JANUARY 13, 2014
WE ALL SEEM TO BE prone to excess, even in discussions of how prone to excess we really are. There is no shortage of books, blogs and articles bemoaning our overweight, over-stressed and under-exercised society. It’s enough to make one think we are on the brink of apocalypse, with apparently our only consolation being that said apocalypse will be the most, well, apocalyptic imaginable.
The horrifying statistics are easy to come by. A 2008 paper in the aptly named journal Obesity predicted that within a decade and a half, virtually all Americans could be overweight or obese. Nearly one third of Americans have high blood pressure, according to the Centers for Disease Control and Prevention, which costs $47.5 billion annually in direct medical expenses and $3.5 billion each year in lost productivity. And a 2010 publication from Harvard Health frets that only one in six of us gets anywhere close to the recommended amount of exercise. On top of all that, rates of autism, autoimmune disorders, and food allergies are escalating, with the former alone costing us an estimated $137 billion per year.
How did we get into this mess? And is there anything we can do to stop the catastrophe from getting even worse? Again, everyone seems to have a solution, often contradicting someone else’s – we should all be taking supplements, we should all stop taking supplements, we should run barefoot, we shouldn’t run at all, we should eat like cavemen, we should eat like rabbits, we should eat like rabbits from the time of the cavemen. Behind many of these proscriptions is the implication that the human body simply isn’t “meant” to do the things we ask it to do every day, from driving a car to sleeping past dawn to eating food that comes from a factory.
Into the chaos steps Daniel Lieberman and his book The Story of the Human Body. Lieberman, an evolutionary anthropologist at Harvard and an expert on how humans run, among other things, provides a blessedly nuanced look at how evolution makes the body what it is. Note the present tense: he recognizes, as we all should, that evolution did not act long ago to produce our current minds and bodies as a finished product suited to a single point in time. Instead, evolution has never stopped, and what’s more, evolution has never given us perfection.
Rather than claiming to have found the secret to wellness or the latest trick to make our bodies shapelier and our minds less sluggish, Lieberman examines how the aforementioned mess came about via a combination of our evolutionary history and our culture. First, he points out, before we start waxing nostalgic for a prehistoric past in which obesity was unknown and we existed in harmony with the environment, it’s important to remember that evolution has never been about perfection. If something results in the production of more children, natural selection is all over it, regardless of whether it makes you feel miserable or look funny. As Lieberman puts it, “we didn’t necessarily evolve to be healthy.”
The Story of the Human Body is a story about evolutionary mismatches; these occur because the environment many of our body parts evolved in is no longer suited to the one we live in. Although we were never perfect, some environments let us get a bit closer to perfection than others. Human innovations like agriculture, with its ready supply of food, have been boons for us in some ways, enabling far more children to survive, but detrimental in others. Craving high-calorie or sweet foods makes sense in an environment where those foods are in short supply and obtaining them can mean the difference between starvation and survival, but their abundance is such a recent novelty that natural selection has not provided us with the ability to put a brake on our desires.
Mismatches have been discussed before, but Lieberman’s approach differs from the usual gloom and doom narrative in two important ways. First, he steers clear of proclaiming that these mismatches mean we made a terrible mistake, whether by adopting agriculture, living in cities, or by some other behavior along the way to our present. He notes that our evolutionary history has been filled with compromises ever since we became human. For example, our vocal apparatus is essential for clear speech, one of our uniquely human attributes, but it also means that the epiglottis is too low in the neck to contact the soft palate, which sharply increases our risk of choking on our food. One could bemoan this ill-designed bit of anatomy, but, well, one couldn’t do much bemoaning, at least not aloud, without it. And adopting some of the practices some associate with our downfall, like agriculture, simply aren’t necessarily all that bad—in fact, before the feudal system took hold, early farmers probably had far longer life expectancies, and had to work less, than some of the foraging people that came before them. As Lieberman puts it, “your distant ancestors who gave up hunting and gathering weren’t so crazy after all.”
