Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live (17 page)

Suppose that a new gene allowing better digestion of grains arises in a population. Such a gene would confer an advantage to its bearer and hence is expected to spread quickly, as the children of the cereal lover, and then their children, pass on the gene. As they do so, however, these individuals also inherit the DNA that was located on either side of the gene, which is less likely to have been broken up in the process of recombination, simply because strong selection on the grain-digesting gene has taken its neighbors along with it.

What this means is that scientists can scan the genomes of people living in different parts of the world who have historically eaten different diets, or the genomes of humans and our close relatives the apes, and look for signs of suspiciously large chunks of DNA that seem to have been passed along intact, as you would expect only if selection favored a gene and then its neighbors were carried along with it in the haste of evolution. After these chunks are identified, it’s possible to determine what the genes within the chunks can do: Are they associated with metabolizing protein? Do they detoxify plant compounds? Finding a large number of these changes means that our genes have changed relatively rapidly, in response to the new selection pressures caused by shifting diets.

Using these and similar techniques, Anna Di Rienzo at the University of Chicago and her colleagues found that humans show different genes associated with metabolizing food and accommodating the environment depending on where they live, or at least on where their ancestors came from.
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Polar people’s genes, for example, reflect an ability to deal with cold stress, and foragers appear to be adapted to a wider variety of foods than are the more specialized agricultural peoples. Liver enzymes associated with detoxifying the compounds found in roots and tubers are more prevalent in the nonpolar regions, where hunting is not the main source of food, as are genes associated with digesting cereals.

This branch of science, while attracting a great deal of attention, is still in its infancy; though researchers can identify and sequence stretches of DNA, assigning those sequences to a given function is not at all straightforward. And when many genes all contribute a small amount to a particular dietary ability, unlike the situation for lactase persistence or starch digestion, it is extremely challenging to chase them all down. Nonetheless, a strong body of evidence points to many changes in our genome since humans spread across the globe and developed agriculture, making it difficult at best to point to a single way of eating to which we were, and remain, best suited.

Sushi and dietary companions

The very newest discoveries about our dietary evolution may lie not in our own genes, but in those of the microbes we carry with us—in our guts, on our skin, and in the soil and air around us. When we digest food, we do not do it by ourselves, and I am not referring to having the family sit around the dinner table. We’ve all heard that some gut bacteria aid digestion, and perhaps that we can influence such bacteria by eating fermented foods like yogurt. But the real story is much more dramatic. The human cells of our bodies are outnumbered by microorganisms by a ratio of about ten of them to every single cell of ours. The microbes are part of our immune system and our sense organs, as well as our digestive tracts, and we could not survive without them. Pieces of their DNA lie intermingled with our own.

This microbial companionship might even have directed our dietary evolution. A recent study led by Jan-Hendrik Hehemann and his colleagues suggests a previously unconsidered mechanism for changing our genes under different dietary regimes.
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The Japanese eat rather more seaweed than do other groups, averaging just over 14 grams per person each day. Seaweeds in turn harbor microorganisms of their own, including some that can help break down some of the carbohydrates in the seaweed. Unlike larger and more complex organisms, microbes from rather different groups can transfer genes to each other; the two types don’t have sex exactly, but undergo a process called horizontal gene transfer. By this means, the genes from the seaweed microbes appear to have been transferred to the gut bacteria of people who tend to consume a lot of seaweed. The seaweed microbe genes thus were found in the intestinal flora of Japanese but not North Americans. It is clear that the Japanese did not get the genes simply by eating sushi, because one of the Japanese study subjects was a still-nursing infant who was not consuming solid food at all.

Other hints that intestinal microbes could affect the ability to consume new foods come from the finding that some people who do not have the lactase persistence gene can digest milk products; it is possible that they acquire microbes that do the job for them. Such gene transfer, and the interaction between us and our invisible companions, poses an extraordinary new avenue for rapid adaptation to the environment. Microbiologist Jeffrey Gordon says, “The gut microbial community can be viewed as a metabolic organ—an organ within an organ . . . It’s like bringing a set of utensils to a dinner party that the host does not have.”
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As our diets change, so does our internal menagerie, which in turn allows us to eat more and different kinds of foods. The caveman wouldn’t just find our modern cuisine foreign; the microbes inside of us, were he able to see them, would be at least as strange.

