Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live (18 page)

Even if our genes themselves have not changed in the last century, our prolonged periods of sitting might change the way those genes function in the body. Frank Booth and his colleagues at the University of Missouri in Columbia examined the physiological consequences of inactivity in both humans and rodents, with the latter serving as good experimental models because of their willingness to exercise on running wheels in cages. They concur with the idea that humans evolved to deal with a feast-famine cycle.
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When rodents are suddenly prevented from running, they begin to store fat, particularly visceral fat, and their insulin sensitivity, an indicator of diabetes risk, returns to its low pre-exercise levels in an astonishingly short time, merely days or weeks. Studies of athletes who stop training suggest similar patterns in humans.
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It is possible that the genes responsible for metabolizing glucose behave differently in the body depending on whether that body is active or sedentary. A couch potato body sends the wrong signals to the genes, which behave as if a famine were imminent, since inactivity is historically associated with not having any food out there to go dig up, hunt down, or gather. The changes in “gene expression” (the scientific term for the actual behavior of genes under a particular set of environmental circumstances) affect a range of body traits, from muscle size to the constitution of the blood vessels. Scientists feel that exercise deficiency plays an important role in the modern prevalence of many diseases, from hypertension and atherosclerosis to Alzheimer’s. Being still potentially sends too many inappropriate messages—“A crisis nears! Husband your resources for the short term, even if it means your body pays for it later!”—to a host of physiological systems.

Fitness and the snake in the library

The idea that being sedentary is bad for our health is one thing. It’s another to suggest, as De Vany, O’Keefe, Cordain, and others do, that our hunter-gatherer ancestry means that short-term and variable exercise is good, while sustained-endurance activity, such as that performed by marathon runners, is harmful. De Vany recommends power walking with weights, to simulate our ancestors lugging chunks of mastodon from the kill site back to camp. He also suggests that this routine is less likely to incur the “pounding and damage” of jogging, of which he takes a dim view. Setting exercise or weight goals is, he claims, counterproductive because they “do not relate to function and process.”
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De Vany also admiringly recounts a story about “Indian braves” who killed five bison by driving them into a pit reported to be over 10 feet deep, after which the hunters pulled the 2-ton animals out and butchered them; the article emphasizes the strength involved in hauling the bison whole out of the pit, though I personally would have thought that a sensible hunter, Stone Age or otherwise, would have thought to render them into manageable pieces before doing all that heavy lifting.

O’Keefe and his coauthors provide a list of “essential features of a hunter-gatherer fitness regimen,” which includes recommendations for exercising outdoors, including sexual activity for cardiovascular health. They are somewhat less negative about running than De Vany, but still state, “Humans in the wild were almost never walking or running on solid flat rock for miles on end.” They are somewhat more in favor of “relatively softer natural surfaces such as grass and dirt,” and also emphasize interval training, weight training, and alternating exercise days with rest days.
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The paleo online bulletin boards are rife with discussion of how “authentic” running really is, with shamefaced confessions by runners unwilling or unable to give up their sport. Some refer disparagingly to “chronic cardio,” as though it were a shameful medical condition. The followers are often particularly uneasy about the high-carbohydrate diets favored by marathon aficionados.

Both O’Keefe and colleagues and De Vany advocate group activities, with De Vany going so far as to note that “the evolutionary basis of sport seems clear. For example, the number of players in most popular team sports today is about equal to the number of prime age males that would be alive in a typical Paleolithic band of hunter gatherers.”
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This latter point, of course, is most charitably described as highly speculative, because both the number of “prime age males” in a given hunter-gatherer society and the team composition in many sports vary widely. Presumably De Vany was not thinking of the venerable sport of lacrosse, historical versions of which featured between 100 and 1,000 players.

Nassim Taleb, economist and author of the best-selling book
The Black Swan
, is also a fan of evolutionarily based exercise. He emphasizes “randomness” in movement,
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which blogger and Taleb fan Daniel Patrick Johnson describes as similar to “when cavemen would spend most of their time meandering around looking for food, and then every once in a while they’d have to sprint away from a prehistoric tiger or chisel out a tool from a heavy boulder.”
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They would not have been lifting those boulders, however; Taleb suggests that “our ancestors had to face most of the time very light stones to lift, mild stressors, and, once or twice a decade, encountered the need to lift a huge one.”
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Most anthropologists would hesitate to characterize the foraging behavior of modern hunter-gatherers as “meandering around,” and even De Vany proposes that foraging could have substituted for today’s aerobic exercise. I also couldn’t help wondering what these hypothetical early humans would have used to “chisel out” a tool in the first place, since they would have needed, well, a tool, presumably made from yet another rock, to do so—which leads to a sort of Paleolithic infinite regression. In addition, the more standard method of manufacturing stone tools by flaking relatively small rocks, while requiring a fair amount of skill, does not seem as if it would require much exertion. Taleb also feels that the motivation behind exercise is important, so, for example, the aforementioned sprints are better if one imagines being angry or frightened, as if one were fleeing a predator or chasing a rival; he frets, “The only thing currently missing from my life is the absence of panics, from, say, finding a gigantic snake in my library.”
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Born to run: Hunting, sweating, and hairlessness

Whether or not one views a paucity of large reptiles in the house as a problem, the ideas of Taleb and the other authors noted in the previous discussion are in direct opposition to a growing number of scientists and exercise fans who believe not only that running long distances is healthy, but that humans evolved doing just that.

