Tucked in the back of a concrete industrial building, the workout space is mostly bare: the exercise equipment consists of a cage-like network of pull-up bars, some gymnastic rings, racks of weight plates and medicine balls, and a few stationary bikes lining one wall. During hour-long classes, metal-punk music blares as a coach guides clients through a dynamic series of exercises, ranging from the familiar (jumping rope) to the slightly convoluted (a combination pushup-handstand maneuver against a wall). As fitness routines go, it’s pretty hardcore.
For some CrossFit enthusiasts, the studio is simply a place to get a good workout. For others, CrossFit quickly becomes part of a lifestyle that evokes a time, tens of thousands of years ago, when our ancestors stayed fit and strong simply by living in a self-sufficient manner, hunting and gathering all of their food. And at some point, many CrossFit members experiment with eating Paleo.
Also known as the caveman diet, Paleo eating emphasizes meat, fruits, and vegetables while excluding grains, legumes, dairy, and added sugars. In terms of meal planning, it’s more well-rounded than the baby-food or grapefruit diets but stricter than Weight Watchers or South Beach. Though the media stereotype of a so-called “ancestral health” participant is a young, male gym rat with a serious carnivorous streak, recent studies show that most Paleo proponents are actually middle-aged women motivated by weight loss and health concerns.
The fundamental philosophy, according to evolutionary nutritionist Loren Cordain, author of the Paleo Diet: Lose Weight and Get Healthy by Eating the Foods You Were Designed to Eat and a related empire of books, is that our processed-food-filled modern diets have strayed far from what our pre-agricultural, hunter-gatherer genes were fine-tuned to use as fuel during the Paleolithic period. The result: diabetes, heart disease, and all sorts of other modern health woes. If we did what our bodies were meant to do, in terms of diet and exercise, Cordain argues, we would be healthier.
A few years ago, Cordain was at an evolutionary-medicine conference talking about a grim-sounding condition called leaky gut syndrome. Digestive difficulties occur, he explained, when people with a particular form of an immune-system gene eat any one of a number of foods commonly found in modern diets—bread, potatoes, and root beer among them.
At least one of the conference attendees was skeptical. Marlene Zuk, a trim, middle-aged evolutionary biologist who studies, among other things, the sex lives of crickets, raised her hand.
Why, the insect researcher asked the nutritionist, did he think this gene form still existed when it was so disadvantageous? Shouldn’t natural selection have eliminated it from the human population by now? “There hasn’t been enough time,” Cordain answered. Zuk, who is now based at the University of Minnesota, pressed him to explain. He responded that people started eating these foods only after the agricultural revolution 10,000 years ago, and that genes couldn’t change that quickly. “Plenty of time,” Zuk challenged. Cordain protested. She repeated herself.
It was an unlikely confrontation between experts from two very different academic worlds, and their disagreement has only intensified since the publication of Zuk’s new book Paleofantasy: What Evolution Really Tells Us About Sex, Diet, and How We Live.
Zuk’s office on the U of M’s St. Paul campus is filled with books about insects and evolution, including her own Sex on Six Legs, which came out in 2011. When I show up, she has just finished eating a lunch of cheese, crackers, and carrots. Zuk refuses to tell me what kind of cheese or what kind of crackers, insisting that the details have nothing to do with her book, which is not a diet book.
Zuk wears her brown hair cut short, and her no-nonsense demeanor is friendly but focused. She seems like the sort of person who would rather talk about why the sky is blue than chitchat about the weather. Scientific questions fascinate Zuk, whose rapid-fire sentences are packed with information and punctuated with wry humor. At one point, she tells me she used to think all biologists should study parasitoids, organisms that feed off of their hosts and often end up killing them. “It is an extraordinary life cycle,” she says, and then doles out a little advice with a matter-of-fact, scientist’s manner. “Now I think everybody, whether they are biologists or not, should work on parasitoids at some point. It should just be, like, a requirement in life.”