I realize that most people, whether or not they find the idea of genes hopping between the seaweed’s internal flora and our own of interest, really want to know what these findings about the recent evolution of our diets mean for what they eat. The answer is that no one, whether a low-carb enthusiast, a proponent of bacon fat, or a fan of organic food, can legitimately claim to have found the only “natural” diet for humans. We simply ate too many different foods in the past, and have adapted to too many new ones, to draw such a conclusion. If the genetic tools to characterize each individual’s diet-related genes are developed, we might be able to examine the suitability of particular diets for certain people. But that day has not yet come.

6

Exercising the Paleofantasy

W
hat we eat is obviously a defining feature, not just for humans but for other creatures; we refer to animals as carnivores or herbivores. But what do funny-looking, glove-like shoes have to do with humans having bigger brains, less body hair, and a greater ability to sweat than other mammals? According to at least some scientists, quite a lot. And according to at least some influential runners, less is more when it comes to footwear, with no or minimalist shoes said to result in a more natural gait and fewer injuries. What’s more, the way we move—in particular, the way we run—may be one of the most commonplace ways that humans show their unique evolutionary heritage. But are we best suited to run marathon-like distances, short sprints, or neither? And whether better distance runners or sprinters, were we necessarily faster, stronger, and fitter back—way back—in the day?

Are we being “seduced to sit”?

The “new age cavemen” profiled in the
New York Times
strive to emulate our ancestors not only in diet but also in exercise regimens.
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Some diligently follow a fitness program called CrossFit, which combines strength and cardiovascular training, including gymnastics, in a less structured manner than other workout routines; the founding principles include “Routine is the enemy” and “Keep workouts short and intense.”
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Although not explicitly attempting to mimic ancestral conditions or a paleo lifestyle, CrossFit touts itself as useful in “combat, survival, many sports, and life,”
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and it advocates against specializing in a particular sport or workout type. Typical routines contain pull-ups, push-ups, squats, and running, as well as more specialized exercises called “pistols” and “Samsons,” with the intensity and load varied for people of different abilities.

Other proponents of a more natural manner of activity include Arthur De Vany, whose blog is called
Evolutionary Fitness
, and who, in his mid-seventies, has a startlingly fit appearance that many half his age would envy. De Vany, like so many of the paleo followers, “begins with the premise that our bodies and minds are adapted to an ancient environment that passed more than 10,000 years ago,” and that “by understanding the hunter-gatherer adaptation and incorporating the activity and eating patterns of our ancestral lifeway . . . we can live a natural and healthy life.”
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De Vany emphasizes exercises that he feels emulate “the activities that were essential to the emergence and evolution of the human species. High intensity, intermittent and brief training mixed with power walking and play is closer than aerobic exercise, high volume weight training, or sedentism to how our ancestors lived.”
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Only with such behavior can humans be healthy and content, he claims. De Vany urges us to “embrace randomness and variety within the context of structured repetitiveness,” which to him means eschewing daily long-distance running, for example, in favor of irregularly performed high-intensity sessions of weight lifting.

This idea of “exercising like a hunter-gatherer” is echoed by Loren Cordain of paleo-diet fame. He and others suggest that modern-day humans should attempt to perform activities that had equivalents in the Stone Age.
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They provide a table outlining such parallel exercises, including “carrying a stacking rock,” which, for those of us not constructing rock walls, can be handily translated into “lifting weights,” while “gathering plant foods” is sensibly equated with “weeding a garden.” “Carrying a young child” is considered, not surprisingly, an enduring form of exercise then and now, although “splitting wood with an axe” is said to be the modern-day equivalent of “butchering a large animal,” which seems to me to substitute one difficult-to-replicate activity with another, at least for those of us in more urban settings. Like De Vany, James O’Keefe and company recommend alternating strenuous days with resting, to more closely replicate a life in which a group hunt, for example, would be followed by feasting and dance.
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The viewpoint that our lives would be better off if we moved as people did many thousands of years ago actually contains two different ideas, one of which is virtually unarguable, while the other is rather more controversial. The unarguable part is that many human beings today are relatively inactive, and this lack of activity is terrible for our health. Research about the detrimental effects of sedentary life on our health is accumulating from many quarters. Searching online for “sedentary lifestyle health risks” yields hundreds of thousands of hits, with couch potatoes suffering higher rates of mortality from various sources, as well as exhibiting more depression, anxiety, and other mental ailments.

