Read Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live Online
Authors: Marlene Zuk
Many species can survive quite well alongside humans, but even when they do, they may evolve new characteristics. We all know that a noisy highway makes it hard to sleep, but in addition, animals have a difficult time hearing each other when traffic and other noises of civilization are nearby. Hans Slabbekoorn at Leiden University in the Netherlands found that traffic noise means that songbirds such as European blackbirds and great tits don’t communicate as well. They also have fewer offspring in noisy environments. In some cases, the birds shift the pitch of their songs so that they are more audible in a world filled with honking trucks and screeching brakes, but not all species are able to accommodate such intrusions.
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We don’t know whether all of the changes that Slabbekoorn and his colleagues have documented represent genetic changes in the birds, and hence rapid evolution, but the opportunity for human-induced evolution is clear.
Perhaps the most recent major environmental shift that has caused rapid evolution in many kinds of plants and animals is global climate change. The environments of many species around the world have been getting warmer, and quickly. In turn, birds are breeding earlier in many parts of Europe, and pitcher plant mosquitoes have shifted the time of year when they emerge from dormancy to become adults. Yukon red squirrels can have litters earlier in the season because the spruce cones they eat are ready sooner in the spring. Scientists are documenting many such changes in the timing of life events, as well as in the shape, size, and color of individuals.
Some of those changes are simply on-the-spot responses to the current conditions, rather than inherited changes passed on to offspring, but others are truly evolutionary in nature, permanently altering the genes of the individuals that make up the population. Still other species, of course, have not changed in the face of rising temperatures or melting glaciers, and face the risk of extinction. The potential effects of climate change on the world’s animals and plants means that understanding rapid evolution, and the reasons why some species change while others do not, is more important than ever. At the same time, we should not use the evidence of rapid evolution to become complacent about the ability of animals to deal with human-caused disturbances; not every species is capable of shifting its schedules or acquiring larger beaks.
Finally, what about people? Where do humans fit on the list of species most likely to evolve quickly in the face of selection? It is ironic that we have induced rapid evolution in so many other species but, as I noted in the previous chapters, seem to doubt its occurrence in our own. Do we have what it takes? Criterion number one is strong natural selection, often via human-caused forces such as pollution or urbanization, but also through sieves such as epidemics, floods, and famines. We certainly qualify on those grounds.
Rapid evolvers also need the genes that will enable them to respond to such selection, and not all genes are equal in this regard. Biologists sometimes talk about “genetic architecture,” a phrase I’ve always found rather elegant. It means the way that the DNA itself translates to what we see, with some traits, like the chirpless crickets, relying on a single gene that changes a pathway during adult development, and others requiring a group of different genes in different places to all work together. In Chapters 4–6 we will explore how selection, together with the buttresses and scaffolding of our own genes, leads to rapid evolution in people.
P
eople are often passionate about dairy products. As I mentioned in the Introduction, my friend Oddný Ósk Sverrisdóttir and her colleagues are busy searching through the bones of long-dead Europeans for clues to the likelihood that milk was a part of their diet. And arguments about milk being a “natural” or even a healthy food abound. The website Notmilk.com, which, as the name suggests, is not a fan of the substance, intones, “Human milk is for human infants, dogs’ milk is for pups, cows’ milk is for calves, cats’ milk is for kittens, and so forth. Clearly, this is the way nature intends it.”
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It also contains instructions on how to “detox” from dairy, promising that after just one week of abstention, “one gallon of mucus will be expelled from your kidneys, spleen, pancreas, and other internal organs,”
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followed by an improvement in your sleep, mood, and sex life, though presumably the mucus needs to be dealt with before this rejuvenation can be realized.
This rather unappetizing image aside, and recognizing the considerable disagreement by medical experts with such an extreme view, it is undeniable that milk consumption is a novelty in human existence. Even today, the majority of people on Earth do not consume dairy foods after childhood. And that is just the way it should be, according to some readers of the
New York Times
Well
blog and the website Cavemanforum.com. One of the former stated:
Cheese is dairy. People aren’t designed to eat dairy. Baby cows are. Simply put, it’s a large reason people are obese and suffer western diseases like diabetes, heart disease, cancer and auto-immune diseases. There’s a HUGE body of research behind the evils of cow proteins. Cow milk (and the cheese made from it) is designed to make baby cows fat quickly. Guess what it does to humans?
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On the forum, one response to the question “What’s wrong with cheese?” was:
I don’t eat it because it’s not paleo, as Paleo Dude says. The basic idea behind paleo is that we are adapted to what we ate during the 2,000,000 years of the paleolithic, so those things are safe. Anything else is questionable.
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It is precisely this novelty that makes milk, or more accurately, the ability to digest it, the poster child for rapid evolution in humans. We understand more about how this ability came about, what it means at a genetic level, and what its consequences have been, than we do about virtually any other change in the human genome.