To see why that might be the case, take a look at the human skeleton, preferably next to that of a chimpanzee, our closest living relative. Unlike the chimp, we are, of course, bipedal (though the chimp can assume that posture for short periods), and our bodies bear many signs of adaptations to walking on two legs: knees that can straighten out, a relatively flat “platform” foot, a curved lower spine, and well-developed gluteal muscles. When we walk, each leg acts like a pendulum, with energy stored in the body’s center of gravity rising up during a step and then shifting to the opposite leg. Because of its hip bone and pelvic structure, the chimp’s feet are farther apart, and when the chimp walks, its comparatively shorter legs don’t allow the same swinging gait that we humans exhibit. Humans are unique in keeping our legs relatively straight while we walk, yielding a characteristic bobbing motion that is not apparent in the movement of other bipedal animals.

Just how and why humans evolved the ability to move on two legs rather than four is still the subject of debate, with theories ranging from freeing the upper limbs for tool use or finding food in trees to better heat loss on the savanna. But as evolutionary biologist David Carrier pointed out in an influential 1984 article, when they think about the evolution of human locomotion, people usually think about walking, and rarely consider that we can also use those legs to run. To his mind, “The feature that differentiates hominids from other primates is not large brain size, but the set of characters associated with erect bipedal posture and a striding gait.”
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Humans are often considered to be poor runners, at least in comparison to quadrupeds like horses and dogs, and it is true that our sprinting abilities pale next to theirs. We can’t run nearly as fast as other animals (a greyhound, for example, can run 40 miles per hour, and the champion cheetah attains speeds of 70–75 miles per hour, compared with our paltry sprint times of 20-something miles per hour). And we burn more calories doing it; our “cost of transport,” a term from animal physiology that refers to the amount of oxygen consumed per unit of body mass for every unit of distance traveled, is up to twice that of other mammals our size. This means that human beings are not particularly efficient runners either.

And yet, if you could hold a marathon for all the world’s beasts, in a Noah’s Ark of long-distance races, somehow compensating for differences in body size, the human runners would be close to breaking the tape first, leaving all but a few other species in the dust, including all of our closest relatives, the primates. Wild dogs and hyenas can run for long periods, as can the migrating ungulates such as wildebeests, but they are the exceptions. Monkeys and apes just can’t compete when it comes to so-called endurance running. Even horses, well-known for their running feats, can falter when racing against humans; the horses need to rest more often, so a human runner can win if the race continues for a sufficient distance, particularly over uneven surfaces. On a more practical note, people from various parts of the world, both historically and today, have been able to hunt animals such as antelopes or kangaroos, simply by running after them until the prey collapse from exhaustion.

Carrier saw a conundrum: Why would people have evolved to do something that in many ways they are so bad at? Barefoot-running aficionado Christopher McDougall turns the question around, saying, “I never thought to ask, ‘Hang on—if running is bad for humans, why isn’t it bad for every animal?’”
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In other words, either humans are in fact adapted to running, or antelopes need Nikes.

The answer seems to be that humans are indeed poor sprinters, but that this lack is more than made up for by our skills at distance running. In particular, Carrier cites two problems faced by the long-distance runner: getting rid of enough of the heat produced by the body, and storing enough energy to fuel the run.
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The latter can be partially ameliorated with diet, but the former is more problematic. If you are a warm-blooded creature, vigorous exercise means that your metabolism generates quite a bit of heat, as any sweaty athlete knows. This heat production is why exercise warms you up on a cold day, but it also poses a risk of hyperthermia, or heat exhaustion, when the excess cannot be released quickly enough. That flash-on-the-savanna cheetah stores heat as it runs, raising its body temperature to dangerous levels in a relatively short time. Once that temperature exceeds 40.5°C, the cheetah won’t continue to run. If it did, it would sprint for only about one kilometer—interestingly enough, about the same distance that cheetahs pursue their prey.

Humans, of course, have far less stringent limitations. As anyone who has gone for a run with a dog knows, when humans get hot, we sweat, but when dogs get hot, they pant, along with most other mammals. (Horses and camels are among the exceptions.) Sweating and panting both work to cool the body by evaporating water from its surface, but sweating has two advantages that are germane to a running animal. First, panting uses only the surface of the mucous membranes such as those inside the mouth, while sweating can take place over a large area of the body. Second, panting interferes with breathing, while sweating does not—an important consideration during strenuous exercise. What’s more, being mostly hairless means that humans can off-load heat relatively rapidly while moving. As McDougall says in his book
Born to Run
, “humans, with our millions of sweat glands, are the best air-cooled engine that evolution has ever put on the market.”
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Human running has another quirk: it lacks an optimal speed—a single rate of movement at which the cost of transport is the lowest. Horses, for example, need to switch gaits from trot to gallop or back once they reach a particular speed, while humans can adjust how fast they run relatively seamlessly and have about the same cost of transport for each speed. What’s more, humans can alter their breathing during running to accommodate those different speeds, with, say, two steps per breath or three, depending on the need to oxygenate the lungs.

All of these characteristics led Carrier to propose an idea that was later further developed by Dennis Bramble of the University of Utah and Daniel Lieberman from Harvard University: the Endurance Running Hypothesis. To set the stage, Bramble and Lieberman list a variety of features of humans that are not shared with our recent ancestors and that aid 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 helps keep the body upright during running.
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The existence of these differences between our muscles and bones and those of our closest evolutionary ancestors is important. Suppose we had elastic-like tendons in our legs and feet, but chimpanzees and gorillas did too. In that case, it would be hard to make the argument that those structures evolved because of natural selection on humans in particular to be good at running, since an obvious alternative argument is that we have them because the ancestor of all apes and humans had them too. A trait that is unique to a species doesn’t always mean that it evolved through selection to perform a particular function, but it’s a hint in that direction.

On the right track?