The problem is more complex than merely a lack of exercise—whether exercise means hunting a gazelle, digging tubers out of rocky soil, or putting in forty-five minutes on the elliptical trainer; it’s that people are spending vast amounts of time sitting. According to researchers led by Neville Owen at the Cancer Prevention Research Centre in Brisbane, Australia, “
too much sitting
is distinct from
too little exercise
” (emphasis in the original).
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Recent studies suggest that even for people who manage to obtain the recommended amount of daily exercise each day, the deleterious effects of being sedentary can include larger waist circumference, higher blood pressure, and problems with blood sugar levels. The television appears to be a particular villain in this regard; the more time in front of the tube, the more drastic these adverse health effects were. The Australian researchers dubbed the conundrum of people who work out vigorously for a short period each day, but sit much of the remainder of the time, the “Active Couch Potato” phenomenon.

Active Couch Potatoes, or at least those from the population of white adults of European descent who were the objects of study, suffer higher-than-expected rates of mortality, even when some other potentially confounding factors, like the type of work they do, are taken into account. A survey of Canadians found that those who had reported spending most of the day seated had a higher risk of dying than did those who were even slightly more active. Although self-reporting is always a bit problematic for drawing conclusions, it is unlikely that the correlation arose out of a spurious association of, say, people who were simply too ill to move around being more likely to die. Among the Australians from the study that gave the Active Couch Potato its name, just a one-hour increase in the time spent watching television was connected to an 11 percent increased risk in mortality from all causes, and an 18 percent increase in dying from cardiovascular disease.

According to James Levine, a physician and researcher with the Mayo Clinic in Minnesota, such a sedentary existence is responsible for much of “the pandemic of obesity,” as he terms it, in many parts of the world.
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As a solution, he and his colleagues recommend increasing what they call NEAT (non-exercise activity thermogenesis), a fancy term for all of the things we do that expend energy but aren’t officially considered “exercise,” such as mowing the lawn, using the stairs, and walking to the kitchen. Such small actions add up through the day, burning what amounts to a significant number of calories as time goes on. Levine points to studies showing that highly active people—not athletes, just people who move around as part of their daily lives—have triple the energy output of inactive people. In addition, a sedentary person’s metabolism changes in ways that appear to favor weight gain and alter levels of blood components like triglycerides that are risk factors for heart disease. In a 2010 article, Levine champions a “science of sedentary behavior,” noting that we understand relatively little about either the ways to measure sedentary behavior (whether the physiological consequences of television watching differ in important ways from those of sitting in a car, for example) or the best interventions to take to reduce our tendency to be couch potatoes, whether active or not.
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Levine also “wonders whether exercise is a modern surrogate for the hunter-gatherer or agriculturalist life style,” and he points out that although a tendency toward obesity has a significant genetic basis, those genes cannot have changed all that drastically over the last century, when the obesity crisis has become apparent.
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Even an enthusiastic proponent of rapid evolution in humans and other animals such as me has to agree—it took thousands of years for lactase persistence to spread, not decades, and many more genes are involved in obesity. So if obesity is bad for your health and is influenced by genes, why weren’t those genes selected out of the human population eons ago?

