Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live (2 page)

We can acknowledge that evolution is continuous, but still it seems hard to comprehend that this means each generation can differ infinitesimally from the one before, without a cosmic moment when a frog or a monkey looked down at itself, pronounced itself satisfied, and said, “Voilà, I am done.” Our bodies therefore reflect a continuously jury-rigged system with echoes of fish, of fruit fly, of lizard and mouse. Wanting to be more like our ancestors just means wanting more of the same set of compromises.

When was that utopia again?

Recognizing the continuity of evolution also makes clear the futility of selecting any particular time period for human harmony. Why would we be any more likely to feel out of sync than those who came before us? Did we really spend hundreds of thousands of years in stasis, perfectly adapted to our environments? When during the past did we attain this adaptation, and how did we know when to stop?

If they had known about evolution, would our cave-dwelling forebears have felt nostalgia for the days before they were bipedal, when life was good and the trees were a comfort zone? Scavenging prey from more formidable predators, similar to what modern hyenas do, is thought to have preceded, or at least accompanied, actual hunting in human history. Were, then, those early hunter-gatherers convinced that swiping a gazelle from the lion that caught it was superior to that newfangled business of running it down yourself? And why stop there? Why not long to be aquatic, since life arose in the sea? In some ways, our lungs are still ill suited to breathing air. For that matter, it might be nice to be unicellular: after all, cancer arises because our differentiated tissues run amok. Single cells don’t get cancer.

Even assuming we could agree on a time to hark back to, there is the sticky issue of exactly what such an ancestral nirvana was like. Do we follow the example of the modern hunter-gatherers living a subsistence existence in a few remaining parts of the world? What about the great apes, the animals that most closely resemble the ancestors we (and they) split off from millions of years ago? How much can we deduce from fossils? People were what anthropologists call “anatomically modern,” meaning that they looked more or less like us, by about 200,000 years ago, but it’s far less clear when “behaviorally modern” humans arose, or what exactly they did. So, trying to deduce the classic lifestyle from which we’ve now deviated is itself a bit of a gamble. In his book
Before the Dawn
, science writer Nicholas Wade points out, “It is tempting to suppose that our ancestors were just like us except where there is evidence to the contrary. This is a hazardous assumption.”
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You might argue that hunter-gatherers, or the cavemen of our paleofantasies, were better adapted to their environment simply because they spent many thousands of years in it—much longer than we’ve spent sitting in front of a computer or eating Mars bars. That’s true for some attributes, but not all. Continued selection in a stable environment, as might occur in the deep sea, can indeed cause ever more finely honed adaptations, as the same kinds of less successful individuals are weeded out of the population. But such rock-solid stability is rare in the world; the Pleistocene varied considerably in its climate over the course of thousands of years, and when people move around, even small shifts in the habitat in which they live, going from warm to cool, from savanna to forest, can pose substantially new evolutionary challenges. Even in perfectly stable environments, trade-offs persist; you can’t give birth to large-brained infants and also walk on two legs trouble-free, no matter how long you try.

Incidentally, it’s important to dispel the myth that modern humans are operating in a completely new environment because we only recently began to live as long as we do now, whereas our ancestors, or the average hunter-gatherer, lived only until thirty or forty, and hence never had to experience age-related diseases. While it is absolutely true that the average life span of a human being has increased enormously over just the last few centuries, this does not mean that thousands of years ago people were hale and hearty until thirty-five and then suddenly dropped dead.

An average life expectancy is just that—an average of all the ages that the people in the population attain before they die. A life expectancy of less than forty can occur without a single individual dying at or even near that age if, for example, childhood mortality from diseases such as measles or malaria is high—a common pattern in developing countries. Suppose you have a village of 100 people. If half of them die at age five, perhaps from such childhood ailments, twenty die at age sixty, and the remaining thirty die at seventy-five, the average life span in the society is thirty-seven, but not a single person actually reached the age of thirty hale and hearty and then suddenly began to senesce. The same pattern writ large is what makes the life expectancy in developing countries so shockingly low. It isn’t that people in sub-Saharan Africa or ancient Rome never experienced old age; it’s that few of them survived their childhood diseases. Average life expectancy is not the same thing as the age at which most people die. Old age is not a recent invention, but its commonness is.

The pace of change

If we do not look to a mythical past utopia for clues to a way forward, what next? The answer is that we start asking different questions. Instead of bemoaning our unsuitability to modern life, we can wonder why some traits evolve quickly and some slowly. How do we know what we do about the rate at which evolution occurs? If lactose tolerance can become established in a population over just a handful of generations, what about an ability to digest and thrive on refined grains, the bugaboo of the paleo diet? Breakthroughs in genomics (the study of the entire set of genes in an organism) and other genetic technologies now allow us to determine how quickly individual genes and gene blocks have been altered in response to natural selection. Evidence is mounting that numerous human genes have changed over just the last few thousand years—a blink of an eye, evolutionarily speaking—while others are the same as they have been for millions of years, relatively unchanged from the form we share with ancestors as distant as worms and yeast. The pages to come will explore which genes and traits have changed, which have not, how we know, and why it matters.

What’s more, a new field called experimental evolution is showing us that sometimes evolution occurs before our eyes, with rapid adaptations happening in 100, 50, or even a dozen or fewer generations. Depending on the life span of the organism, that could mean less than a year, or perhaps a quarter century. It is most easily demonstrated in the laboratory, but increasingly, now that we know what to look for, we are seeing it in the wild. And although humans are evolving all the time, it is often easier to see the process in other kinds of organisms. Humans are not the only species whose environment has changed dramatically over the last few hundred years, or even the last few decades. Some of the work my students and I have been doing on crickets found in the Hawaiian Islands and in the rest of the Pacific shows that a completely new trait, a wing mutation that renders males silent, spread in just five years, fewer than twenty generations.
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It is the equivalent of humans becoming involuntarily mute during the time between the publication of the Gutenberg Bible and
On the Origin of Species
. This and similar research on animals is shedding light on which traits are likely to evolve quickly and under what circumstances, because we can test our ideas in real time under controlled conditions.

