Reading notes: Exercised
Humans evolved to be physically active by necessity. We had to search for and chase our food for many millennia. Only very recently (evolutionarily speaking) has our food come to us.
Evolving in a calorie-scarce environment meant that conservation of energy is key. When given the opportunity, humans prefer to sit around and do nothing. In the modern world, sedentary behaviour coupled with nearly unlimited access to calories leads to diseases such as obesity, diabetes, and hypertension. We now find ourselves in a situation where voluntary physical activity, exercise, is necessary for good health. Exercised by Daniel Lieberman explores the evolutionary anthropological basis for our need to exercise despite our most basic instincts.
Exercised is meticulously crafted. I found its style similar to Jared Diamond's Guns, Germs, and Steel, another book on human evolution and anthropology that I enjoyed many years ago now. Overall, I recommend the book to anyone who is interested in fitness, anthropology, or evolutionary biology. The ideal reader would be interested in all three (that includes me). What follows are a few of the main points that I took away from the book.
- Humans did not evolve to exercise voluntarily. We did evolve in an environment where our ancestors needed to undertake consistent levels of physical activity for 6-8 hours per day: walking, running, digging, carrying, etc.
- Our body systems are well suited for aerobic activities. We have extraordinary stamina on our feet; our two-legged stature and capacity to sweat (heat dissipation) allow us to outrun most four-legged species over long distances. Originally skeptical, Lieberman himself ran a marathon against 50 horses, of which he beat ~44 (don't quote me on the exact number).
- Studies on optimal diet and sleep patterns for humans are flawed at best and misleading at worst. The truth is, no one really knows how much sleep one should get (the "eight-hour rule" is not a rule). It's impossible to state that any diet is preferable over another, insofar as the comparative diets are relatively balanced and contain enough macronutrients to sustain our bodies (i.e. protein, fat, carbohydrates). Most likely, optimal diet and sleep patterns depend on the individual.
- When evaluating fad diets and similar health trends, beware of the "noble savage" fallacy. It's tempting to ascribe our modern diseases (physical and mental) to our modern way of life. In this view, reverting to the diets of our ancient ancestors should provide health benefits. There is some truth to this; avoiding modern processed foods that are extremely calorie dense (fat and sugar) should help prevent obesity and diabetes. However, ancient human diets were not optimized for health. Rather, they were optimized for simply not dying. Our ancestors ate what they could get. While they likely had excellent aerobic capacity and were generally free of heart disease, diabetes, and obesity, they weren't necessarily the pinnacle of the human form.
- Humans did not evolve to sit in chairs. However, the recent claims of the deadly effects of sitting for long periods at a time are likely overblown. Any negative health impacts attributed to sitting are more likely due to insufficient physical activity.
- Our modern invention of exercise fulfills our physiological need for physical activity. Aerobic activity and moderate weight lifting has enormous health benefits, far exceeding the gains from an optimal diet or sleep regimen. Like diet and sleep, we don't know the optimal amount of exercise for a given person. However, "more than nothing" is a good rule of thumb. It's very difficult to overtrain, unless you're doing a specialized activity that has a large effect on a single body part and no evolutionary basis (e.g. throwing a baseball).
- There exists a "fitness paradox": Our body makes endorphins (pleasure-inducing chemicals) when we undertake high-intensity exercise. This pathway reinforces the exercise habit pathway. However, one needs to be in relatively good shape (high cardiovascular fitness) to exercise at a high enough intensity to stimulate this reward pathway. That makes it hard to get off the couch, but once an exercise habit forms it can be quite sticky.
- The "fit grandparent hypothesis": Humans are a bit unusual in the animal kingdom in that we can live long, healthy lives after we pass child-bearing age. For many animal species, there is no reason to live long once an individual can no longer pass on their genes. Especially in social animals, there is selective pressure against slow, needy, elderly members of the herd. In human hunter-gatherer societies, many individuals live well into their 70s, still in good health and decades past the point of child rearing. Lieberman hypothesizes that these "fit grandparents" still played a crucial role in passing on their genes by providing calories for their children and grandchildren. The ability of a young hunter-gatherer woman to get pregnant is limited by her access to steady calories; if she is undernourished her body will not be fertile. If a grandparent can collect more food than they need, they can increase their daughter's chances of bearing a child, indirectly increasing their evolutionary success. To provide excess calories to their children and grandchildren, elderly humans needed to maintain high levels of fitness well into old age.
- While it's true that lifespan has greatly increased in our modern world -- mostly thanks to reduced child mortality and modern medicine -- our "healthspan" hasn't as much. If an ancient human survived to adulthood, there was a good chance of them living into their sixties or seventies. Not quite as long as people can expect to live today, but still a good, long life. Unlike modern people, ancient hunter-gatherers were typically in good health and physically fit until the end ("fit grandparents"). They had a "healthspan" that rivals modern humans. People today often lose a significant amount of aerobic fitness in their seventies and by their eighties, they begin to experience adverse health events related to low levels of physical activity. They are less mobile, need more care, and are generally in poorer health.