For tens of thousands of years the reality of human existence was discomfort. It is only in recent years—evolutionary speaking—that homo sapiens have been able to kick back and relax. In an excerpt from his new book, The Story of the Human Body: Evolution, Health, and Disease, evolutionary biologist Daniel Lieberman explains why this new phenomenon of being comfortable is hazardous to your health. In the late 1920s, two enterprising young men from Michigan held a contest to name the upholstered reclining chair they had invented. From the many submissions, they chose La-Z-Boy (other entries were Sit-N-Snooze and Slack-Back), and the company is still producing luxury chairs of the same name.Yet for the same price as some La-Z-Boy chairs, you could buy a round-trip airplane ticket to the Kalahari Desert, where you’ll be hard-pressed to find chairs, let alone ones with cushioning, reclining backs, and leg rests. But this doesn’t mean you won’t find anyone sitting. Hunter-gatherers and subsistence farmers work hard to obtain every calorie they eat.
When hardworking people with limited food have the chance, they sensibly sit or lie, which costs much less energy than standing. However, when they sit, they usually squat, or they rest on the ground with their legs folded or straight out.To those of us reading this book, sitting in a comfy chair is an utterly normal and pleasant activity, but an evolutionary perspective teaches us that this kind of sitting is unusual. But are chairs unhealthy? Should we throw out our mattresses and sleep like our ancestors on hard mats?For the record, I have no intention of getting rid of the chairs in my house. But there may be reasons to be concerned about the amount of time you spend in chairs, especially if you are inactive for the rest of the day. For every hour you sit at a desk, you spend about 20 fewer calories than if you were to stand, because you are no longer tensing muscles in your legs, back, and shoulder. Standing for eight hours a day adds up to 160 calories, the equivalent of a half- hour walk.
Over weeks and years, the energetic difference between mostly sitting and standing is staggering.In terms of muscle activity, sitting in a chair is not much different from lying in bed. It is commonly appreciated that prolonged bed rest has many deleterious effects on the body, including a weaker heart, muscle degeneration, bone loss, and elevated levels of tissue inflammation. Prolonged chair rest has almost the same effect because you also don’t use any leg muscles to support your weight, and if the chair has a backrest, a headrest, and armrests, you may not be using as many muscles in your upper body either. Muscles deteriorate in response to prolonged periods of inactivity by losing muscle fibers, especially the slow-twitch fibers that provide endurance. Months and years of sitting with poor posture in comfortable chairs combined with other sedentary habits therefore allow trunk and abdominal muscles to be weak and to fatigue rapidly.Muscle imbalances caused by hours of sitting in chairs have also been hypothesized to contribute to one of the most common health problems on the planet: lower back pain.
Depending on where you live and what you do, your chances of getting lower back pain are between 60 and 90 percent. The majority of lower back pain is diagnosed as “nonspecific,” a medical euphemism for problems whose causes are poorly understood. Despite decades of intense research, we remain woefully ineffective at diagnosing, preventing, and treating lower back pain. Many experts have therefore concluded that lower back pain is a nearly inevitable consequence of evolution’s unintelligent design of the human lumbar curve.But is this conclusion true? Today we have painkillers, heat pads, and other largely ineffective ways to alleviate back pain, but imagine how a serious back injury would have affected a Paleolithic hunter-gatherer. Even if our ancestors simply suffered through the pain, back troubles would surely have lessened their ability to do the many tasks that affect reproductive success. Natural selection is therefore likely to have selected for individuals whose backs were less susceptible to injury.
Selection to strengthen the spine may also explain why humans today tend to have five lumbar vertebrae, one fewer than early hominins. Perhaps the lumbar spine is a much better adapted structure than we realize. If so, then is the high incidence of lower back pain today an example of an evolutionary mismatch in which our bodies are not well adapted to the way we use them? Could it be that we are simply poorly adapted to inactivity?Comprehensive analyses of the incidence of back pain around the world consistently find that back pain is twice as high in developed versus less developed countries; further, within low-income countries, the incidence is roughly twice as high in urban versus rural areas. For example, lower back pain afflicts about 40 percent of farmers in rural Tibet but 68 percent of sewing machine operators in India. Neither of these populations lounges about in La-Z-Boys, but a general trend is that people who frequently carry heavy loads and do other “back-breaking” work get fewer back injuries than those who sit in chairs for hours bent over a machine.
From an evolutionary perspective, none of the populations so far studied use their backs in a normal way. No one has yet quantified the incidence of lower back pain among hunter-gatherers, but foragers rarely sit in chairs, they never sleep on soft mattresses, they often walk while carrying moderate loads, and they also dig, climb, prepare food, and run. They also don’t engage in hours of strenuous work such as hoeing or lifting that repetitively load the back. In other words, hunter-gatherers use their backs moderately— neither as intensively as subsistence farmers nor as minimally as sedentary office workers.Since people who are mostly sitters tend to have weak and inflexible backs, they are more likely to experience muscle strains, torn ligaments, stressed joints, bulging disks, and other causes of pain if and when they subject their backs to unusual, stressful movements. As predicted, people in developed countries who suffer from back pain tend to have a lower percentage of slow twitch fibers, which means that their backs fatigue more rapidly, and they also have lower core muscle strength, reduced flexibility in the hip and spine, and more abnormal patterns of motion.
At the other end of the spectrum are people whose livelihoods require lots of heavy lifting and other stressful activities that cause repetitive stress damage to the back’s muscles, bones, ligaments, disks, and nerves. For this reason, subsistence farmers in Tibet, who dig their fields and harvest crops for weeks on end, and furniture movers, who carry enormous loads, both suffer from back injuries, but their injuries have a different set of causes than those suffered by people who sit all day long hunched over computers or sewing machines.In short, there is probably a balance between how you use your back and how healthy your back is. A normal back is used with varying degrees of moderate intensity all day long, even during sleep. The adoption of agriculture was probably bad news for human backs. Now we face the opposite problem. Liberated from overstressing our backs, we suffer from weak and inflexible backs. The resulting scenario is all too common: for months or even years, you may be pain-free, but your back is weak.