Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live (16 page)

Of course, the news isn’t all bad. Milton examined eighteen species of wild Panamanian fruits eaten by several monkeys, and found that they averaged 6.5 percent protein, compared with just over 5 percent protein for cultivated fruits from the grocery store.
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A similar analysis of fruits eaten by chimpanzees in Africa showed a protein content of over 10 percent.
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Wild fruits also are more apt to contain worms and other insect infestations, probably adding negligibly to the protein content but, according to Milton, potentially increasing vitamins, amino acids, and other micronutrients not usually available in the fruit itself. And some fruits are quite large, sweet, and juicy even in an unmodified form, particularly tropical varieties like relatives of the soursop.

The point is that most modern foods, whether processed or not, are a far cry from their wild ancestors, having been enormously modified by people seeking a more calorie-rich, more easily transportable, or simply tastier version of the original. Many of the paleo-diet enthusiasts realize this, and the discussion boards are full of debates on whether to therefore stick to less sweet fruits such as avocados (to which other posters respond by noting that early forms of avocado are basically a large pit surrounded by a thin layer of, well, avocado flesh, which doesn’t have much substance to it). The reality is that we are not eating what our ancestors ate, perhaps because we do not want to, but also because we can’t.

In addition to shunning grains and the foods made from them, paleo-diet followers are concerned about eating starchy vegetables and tubers, such as potatoes. Sweet potatoes seem marginally better than the rest, but differences of opinion abound. One blog contributor eschewed all forms of potatoes, concluding by saying, “To summarize, there’s 3 problems with tubers: a) Poisonous substances; b) Carb load; c) Problems we are yet to discover.”
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Arguably, this list contains either two problems or an infinite number, depending on your interpretation of point (c), but the interesting issue from my perspective is the first: the existence of toxins in tubers and many other wild foods.

Potatoes are from the same plant family as deadly nightshade, and the leaves and fruits contain several poisonous compounds, including solanine, which can cause digestive illnesses and, in large amounts, coma and death. Eating the potato itself avoids these substances, and one would have to consume quite a bit of the leaves or other parts of the plant containing these poisons to suffer ill effects. Nevertheless, the possibility of getting sick from one’s food source unless one knows exactly which part to eat raises an interesting question: How did people figure out that the ancestors of potatoes were edible to begin with? Some other plant foods, such as acorns, require substantial treatment before they become nontoxic; Native Americans leached the poisonous tannins from acorns with successive washes before grinding them into acorn meal.

How in the world did people figure out what to do to render such initially inedible, even dangerous, plant products into palatable food? One assumes that necessity was the mother of invention, but the details of discovery, and the missteps—perhaps deadly—along the way, remain unknown. George Armelagos, an anthropologist at Emory University, suggests that distinctive cuisines developed as a way of restricting the potential for eating poisonous or otherwise unsafe foods in the environment.
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That, too, however, underscores the flexibility of ancestral eating. We did not have a single diet, regardless of the relative compositions of fruits, nuts, tubers, and meat, throughout the period between the evolution of the genus
Homo
and the development of agriculture. Instead, people in different places ate different foods, modified them to differing degrees, and thrived on the variety.

The pot and the spit

Although the ability to digest the wide variety of carbohydrates available is more complicated than lactase persistence, involving more biochemical and physiological processes, in at least one respect humans have also evolved adaptations to eating starches, and relatively recently at that. Humans in different places have been eating different diets for thousands of years. The Japanese, for example, have been consuming starches in the form of rice for thousands of years, while people of the far north, such as the Yakut of Siberia, rely on hunting and fishing for sustenance.

A recent study by George Perry and colleagues asked the deceptively simple question of whether these two groups of people, and others like them, had evolved any adaptations to their different diets. As your mother may have told you, chewing food before swallowing it is important for proper digestion, in part because the enzyme amylase in saliva helps to break down starch. Salivary amylase also remains in the stomach and intestine after the food is swallowed, aiding the further digestion of starch by additional amylase present in the pancreas. Salivary digestion of starch, and therefore the presence of amylase, is crucial if an individual is suffering from diarrhea, since food is insufficiently digested in the intestine during such illnesses.

Perry and his coworkers looked for genetic differences in amylase genes from human populations with relatively low and high historical starch consumption: two African hunter-gatherer groups, an African pastoralist group, and the Yakut for the former; and European Americans, Hadza hunter-gatherers (who rely on starchy tubers) from Tanzania, and Japanese for the latter.
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The researchers acknowledge that the diets are not mutually exclusive in their ingredients, but the two categories still show substantial differences.

The scientists examined not the structure of the amylase gene itself, but the number of copies of it in the genomes of the two sets of populations. Like many genes, the amylase gene is prone to duplication, with multiple copies existing in some individuals but not others. A person inherits the number of copies from his or her parents—in other words, the duplication does not occur during each individual’s lifetime—but the likelihood that a duplication event will spread depends on whether or not it is advantageous. Gene duplication may have little or no effect, depending on what the gene does, but in the case of amylase, having more copies means that the individual is better able to digest starchy foods—an obvious advantage for people whose diet includes them. Perry predicted that the high-starch-consuming people would possess more copies of the amylase gene than the meat and fish eaters had. He was right; in the high-starch populations, 70 percent of the people had at least six copies of the amylase gene, but in the low-starch populations, that proportion dropped to just 37 percent.