The answer, according to Levine, is NEAT, or the absence thereof. Suppose that some people are programmed to be “NEAT conservers” and don’t move around much in situations when food is scarce, which would be adaptive if it enabled them to hang on to their fat stores until food became available again. Others are “NEAT activators” and increase their activity during famine, perhaps to seek food outside the usual hunting range. Until about a century or a century and a half ago, nearly everyone had to move around to live, using their bodies to do everything from washing clothes to visiting friends, so even though food was more readily available than it had been in previous millennia, most people burned off enough calories to avoid becoming overweight. With industrialization and the reduced need to expend energy, Levine argues, the NEAT activators still found a way to move around, whether by pacing, painting the walls of their kitchen, or going to the gym. But the NEAT conservers felt no such need, and, well, the rest is an ever-plumping history, at least for the last 100 years. “Over the last century,” Levine says, “environmental cues have been so overwhelming that ambulatory individuals have been seduced to sit.”
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This notion echoes the “thrifty genotype” explanation for the increased prevalence of diabetes in modern humans that was first promoted by anthropologist James Neel. Diabetes has become one of the major diseases of our time, costing the United States more than 15 percent of its total health care budget. Though diabetes occurs in people from all ethnicities and walks of life, it has risen particularly quickly among those of non-European descent, particularly in urban areas. While diet and exercise play an important role in when and even whether diabetes manifests itself in any particular case, genetics also plays a role, with people from those non-European backgrounds being more likely to develop the disease.

Those people, according to the thrifty-genotype hypothesis, have genes that efficiently utilize sugar and store fat during relatively infrequent times of abundance, which also enable them to survive during famine. The problem arises when ample food is always available; those same thrifty genes then promote hyperglycemia and problems with insulin regulation. People of European descent are less likely to have such genes because the boom-and-bust periods of food availability that selected for thrifty genes happened several hundreds of years ago, and people with a tendency toward diabetes were selected out. Furthermore, the frequency of famines decreased gradually. Among Pacific islanders and Native Americans, by contrast, particularly in urban areas, populations went from subsistence farming to convenience stores in a single generation, giving them little opportunity to adapt to continual plenty.

The basic idea in both cases—inactivity-induced obesity and overabundant food–triggered diabetes—is that behaviors adaptive in our ancestral conditions have become detrimental. Note that I am not advocating an exercise paleofantasy here; we are still not exactly like our Pleistocene ancestors, and evolution is still occurring. Just as it makes sense to eat foods that are not intensely calorie-rich all the time without promoting a diet of venison and tubers, it is reasonable to suggest that we are adapted to more movement without having to assume that humans are entirely unchanged since the Pleistocene. And Levine doesn’t advocate a slavish return to activities that mimic those of a hunter-gatherer; spending the weekend putting up shelves rather than watching televised sports is fine with him, without having to try and run down a rabbit for dinner. This approach makes more sense to me than the rather literal-minded proscriptions of De Vany and the other paleo devotees. I do suspect it is more likely that the variation Levine notes in NEAT patterns is continuous, rather than falling into two neatly labeled categories. Levine himself hints at this in some of his writings, but as he points out, we need to start looking at how people move in a different way before we can draw firm conclusions.

Levine does not see the same sharp distinction between pre- and postagricultural people that the paleo crowd does; to him, and to many of those concerned about the health consequences of being sedentary, the problem came much later in our history, when mechanized tools and modes of transportation made that seduction to sit ever easier. Working on a farm, or keeping house without a dishwasher, still uses more calories than does commuting to a cubicle, even if one is not hunting prey with a spear or piling up those rocks.

What’s more, the differences between mostly active and mostly sluggish do not apply solely to then versus now; they are just as stark in modern agricultural denizens versus city dwellers. For example, a 2011 study comparing rural and urban people in Jamaica and the United States found that the rural Jamaicans walked or ran 60 percent more than those living in the city of Kingston. When the US sample was divided into “lean” and “obese” categories, the latter were found to be spending nearly ten hours per day sitting, compared with only seven and a half hours for the urban Jamaicans and five and a half for the rural Jamaicans.
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(Movement was measured using an elaborate electronic system of underwear sensors, which in one case resulted in a Jamaican subject being “mistakenly arrested because of the wired nature” of the apparatus.
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One can only imagine the difficulty in explaining the study to the police.) Clearly, moving around more does not require dwelling in a cave and dressing in hides.