Authors: Marlene Zuk
WHAT EVOLUTION REALLY TELLS US
ABOUT SEX, DIET, AND
HOW WE LIVE
The first thing you have to do to study 4,000-year-old DNA is take off your clothes. I am standing with Oddný Ósk Sverrisdóttir in the air lock room next to the ancient-DNA laboratory at Uppsala University in Sweden,
preparing to see how she and her colleagues examine the bones of human beings and the animals they domesticated thousands of years ago. These scientists are looking for signs of changes in the genes that allow us to consume dairy products past the age of weaning, when all other mammals lose the ability to digest lactose, the sugar present in milk. After that time, dairy products can cause stomach upsets. But in some groups of humans, particularly those from northern Europe and parts of Africa, lactase—the enzyme that breaks down lactose—lingers throughout life, allowing them to take advantage of a previously unusable food source. Oddný and her PhD supervisor, Anders Götherström, study how and when this development occurred, and how it is related to the domestication of cows for their meat and milk. They examine minute changes in genes obtained from radiocarbon-dated bones from archaeological sites around Europe.
The first step is to extract the DNA from the bones. But when examining genes from other humans, you must avoid contaminating the samples with your own genetic material. Suddenly I feel sullied by my own DNA and imagine it floating all around me, like infestive dust motes, needing to be contained as if it were the miasma of a terrible plague. Oddný, a tall, blonde Icelandic woman who looks like my image of a Valkyrie, at least if Valkyries are given to cigarette breaks and bouts of cheerful profanity, has brought a clean set of clothes for me to put on under the disposable space-suit-like outfit I need to wear in the lab. I have to remove everything except my underwear, including my jewelry. Götherström says it is the only time he ever takes off his wedding ring. I don the clean outfit, followed by the white papery suit, a face mask that includes a transparent plastic visor over my eyes, latex gloves, and a pair of slip-on rubber shoes from a pile kept in the neverland between the lab and the outside world. Anything else that goes into the lab—a flash drive for the computer, say—cannot go back in once it has gone out, to prevent secondary contamination of the facility. Finally, I put on a hairnet and tuck my hair underneath.
We enter the lab, where the first thing we do is stretch another pair of gloves over the ones we just put on. Oddný takes out a plastic bin of bone samples, each in its own zip-top bag. The bones themselves have been bleached and then irradiated with ultraviolet light to remove surface contamination. Before setting the bin on the counter, she wipes the surface with ethanol, followed by a weak bleach solution, and then with more ethanol. Apparently the saying that one can’t be too careful is taken literally in this lab. “We all have to be kind of OCD to do this work,” says Oddný, smiling. Or at least I think she is smiling under her mask.
To obtain the DNA, the bones are drilled and the powder from the interior is processed so that the genetic sequences inside are amplified—that is, replicated to yield a larger amount of material for easier analysis. Some bones are more likely to be fruitful than others; we heft the samples, since Oddný says that the most promising ones are heavy for their size, and glossy. Most of the samples are about 4,000 years old, but one of them is around 16,000 years old. It has already been rendered into powder, and I look at it closely, but it doesn’t seem any different from the others. One of the pieces is a flat section of skull, while others are sections of leg or arm bones, or a bit of pelvis. Oddný and I wonder briefly who all these people were, and what their lives were like. The details of their experiences, of course, are lost forever. But the signature of what they were able to eat and drink, and how their diet differed from that of their—our—ancestors, is forever recorded in their DNA.
Other than simple curiosity about our ancestors, why do we care whether an adult from 4,000 years ago could drink milk without getting a stomachache? The answer is that these samples are revolutionizing our ideas about the speed at which our evolution has occurred, and this knowledge, in turn, has made us question the idea that we are stuck with ancient genes, and ancient bodies, in a modern environment. We can use this ancient DNA to show that we are not shackled by it.
The speed of evolution and our cave dweller past
Because we often think about evolution over the great sweep of time, in terms of minuscule changes over millions of years when we went from fin to scaly paw to opposable-thumbed hand, it is easy to assume that evolution always requires eons. That assumption in turn makes us feel that humans, who have gone from savanna to asphalt in a mere few thousand years, must be caught out by the pace of modern life, when we’d be much better suited to something more familiar in our history. We’re fat and unfit, we have high blood pressure, and we suffer from ailments that we suspect our ancestors never worried about, like posttraumatic stress disorder and AIDS. Dr. Julie Holland, writing in
magazine, counsels that if you “feel less than human,” constantly stressed and run-down, you need to remember that “the way so many of us are living now goes against our nature. Biologically, we modern
are a lot like our cavewoman ancestors: We’re animals. Primates, in fact. And we have many primal needs that get ignored. That’s why the prescription for good health may be as simple as asking, What would a cavewoman do?”