Confirming the idea that the humans eating more starch evolved their higher number of copies of the amylase gene is the discovery that the levels of salivary amylase in chimpanzees are one-sixth to one-eighth what they are in humans. Chimpanzees eat a very low-starch diet, and geneticists Etienne Patin and Lluís Quintana-Murci from the Pasteur Institute in Paris suggested that a low copy number would have been found in our common ancestor, as well as in early humans.
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(A section of their paper is titled “Adjusting the Spit to the Pot,” which I stole as the heading for this section.) Then natural selection would have favored individuals with more copies as the amount of starch in the diet increased, so that the early agriculturalists could take advantage of the new foods. Cereal crops such as barley or rice probably were not domesticated until people had evolved more efficient starch digestion.

Interestingly, although chimpanzees and their close relatives the bonobos do not produce significant amounts of amylase, the Old World monkeys called cercopiths, a group that includes macaques and mangabeys, do. Perry and his colleagues speculate that the monkeys might use the enzyme to help them digest starchy foods, such as unripe fruit, that they store in their cheek pouches—a habit that only this group of monkeys possesses.
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Although we cannot go back in time to test early humans for the presence of amylase gene copies, an opportunity might exist, at least in theory, to do the next best thing. Until very recently, a small group of people in the remote mountains of northern Thailand and western Laos, the Mlabri, lived a nomadic hunter-gatherer lifestyle. But their language and culture, as well as the similarity between their genes and those of related peoples in the region, suggest an intriguing history. Hiroki Oota and colleagues believe that the Mlabri used to be agricultural but “reverted,” in their words, to foraging, perhaps because the founding group of people was simply too small to support planting and harvesting of crops.
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Depending on how recently the shift occurred (the data suggest anywhere between 500 and 1,000 years ago), it is possible that the Mlabri have a higher-than-expected number of amylase gene copies—a holdover from their farming past and a departure from other hunter-gatherer groups.

Agriculture’s bitter legacy

Which other human genes associated with diet may have changed since the development of agriculture? Lluís Quintana-Murci of the Pasteur Institute has also studied an enzyme called NAT2, which was first described because of its role in metabolizing drugs used in treating hypertension and tuberculosis, but which is also important in breaking down toxins from plants and cooked meat. The gene that codes for NAT2 comes in several forms, with one form more common among hunter-gatherers and the other more common in people whose ancestors were agriculturalists.

Quintana-Murci and his colleagues believe that the variation in NAT2 has to do with the availability of folate in the two types of populations.
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Folate is the naturally occurring form of folic acid, a substance known to many people because of its role in preventing miscarriage and birth defects, particularly spina bifida. Pregnant women are often encouraged to take folic acid supplements to ensure that their diets have enough of the compound. Folate is found in leafy greens or liver, neither of which are likely to be eaten in large amounts by agricultural peoples relying on grains for much of the diet.

NAT2 is also used to break down folate in the body, which means that having a gene variant that increases the rate of folate processing is a disadvantage if folate supplies in the diet are low; it’s like enlarging the holes of a sieve when the flow of water is decreasing. So it’s possible that natural selection favored people with less active forms of NAT2 after agriculture arose. At the same time, if folate is plentiful, metabolizing it more effectively could have helped reduce birth defects and the loss of pregnancies in hunter-gatherers. Support for this idea was found by a French research team led by Audrey Sabbagh, who surveyed the NAT2 variants of over 14,000 people from 128 different populations with a variety of diets. As expected if NAT2 is helpful when folate is abundant but harmful when it is scarce, foraging people were far more likely to possess the active form of the gene.
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Similar trade-offs between the advantages and disadvantages of genes associated with the diet are seen in the way we taste our foods. Humans have a remarkable sensitivity to bitterness—a useful ability, given the numerous bitter-tasting toxic compounds in wild plants—and several genes are responsible for the variation among people in their sensitivity to bitter flavors. Ordinarily, being more sensitive to bitterness is better, because it enables the taster to reject a potentially poisonous food source before eating too much of it. Among some people of central Africa, however, a gene variant that makes the bearer less able to taste bitterness is extremely common, despite the use of cassava, a starchy (and somewhat bitter-tasting) root, in that region. When some compounds in cassava are broken down in the gut, they release cyanide, which is obviously toxic in large quantities. If enough protein is present in the diet, however, people can tolerate small amounts of cyanide, so it is not automatically deadly. The less discerning gene variant may persist because it also renders those who carry it more resistant to malaria, which is endemic in the region. The distribution of the less sensitive variant mirrors that of the malaria parasite in Africa. Interestingly, some bitter-tasting plants in the region also have antimalarial activity, suggesting that humans with less sensitivity to the plants’ taste may be more willing to ingest them.

Dietary signatures

Although the method of analyzing a gene here and an enzyme there has its appeal, many researchers are now taking a broader approach to the question of recent diet-induced changes in our genes. Modern genetic tools allow scientists to survey the whole genome—our entire collection of genes—at once, or at least to compare large chunks of it among different populations with different diets or ancestry. These techniques can uncover more subtle genetic changes than those affecting lactase persistence or amylase—changes that are caused by tiny alterations in many genes acting together, rather than by a single dramatic shift.

The basic idea is simple. Genes occur together on the chromosomes like houses on a street, with some neighbors closer than others. Like other organisms, humans have chromosomes in pairs, with alternate forms of a gene on each member of the pair. When sperm and eggs are produced, they each have only one member of the pair, so that when they join to make a baby, the child has the normal number of chromosomes. But before the chromosome pairs unzip, they swap some of the genes at locations across from each other; to continue with the house analogy, it is as if one of the even-numbered houses on one side of the street were exchanged for one of the odd-numbered houses on the opposite side. This genetic recombination is why children produced by the same parent differ from one another, even though they each get half their DNA from their mother and half from their father. Usually genes are swapped in big chunks, with several sequences of DNA linked together. The closer any single piece of DNA is to another, the more likely the two will be recombined together, so—returning to the house analogy—houses on the same city block tend to stay together for longer than do houses on adjoining blocks.