Exercise confers huge health benefits, so why does it often feel like such a chore? Evolutionary biologist Daniel Lieberman of Harvard explains the paradox. He specializes in research on human movement and endurance running and is a keen long-distance runner.
Anil Ananthaswamy: Why did you start to study the evolution of running and exercise?
Daniel Lieberman: I got interested in how we hold our heads still when we run. It began when my colleagues and I were doing some experiments with pigs as models. It is very uncomfortable to watch a pig run: Its head bobs all over the place. But animals that are good at running, like us, are extremely good at keeping the head still, because it is important for gaze stabilization. We started thinking about humans and chimps, and came up with hypotheses about how we evolved head stabilization to run.
AA: Why do you think head stabilization evolved for running, and not another form of movement?
DL: If you watch someone with a ponytail running, the ponytail bobs up and down. That's because of the pitching forces acting on the head. The head itself stays very stable. There are special mechanisms—the semicircular canals in human heads are greatly enlarged relative to apes, for instance—that give us a much greater ability to perceive and react to rapid accelerations of the head.
Walking does not create such accelerations. And I don't think our ancestors were jumping on trampolines or hitting each other on the head so much. The only explanation we can come up with is running.
AA: Being able to run is one thing—how did we then go on to become endurance athletes?
DL: We evolved from very non-active creatures. A typical chimp will walk 2 to 3 kilometers a day, run about 100 meters and climb a tree or two. Your average hunter-gatherer walks or runs 9 to 15 kilometers per day, and we have all these features in our bodies, literally from our heads down to our toes, that make us really good at long-distance walking and running.
I and my colleagues at the University of Utah, Dennis Bramble and David Carrier, think the key advantage for humans was persistence hunting, whereby you run very long distances to chase animals in the heat and run them into heat stroke. We can run for very long distances, marathons in fact, at speeds at which other animals have to gallop. That's not an endurance gait for quadrupeds, because they cool by panting—short shallow breaths. You can't pant and gallop at the same time. If you make an animal gallop in the heat for 15 minutes or so, on a hot day, you'll kill it.
AA: But we have adaptations for this kind of endurance running?
DL: Yes. Our bodies are loaded with all kinds of features: short toes that require less energy to stabilize and generate less shock when running; the Achilles tendon that stores and releases energy appropriately as we run; the large gluteus maximus muscles that steady the trunk; and stabilization of the head. I'm a middle-aged professor, I'm not a great specimen of an athlete, but I can easily run a marathon at a speed that would cause a dog my size to gallop.
AA: What's your best marathon time?
DL: [Laughs] 3 hours and 34 minutes. There are guys who can run almost twice as fast as me.
AA: Still, if you made an animal run that far at your speed, you would ...
DL: I'd have dinner.
AA: Why, in spite of our adaptations, have we gone from endurance athletes to couch potatoes?
DL: It was incredibly recently in history that a large number of humans have been freed from having to do physical activity. My argument, from an evolutionary perspective, would be that not having regular physical activity every day is pathological and abnormal. In a lot of medical studies, we compare people who are sick with controls. But who are those controls? They are relatively sedentary Westerners. I'd argue that we are comparing people who are sick to people who are abnormal and semi-pathological.
AA: If being inactive is pathological and abnormal, then how come we hate exercise so much?
DL: There was never any evolutionary selection pressure to make us like exercise. If you are a Neanderthal or Homo erectus or an early modern human, you didn't think, "Gee, I'm going to go for a run so that I'm not going to get depressed." They had to go long distances every day in order to survive. Not exercising was never an option, so there was never any selection pressure to make people like exercise. On the contrary, there was probably selection to help people avoid needless exercise when they could. Some hunter-gatherers had diets of about 2,200 calories a day. When your energy intake is that low, you can't afford to go for a jog just for fun.
AA: So evolution selected for traits that made us relax or be lazy?
DL: Of course. Just like anytime you crave sugary, fatty foods—that would have been advantageous for early humans. It's only now that they have become maladaptive.
When you walk into a train station and there is a staircase and an escalator, your brain always tells you to take the escalator. Given a choice between a piece of cake and a carrot, we always go for the cake. It's not in your best interest, but it's probably a very deeply rooted evolutionary instinct.
AA: What are the consequences of the modern sedentary lifestyle?
DL: It's hard to think of one disease that is not affected by physical activity. Take the two major killers: heart disease and cancer. The heart requires exercise to grow properly. Exercise increases the peripheral arteries and decreases your cholesterol levels; it decreases your risk of heart disease by at least half.
Breast cancers and many other reproductive tissue cancers also respond strongly to exercise. Other factors being constant, women who have engaged in regular vigorous exercise have significantly lower cancer rates than women who have not. Colon cancer has been shown to be reduced by up to 30 percent by exercise. There are also benefits for mental health—depression, anxiety, the list is incredibly long.
AA: What can we do about our maladaptive traits?
DL: If we want to practice preventive medicine, that means we have to eat foods that we might not prefer, and exercise when we don't want to. The only way to do that is through some form of socially acceptable coercion. There is a reason why we require good food and exercise in school—otherwise the kids won't get enough of it. Right now we are dropping those requirements around the world.
If we are going to solve these health problems, we have to push ourselves to act in our own self-interest. As a society, as a culture, we have to somehow agree that it's necessary or face the consequence—which is billions of unfit, overweight people.
AA: Has evolution given us any instincts that promote exercise?
DL: Yes. It's important to recognize that the body isn't adapted only in one way or another. There are multiple competing adaptations. While it's true that many of our instincts are to not like exercise, we also have other adaptations that make us enjoy exercise. The most obvious example is the runner's high.
AA: What's the evolutionary advantage of the runner's high?
DL: Imagine you are chasing an animal, and you have to keep going. When you are chasing, you are usually also tracking, which is all about observation. You are looking for clues in the environment. What does a runner's high do? It makes everything more intense. It stimulates your perception and your sensory awareness.
I can give you an example: I ran the London marathon a few years ago, and as I was nearing the finish I remember running by Big Ben and thinking, "Wow, Big Ben is really big." And then I remember thinking to myself, "Oh, I must be high."
This article originally appeared in New Scientist.