Along similar lines, here are some comments from readers of the
New York Times
Our bodies evolved over hundreds of thousands of years, and they’re perfectly suited to the life we led for 99% of that time living in small hunting and gathering bands.
We are (like it or not) warm-blooded vertebrate mammals, i.e., part of the animal kingdom, and only in a very recent eyeblink of time become [
] relatively free of the evolutionary pressures that shaped this species for millennia.
Probably goes all the way back to caveman days—women out gathering berries, sweeping up the place, generally always on the run. Cave Mr. Man out risking his neck, hunting a sabre tooth tiger or maybe a wooly mammoth, dragging the thing home, and then collapsing in a heap on the couch with a beer. I get it—makes sense.
I am not suggesting that
magazine or the readers of the
New York Times
have pinpointed the modern dilemma in its entirety. But it’s hard to escape the recurring conviction that somewhere, somehow, things have gone wrong. In a time with unprecedented ability to transform the environment, to make deserts bloom and turn intercontinental travel into the work of a few hours, we are suffering from diseases our ancestors of a few thousand years ago, much less our prehuman selves, never knew: diabetes, hypertension, rheumatoid arthritis. Recent data from the Centers for Disease Control and Prevention (CDC) suggest that for the first time in history, the members of the current generation will not live as long as their parents, probably because obesity and associated maladies are curtailing the promise of modern medicine.
Some of this nostalgia for a simpler past is just the same old amnesia that every generation has about the good old days actually being all that good. The ancient Romans fretted about the young and their callous disregard for the hard-won wisdom of their elders. Several sixteenth- and seventeenth-century writers and philosophers famously idealized the Noble Savage, a being who lived in harmony with nature and did not destroy his surroundings. Now we worry about our kids as “digital natives,” who grow up surrounded by electronics and can’t settle their brains sufficiently to concentrate on walking the dog without simultaneously texting and listening to their iPods.
Another part of the feeling that the modern human is misplaced in urban society comes from the realization that people are still genetically close not only to the Romans and the seventeenth-century Europeans, but to Neandertals, to the ape ancestors Holland mentions, and to the small bands of early hominids that populated Africa hundreds of thousands of years ago. It is indeed during the blink of an eye, relatively speaking, that people settled down from nomadism to permanent settlements, developed agriculture, lived in towns and then cities, and acquired the ability to fly to the moon, create embryos in the lab, and store enormous amounts of information in a space the size of our handily opposable thumbs.
Given this whiplash-inducing rate of recent change, it’s reasonable to conclude that we aren’t suited to our modern lives, and that our health, our family lives, and perhaps our sanity would all be improved if we could live the way early humans did. Exactly what we mean by “the way early humans did” is a point of contention, and one I will return to in detail in Chapter 2, but the preconception—an erroneous one, as I will demonstrate—is the same: our bodies and minds evolved under a particular set of circumstances, and in changing those circumstances without allowing our bodies time to evolve in response, we have wreaked the havoc that is modern life.
In short, we have what the anthropologist Leslie Aiello, president of the renowned Wenner-Gren Foundation, called “paleofantasies.”
She was referring to stories about human evolution based on limited fossil evidence, but the term applies just as well to the idea that our modern lives are out of touch with the way human beings evolved and that we need to redress the imbalance. Newspaper articles, morning TV, dozens of books, and self-help advocates promoting slow-food or no-cook diets, barefoot running, sleeping with our infants, and other measures large and small claim that it would be more natural, and healthier, to live more like our ancestors. A corollary to this notion is that we are good at things we had to do back in the Pleistocene, like keeping an eye out for cheaters in our small groups, and bad at things we didn’t, like negotiating with people we can’t see and have never met.
I am all for examining human health and behavior in an evolutionary context, and part of that context requires understanding the environment in which we evolved. At the same time, discoveries like those from Oddný’s lab in Sweden and many more make it clear that we cannot assume that evolution has stopped for humans, or that it can take place only ploddingly, with tiny steps over hundreds of thousands of years. In just the last few years we have added the ability to function at high altitudes and resistance to malaria to the list of rapidly evolved human characteristics, and the stage is set for many more. We can even screen the entire genome, in great gulps of DNA at a time, looking for the signature of rapid selection in our genes.
To think of ourselves as misfits in our own time and of our own making flatly contradicts what we now understand about the way evolution works—namely, that rate matters. That evolution can be fast, slow, or in-between, and that understanding what makes the difference is far more enlightening, and exciting, than holding our flabby modern selves up against a vision—accurate or not—of our well-muscled and harmoniously adapted ancestors. The coming chapters will show just how much we know about that harmony, about the speed of evolution, and what these findings mean about the future of human evolution.
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.