Over the last decade, our understanding of such rapid evolution, also called “evolution in ecological timescales,” has increased enormously. And studying the rate of evolution also has practical implications. For example, fishermen often take the largest specimens of salmon or trout from streams and rivers. Fish usually need to reach a certain size before becoming sexually mature and capable of reproduction, after which growth slows down. Like other animals, fish show a trade-off between large size and time of reproduction: if you wait to be large before producing offspring, you probably will be able to produce more of them, and having greater numbers of offspring is favored by evolution, but you also risk dying before you are able to reproduce at all. But where overfishing has removed a substantial portion of a population, the average size of fish is now substantially smaller, because the fishermen have inadvertently selected for earlier reproduction, and evolution has favored fish that get to the business of sex sooner. It’s not just that the larger fish have all been taken; it’s that the fish are not reaching such sizes to begin with. The genes responsible for regulating growth and size at sexual maturity are now different because evolution has occurred. To bring back the jaw-dropping trophy fish of decades past, scientists say, people will have to change their ways.

It’s common for people talk about how we were “meant” to be, in areas ranging from diet to exercise to sex and family. Yet these notions are often flawed, making us unnecessarily wary of new foods and, in the long run, new ideas. I would not dream of denying the evolutionary heritage present in our bodies—and our minds. And it is clear that a life of sloth with a diet of junk food isn’t doing us any favors. But to assume that we evolved until we reached a particular point and now are unlikely to change for the rest of history, or to view ourselves as relics hampered by a self-inflicted mismatch between our environment and our genes, is to miss out on some of the most exciting new developments in evolutionary biology.

The influential twentieth-century biologist George Gaylord Simpson wrote a book called
Tempo and Mode in Evolution
, published in 1944. It is admirable from many perspectives, not least of which is the distinction it makes between the rate at which evolution occurs (tempo) and the pattern of evolution itself (mode). Simpson, a paleontologist by training, saw the work as an attempt to merge the then-new field of genetics with his own—a procedure he admitted to be “surprising and possibly hazardous”:

Not long ago paleontologists felt that a geneticist was a person who shut himself in a room, pulled down the shades, watched small flies disporting themselves in milk bottles, and thought that he was studying nature. A pursuit so removed from the realities of life, they said, had no significance for the true biologist. On the other hand, the geneticists said that paleontology had no further contributions to make to biology, that its only point had been the completed demonstration of the truth of evolution, and that it was a subject too purely descriptive to merit the name “science.” The paleontologist, they believed, is like a man who undertakes to study the principles of the internal combustion engine by standing on a street corner and watching the motor cars whiz by.
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We still sometimes think that paleontology, or evolution at grand scales—the rise and fall of dinosaurs, the origin of land animals—has little in common with the minuscule goings-on of the genes from one generation to the next. But Simpson recognized that the two processes, while having some distinctive components, are still linked, and that the disporting flies exhibit many of the same characteristics as those million-year-old bones. It’s just that the scale of measurement differs.

The title of Simpson’s book is particularly germane to my argument here, calling up as it does a rather orchestral view of evolution, with allegro and adagio components. Seeing evolution without appreciating its variously fast and slow parts is like making all the movements of a symphony happen at the same pace; you get the same notes, but most of the joy and subtlety are missing. New advances in biotechnology have made the merger of paleontology and genetics more feasible than Simpson could have imagined. We have not yet cloned dinosaurs à la
Jurassic Park
, but we are not too far off. And this merger means that we can examine not only what evolution has wrought, but also the pace at which it operates, just as Simpson hoped.

Change doesn’t always do you good

At the same time that we wistfully hold to our paleofantasy of a world where we were in sync with our environment, we are proud of ourselves for being so different from our apelike ancestors. Animals like crocodiles and sharks are often referred to as “living fossils” because their appearance is eerily similar to that of their ancestors from millions of years earlier that are preserved in stone. But there is sometimes a tone of disparagement in the term; it is as though we pity them for not keeping up with trends, as if they are embarrassing us by walking (or swimming) around in the evolutionary equivalent of mullet haircuts and suspenders. Evolving more recently, so that no one would mistake a human for our predecessors of even a couple of million years back, seems like a virtue, as if we improved ourselves while other organisms stuck with the same old styles their parents wore.

Regardless of the shaky ground on which that impression lies, we don’t even win the prize for most recent evolution; in fact, we lose by a wide margin. Strictly speaking, according to the textbook definition of evolution as a change in gene frequencies in a population, many of the most rapidly evolving species, and hence those with the most recent changes, are not primates but pathogens, the disease-causing organisms like viruses and bacteria. Because of their rapid generation times, viruses can produce offspring in short order, which means that viral gene frequencies can become altered in a fraction of the time it would take to do the same thing in a population of humans, zebras, or any other vertebrate.

Evolution being what it is—namely, without any purpose or intent—evolving quickly is not necessarily a good thing. Often the impetus behind rapid evolution in nonhuman organisms is a strong and novel selective agent: a crop is sprayed with a new insecticide, or a new disease is introduced to a population by a few individuals who stray into its boundaries. Those who are resistant, sometimes an extremely tiny minority, survive and reproduce, while the others perish. These events are not confined to crops, or even to nonhumans. Some estimates of death rates from the medieval outbreak of bubonic plague called the Black Death in Europe have gone as high as 95 percent.