What’s more, the use of dairy as a food source is an illustration of how genes and human cultural activities can influence each other. This gene-culture interaction is also huge: it means not just that humans have evolved, and recently (impressive though that is) or that culture has changed through time (though that, too, occurs), but that both have been altered, one by the other, in a tight coevolutionary spiral that may be continuing right now.
Finally, understanding the evolution of lactose digestion requires the use of every tool that science possesses. We use computer models that treat people and their genes as bits of code in a hypothetical universe. We use the dusty particles of DNA that Sverrisdóttir and her coworkers gouge from bleached bones, as well as the juicier samples extracted from modern Finns and African nomads. We even use microscopic bits of butterfat scraped from shards of ceramic vessels that were used to store milk and cheese thousands of years ago. Who knows—maybe someone millennia from now will be delving into the discarded Häagen-Dazs cartons of our time to find out more about our genes.
The problem
Unlike some animals that specialize in only one or a few foods—for example, anteaters that eat just their eponymous insects or koalas that monotonously munch eucalyptus—human beings can eat and digest a wide variety of foods, both plant and animal. We cannot eat everything, of course; we lack the bacteria and other microorganisms necessary to break down the cellulose in grass and many other plants, which means that we cannot survive by grazing like cattle. But many other foods are fair game.
At first glance, milk would seem to pose little difficulty for omnivores like us. After all, milk, or the ability to produce it, is what defines us as mammals, along with mice, whales, goats, armadillos, and the rest of the variously furred or haired crew. Our group got its name back in 1758, when Carolus Linnaeus, the Swede famous for developing the double-barreled system of Latin names for each organism still in use today, decided to make “the female mammae the icon of that class,” as historian Londa Schiebinger puts it.
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Schiebinger points out the gender politics evident in Linnaeus’s choice, observing that no other group was distinguished by female reproductive parts, but be that as it may, milk is essential to the young mammal’s life. Lactation, the production of milk, provides mammalian offspring with nutrition and essential components of the immune system. Even the monotremes, those oddball mammalian egg layers like the echidna and platypus, rely on milk, with the youngsters lapping it up from specialized skin on their mothers’ bellies. We could, with equal justification, have been dubbed “lactimals.”
All good things must come to an end, however, and so it is with milk consumption, or at least so it is for all mammals other than humans. Each species has its own signature blend of components such as protein, fat, and calories geared to the growth schedule of the young animal consuming it. Cow’s milk is higher in protein but lower in fat than human milk, though it contains nearly the same number of calories. Whales and seals are famous for the high fat content of their milk, essential to the rapid growth of their young in cold environments. A colleague of mine who studied seals in the Arctic says that the milk in some species is so thick with butterfat that it resembles toothpaste squeezed out of a tube into the mouths of the hungry pups. Surprisingly, mouse milk is quite high in fat and calories, though not on a par with the milk of marine mammals; a cup of mouse milk, assuming you had the patience to obtain it, would contain over 400 calories, more than two and a half times the number in cow’s milk.
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Despite these differences, nearly all milk, regardless of the species from which it originates, contains a type of sugar called lactose. Digesting lactose—that is, breaking it down in the small intestine so that it can be used by the body—requires an enzyme called lactase. The ability to produce lactase is genetically controlled, and that ability is present, with extremely rare exceptions, in all mammals at birth. The lactase molecule spends its time tucked into the small intestine, able to encounter lactose as it tumbles by from dairy-rich meals. (A small percentage of babies are allergic to milk at birth, but this allergy is a reaction to the protein in milk, not its sugar, and it is distinct from lactose intolerance.) A funny thing happens on the way to adolescence, however: lactase production in all nonhuman mammals, and in most humans as well, grinds to a near halt sometime soon after weaning. Most adults possess only about 10 percent of an infant’s lactose-digesting ability. Why lactase stops being produced is an interesting question that has received, at least in my opinion, too little consideration. Saying “it’s not needed anymore” isn’t really satisfying, since of course organisms retain many characteristics that aren’t necessary, from human appendices to the tiny vestigial legs in whale skeletons.
Whatever the reason for lactase’s usual disappearance, consuming lactose after the enzyme is no longer active often has unpleasant gastrointestinal consequences. If it is not broken down, lactose passes into the large intestine, which contains the rich stew of microorganisms we rely on for help with digesting many of our foods. When these microbes encounter lactose, they cheerfully ferment it too, causing the production of methane and hydrogen gas. We always produce some of this gas, but in large amounts in the lower gastrointestinal tract, it, along with other by-products of the bacterial activity, causes bloating, abdominal cramps, and diarrhea.
