious. If you’re curious, use a fitness tracker to track new activities you try or routes you explore. This will remind you to exercise and help you keep your exercise routine exciting. If you’re cautious, join a class that always meets at the same time. Knowing that others are expecting you to be there will be a helpful push.

3. Conscientiousness—from organized to spontaneous. If you’re organized, an effective prod might be a calendar that tracks how many workouts you do. If you’re spontaneous, keep a workout outfit in your bag, so you can easily work out when the mood strikes.

4. Agreeableness—from caring to withdrawn. If you’re caring, an effective push might be to work out with a loved one so you can help each other reach your fitness goals. If you’re withdrawn, invest in a home gym to work out without needing to interact with others.

5. Neuroticism—from fretful to assured. If you tend to fret, your prod might be to subscribe to health newsletters that discuss the importance of exercise. Each email will remind you to get a workout in your day. If you’re assured, sign up for a fitness challenge.

What Kind of Exercise Should You Do?

Lieberman emphasizes that a mix of different types of exercise provides the most comprehensive benefits, though the exact balance will vary by individual. He says that mix should include cardio, HIIT, and resistance training.

Moderate-Intensity Cardiovascular Exercise

Cardio should be the bread and butter of your exercise routine. It’s crucial for preventing illnesses and managing a healthy weight. While our ancestors went on hours-long walks and runs, you can aim for 10,000 daily steps, a brisk jog, or other aerobic activities you enjoy.

High-Intensity Interval Training

HIIT can improve endurance and enhance muscle performance. Since humans likely evolved to be capable of both sustained endurance and brief bursts of speed, HIIT satisfies our bodies’ need to perform a variety of moderately challenging physical tasks.

Resistance Training

Weight training prevents and even reverses muscle loss. Lieberman explains that resistance training creates microscopic damage that stimulates muscle growth, helping us build and maintain muscle mass. You can replace our ancestors’ resistance activities, like carrying water for miles, by using machines and free weights at the gym. Finally, instead of bulking up, Lieberman advocates maintaining moderate strength.

How to Exercise Strategically

While Lieberman argues that everyone should do some combination of cardio, HIIT, and resistance training, Tony Robbins (Awaken the Giant Within) suggests a different approach for planning your workouts. He echoes Lieberman’s recommendation to make cardio the foundation of your exercise routine. However, he argues you shouldn’t incorporate anaerobic exercise like weightlifting and HIIT until you’ve hit a plateau in your aerobic training.

Robbins argues that people tend to prioritize anaerobic exercise because they think that more intense workouts and higher heart rates, like those you experience during a HIIT session, create better results. However, intense workouts burn through so much glycogen that your body has to start using blood sugar as fuel. This is a problem because your nervous system consistently needs two-thirds of your blood sugar. So, when you start burning through it with too much anaerobic exercise, you can experience health problems, including fatigue, headaches, and repeated injuries.

You can avoid those health problems by building an aerobic base, which trains your body to burn fat as its primary fuel. To develop your aerobic system, start by doing only aerobic workouts like running, swimming, or biking for at least 30 minutes a day, three times a week, for two to eight months. This will condition your metabolism, which:

• Promotes fat burning

• Boosts your energy

• Supports your immune system

• Minimizes your injuries

• Improves your endurance

After building your endurance through an aerobic base, increase your power by adding anaerobic exercise. Anaerobic workouts increase your levels of human growth hormone (HGH), which:

• Improves muscle tone

• Increases lean mass

• Improves flexibility

Robbins recommends incorporating one to three anaerobic workouts a week, like weight training and HIIT, while also increasing the pace of your aerobic workouts.

How Much Should You Exercise?

Lieberman echoes the widely recommended guideline of doing 150 minutes of moderate exercise per week. This guideline comes from several studies demonstrating that more exercise correlates with lower mortality rates, with 150 minutes of moderate exercise or 75 minutes of more challenging exercise showing the same positive health effects. However, Lieberman notes that even small amounts of exercise provide significant benefits, with diminishing returns as the amount increases. So, if you can’t do 150 minutes of exercise per week, it’s still more beneficial to do a little exercise than none at all—and there’s no health-related reason to exercise much more than 150 minutes a week.

(Shortform note: Lieberman’s recommendations are in line with those of other scientists. According to the World Health Organization (WHO), adults aged 18 to 64 should get at least 150 minutes of moderate physical activity during the week. However, they believe 150 minutes is a healthy baseline and that it’s beneficial to aim for around 300 minutes of weekly exercise. However, John Ratey, author of Spark, echoes Lieberman’s sentiment that doing something is better than doing nothing. He urges readers not to get discouraged if they can’t reach or maintain their workout goals yet. He argues that the brain—and the body, as we’ve seen—is optimized for movement, so the key is to move.)