In her new book Paleofantasy: What Evolution Really Tells Us About Sex, Diet and How We Live, University of Minnesota biologist Marlene Zuk explains why the paleo lifestyle is based on incorrect assumptions about how evolution works. In this excerpt, she lays out her argument — and it's pretty damn compelling.
Our maladapted ancestors
The paleofantasy is a fantasy in part because it supposes that we humans, or at least our protohuman forebears, were at some point perfectly adapted to our environments. We apply this erroneous idea of evolution producing the ideal mesh between organism and surroundings to other life-forms too, not just to people. We seem to have a vague idea that long long ago, when organisms were emerging from the primordial slime, they were rough-hewn approximations of their eventual shape, like toys hastily carved from wood, or an artist’s first rendition of a portrait, with holes where the eyes and mouth eventually will be. Then, the thinking goes, the animals were subject to the forces of nature. Those in the desert got better at resisting the sun, while those in the cold evolved fur or blubber or the ability to use fire. Once those traits had appeared and spread in the population, we had not a kind of sketch, but a fully realized organism, a fait accompli, with all of the lovely details executed, the anatomical t’s crossed and i’s dotted.
But of course that isn’t true. Although we can admire a stick insect that seems to flawlessly imitate a leafy twig in every detail, down to the marks of faux bird droppings on its wings, or a sled dog with legs that can withstand subzero temperatures because of the exquisite heat exchange between its blood vessels, both are full of compromises, jury-rigged like all other organisms. The insect has to resist disease, as well as blend into its background; the dog must run and find food, as well as stay warm. The pigment used to form those dark specks on the insect is also useful in the insect immune system, and using it in one place means it can’t be used in another. For the dog, having long legs for running can make it harder to keep the cold at bay, since more heat is lost from narrow limbs than from wider ones. These often conflicting needs mean automatic trade-offs in every system, so that each may be good enough but is rarely if ever perfect. Neither we nor any other species have ever been a seamless match with the environment. Instead, our adaptation is more like a broken zipper, with some teeth that align and others that gape apart. Except that it looks broken only to our unrealistically perfectionist eyes—eyes that themselves contain oddly looped vessels as a holdover from their past.
Even without these compromises from natural selection acting on our current selves, we have trade-offs and “good enough” solutions that linger from our evolutionary history. Humans are built on a vertebrate plan that carries with it oddities that make sense if you are a fish, but not a terrestrial biped. The paleontologist Neal Shubin points out that our inner fish constrains the human body’s performance and health because adaptations that arose in one environment bedevil us in another. Hiccups, hernias, and hemorrhoids are all caused by an imperfect transfer of anatomical technology from our fish ancestors. These problems haven’t disappeared for a number of reasons: just by chance, no genetic variants have been born that lacked the detrimental traits, or, more likely, altering one’s esophagus to prevent hiccups would entail unacceptable changes in another part of the anatomy. If something works well enough for the moment, at least long enough for its bearer to reproduce, that’s enough for evolution.
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.”
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. 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.
Excerpted from Paleofantasy: What Evolution Really Tells Us About Sex, Diet, and How We Live by Marlene Zuk. Copyright © 2013 by Marlene Zuk. With permission of the publisher, W.W. Norton & Company, Inc.