Some drugs and parasites, such as the protozoan
Giardia
, may damage the intestine so that it no longer digests lactose, but the more usual cause of lactose intolerance is the absence of the enzyme. A common test for lactose intolerance measures the amount of hydrogen gas exhaled in the breath after consumption of milk products; in people who don’t produce lactase, the levels are higher. Even those without lactase can often consume small amounts of lactose, and fermented dairy products like cheese and yogurt are usually tolerated reasonably well, since the lactose has already been partially digested in such foods. But generally speaking, if you lack lactase, dairy products are not a desirable part of the menu.
The solution
Despite these difficulties, many people can continue to consume dairy products throughout their lives. Why is that? The Greeks and Romans noticed that adults differ in their ability to digest dairy products, but not until the last half of the twentieth century did people recognize patterns of lactose tolerance that suggested it was genetically based. In people who can tolerate milk as adults, the gene responsible for producing lactase continues to be active because of a mutation in another genetic region that ordinarily curtails the enzyme. By the 1970s, scientists had determined that lactase persistence is a dominant trait, meaning that only one copy of the gene controlling it, from just one parent, is needed for a child to exhibit the ability to digest dairy. (If it were a recessive trait, in contrast, a child would need to inherit two copies of the gene.) The exact nature of the molecules governing lactase persistence was identified in the early part of the twenty-first century, although work on the details of the alteration continues.
Initially, lactase persistence was assumed to be the “normal” state, with the lactose-intolerant an unfortunately flatulent few. But as more people around the world were surveyed, it became clear that the ability to digest dairy products in adulthood is prevalent in only certain parts of the world: northern Europe, particularly Scandinavia, and parts of Africa and the Middle East. About 35 percent of all the people in the world show lactase persistence, and they are clustered in only a few places.
The distribution in Africa is particularly interesting, because there, ethnic groups that live virtually side by side can have strikingly different proportions of individuals with lactase persistence. In the Sudan, for example, lactase persistence is seen in less than 30 percent of the Nilotic peoples but occurs in over 80 percent of the nomadic Beja. The Bedouin of the Middle East are also much more likely to be able to consume dairy than are their non-Bedouin counterparts in the same regions. What is responsible for the variation? Rapid evolution.
Cows first
To understand the evolution of lactase persistence, and the reason for the odd distribution of the genes that allow it, you first need to think about cows. And not just about the animals themselves, but about the relationship between humans and cattle or other milk-producing hoofed animals—a relationship that extends back at least 7,000 years.
Cattle were originally domesticated for the usefulness of their meat and hides, not their milk, and some cattle-keeping humans still make little or no use of dairy products in their diets, relying on the animals for meat and hides. If you want to obtain milk from cows for human consumption, you have to take the calves away from their mothers while the mothers are still producing milk, and then selectively breed the females that are best able to keep up that production, eventually creating cattle that differ from their ancestors in the genes that control lactation.
To even think of making use of cow’s milk, though, you have to have cows to begin with. And not all early peoples raised cattle. So what gave people the idea to raise cattle in the first place? Or, to put it another way, as Gabrielle Bloom and Paul Sherman of Cornell University asked, what kinds of environments make it too difficult to keep cattle or other ungulates year-round?
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Bloom and Sherman figured that in extremely hot or cold climates, or places where little food for the herds was found year-round, cattle would be too difficult to keep. More important, the prevalence of animal diseases such as anthrax and rinderpest in certain parts of the world can make it virtually impossible to successfully rear cattle and keep them healthy. Bloom and Sherman examined the relationship between where we see ancestral lactase persistence in the world and the geographic distribution of nine of these diseases, as well as the climate in which people with lactase persistence originated.
As they had predicted, people from parts of the world where the diseases had been prevalent were far less likely to exhibit lactase persistence, as were groups from extreme climate regimes, such as deserts or tropical rain forests. A few interesting anomalies emerged from this analysis: some groups of people live in areas that should be inhospitable to cattle, such as near-equatorial Africa or the Middle East, but nonetheless keep dairy herds and have a reasonably high rate of lactase persistence. Bloom and Sherman speculated that these people were able to overcome the barrier posed by the diseases by being nomadic, moving themselves and their cows around to avoid infection.
The cattle of today possess genes that are very different from those of their ancestors. Some of the change is a simple directional one; most modern domesticated dairy cattle produce far more milk over a much longer period, for example, than do either modern beef cattle or the forebears of either type. If you want more milk, you encourage early weaning; cows reared for meat are weaned late because the calves grow larger that way. Careful analysis of the teeth from cattle skulls dating back to the Neolithic reveals that the calves in such early herds were indeed taken from their mothers at a relatively young age, supporting the notion that cows were first domesticated for milk. In addition, where cattle are commonly kept, as in north-central Europe, the genes for different components of milk, such as the specific proteins, are quite variable. According to a research team led by Albano Beja-Pereira of France, this diversity of genetic types reflects the selective breeding of cows by early pastoralists, rather than new mutations arising in the different populations.
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