Second, The Story of the Human Body is much broader in scope than similar volumes. Lieberman covers the obesity epidemic, but he does so with sophistication and sympathy, rather than the it’s-all-because-of-high-fructose-corn-syrup [or gluten, or fruit juice, or any other single food] attitude that seems to have become so prevalent. And there is more to modern life than being, or worrying about being, fat. He also looks – and this is one of the best features of the book – at how our bodies suffer not just from excess, but also from disuse, giving rise to conditions like osteoporosis or impacted wisdom teeth. The problem is that, as Lieberman observes, “growing up needs to be stressful.” Not psychologically (here he is mute), but physically. Bones that experience weight loads and the tiny continual deformities associated with responses to exercise will grow to an optimal size and strength, one that should sustain an adult into old age. Remove those cues, and the cells that make new bone, osteoblasts, cannot stand up to the osteoclasts, cells that remove old bone as it deteriorates. Hormones influence the activities of these cells, which is why postmenopausal women are most at risk for osteoporosis — a skeleton that hasn’t been prepared for the loss of estrogen by early stress on the bones is far more vulnerable than one that’s been exercising during bone growth. An additional but often-overlooked component of susceptibility to osteoporosis is the age at onset of puberty, which has been decreasing rapidly in Western societies. Until recently, girls had their first menstrual period at about age sixteen. Reaching sexual maturity earlier gives the body less time for bone growth, which exacerbates the effects of limited exercise. This is not to suggest that a childhood of unrelenting physical labor would be better for us; the key is enough stress to encourage bone growth but without so much repetitive motion that the skeleton buckles under the strain.
Along the same lines, wisdom teeth, and the trouble they cause, turn out not to be some kind of weird vestige from our days as large-jawed cavemen. Skulls from millennia ago do not show the same kind of dental deformities common in remains from the last few hundred years, apparently because the jaws and teeth were also getting their exercise, by chewing the hard, gristly foods that were commonplace before Cuisinarts and Magic Bullets macerated everything into smoothies. Chewing actually shapes the jaw and the position of the teeth, another case of some stress being useful for appropriate development. Of course, Lieberman is not advocating forcing children to eat only tough foods. However, he does suggest that chewing gum might help ameliorate the problem, though more investigation of the topic is needed.
In short, The Story of the Human Body is full of these intricate stories, showing how evolution has acted on our bodies in complex ways that defy one-liner explanations. The book spends relatively little time on an idea that Lieberman, along with other scientists, is probably best known for: the endurance running hypothesis, which suggests that humans are actually well-adapted for long distance running, marathon style. Being bipedal is usually associated with walking, but in fact our bodies may be running machines. Humans turn out to have a variety of musculoskeletal features that are not shared either with modern apes or our recent ancestors and that aid in running, but not walking. These range from skull and neck structures that stabilize the head during rapid movement to foot bones, including a stable longitudinal foot arch. The human head moves independently from the shoulder girdle, which doesn’t matter when one is walking but which helps keep the body upright when running.
According to the endurance-running-hypothesis, the point of all these characteristics wasn’t to run Stone Age marathons but to hunt prey. When they are first startled by a predator, most prey animals sprint away, relying on speed in order to escape. But they will tire after a relatively short period, and if they have not shaken off the predator, then the predator can start to close in. Furthermore, most prey animals will start to overheat after they have been running for a while, and will attempt to rest. If the predator can keep after the prey, and not overheat itself, eventually it can catch up to its quarry, not because it is faster, but because the prey animal has started to develop heat exhaustion. Humans are really good at sweating, which would have aided in the ability to keep going after other animals cannot.
This emphasis on running and how it evolved also means that moving while barefoot is not as injurious as it may initially seem. Early humans, as well as many people around the world today, didn’t run in shoes. Cave paintings do not show people who look like they have shin splints! People who run in highly structured shoes do not have a lower incidence of injury, either. But this is not to say that everyone could immediately hit the sidewalks barefoot. Feet, and our gait, are influenced by the environment, regardless of what that environment is. People who are accustomed to running barefoot tend to hit the ground with the forefoot, while those who run with shoes are more likely to heel strike. Changing the foot covering won’t make a difference if the gait doesn’t change as well. This underscores the continual interaction between our bodies and the world around us, whether that world is today’s world or one ten thousand years ago. In other words: if you put those adorable tiny shoes on a toddler’s feet, the shoes alter the growth pattern of those feet. If you don’t put shoes on them, the growth pattern is still altered – by the surface the child walks on, by the amount of time the child spends doing different activities, and so on. Anthropologist Greg Downey from Macquarie University in Australia points out that, in people whose arms or hand are disabled, feet can perform all kinds of tasks, including painting or typing on a keypad: “The fact that skills like foot painting or feeding oneself with one’s feet are rare does not mean that our feet were not ‘designed’ to do them.”
Not all of this appears in The Story of the Human Body, but the theme of the book is similar. What we have done, Lieberman argues, is create lives of great comfort and safety, through antibiotics and labor-saving devices, through purified water and artificial light. None of these are inherently bad, and antibiotics alone have doubtless saved many millions of lives. The problem, according to Lieberman, is that “we frequently mistake comfort for well-being,” leading to diseases of disuse, like osteoporosis, or diseases of novelty, like back pain from too much sitting in chairs. The key is not to return to the Stone Age, but to see how best to minimize our mismatches.