For 30 years, most of them spent at the University of California–Riverside, Zuk has been studying birds, insects, and other animals for insights into what makes males and females different and how parasites influence reproduction, with her most recent focus on cricket evolution and mating behaviors. In Paleofantasy, Zuk turns her evolutionary lens on our own species, arguing that there is no such thing as a single “natural” way to eat, exercise, make love, raise babies, live, die, or anything else. The notion of an idyllic time when ancient humans and their genes lived in perfect harmony with the earth is fundamentally hogwash, Zuk argues. There has often been a mismatch between our bodies and the environments we inhabit. And perhaps most importantly, as the world has continued to change, so too has our DNA—more quickly than scientists long thought possible. Our genes don’t necessarily suggest anything about our lifestyles, she says, much less that we should live like cavemen. (While we modern humans share most of our genes in common with our Stone Age ancestors, in fact, there’s also significant overlap with chimpanzees, and even bacteria and sea anemones.) The world is different now, and so are we.
In Zuk’s mind, her book is purely an attempt to correct misguided ideas about how evolution works by highlighting exciting new research on rapid genetic change throughout the animal kingdom. But its provocative title and its chapters on food have sparked debate, even ire. After the first round of Internet commenters posted their thoughts on the tome, Zuk stopped reading the blogs. “I was taken aback by how all these people who didn’t read the book took it as a big attack on their diet,” she says. “Really, it’s not what the book was about. People can eat whatever they like. I just don’t care about people’s diets.”
What she does care about are crickets. Zuk loved animals as a kid, but she grew up in Los Angeles, where wildlife is scarce. (One thing she appreciates about insects is their ubiquity—they’re accessible to just about everyone.) Alongside other projects, she has been delving into questions about how crickets find each other to mate and how parasites can alter their characteristic chirping. Zuk leads me down a hallway to a room lined with loud-humming, refrigerator-shaped machines. When she opens one of the doors with routine practicality, a cacophony of chirps escapes, evoking a post-sunset evening at summer camp. The dozens of plastic containers before us don’t contain the lab staff’s sandwiches and fruit salad. Instead, they host colonies of crickets and the rabbit feed the creatures eat.
“Isn’t that awesome?” she asks, perking up. “I love watching them call!” She takes a moment to point out a female that’s looking around for a place to lay her eggs and some just-hatched tiny babies. “I always think they look almost like little ants with curly antennae,” she says. “Aren’t they cute? Very cute.”
Zuk and her colleagues use these crickets for all sorts of “wacky” experiments, she explains, including raising some in silence and others in a world of sound to see how various auditory environments affect mating behavior. Crickets may seem too different from humans to offer windows into our lives, but that’s the point, she says. You can’t anthropomorphize bugs, which makes it easier to be objective about the scientific insights they offer—including the ones that came out of a startling breakthrough Zuk made a decade ago in a dark, silent field in Hawaii.
On a typically warm, post-sunset evening on Kauai in 2003, Zuk hopped out of a car, strapped on a headlamp, and walked onto a grassy lawn. It was an ideal environment for crickets, but the field was chirp-free. Zuk hadn’t expected to see any of the insects hopping around; over the previous decade, she had watched the island’s cricket populations decline rapidly as a result of parasitic flies that dive-bomb singing males and deposit larvae inside their bodies. Once inside, the wormlike maggots eat their hosts alive and then burrow out, leaving the dying crickets behind.
Zuk’s study of the system began in the early 1990s, when she traveled to Hawaii for a conference and decided to pluck some crickets from a field and dissect them in a “quasi-recreational way.” (It “seemed amusing at the time,” she recalls.) Sure enough, her investigations revealed fat, white larvae wriggling inside some of the crickets. Zuk’s good-spirited husband, John Rotenberry, a bird biologist also at the U of M, was along on the trip and was surprised by the discovery. ”I always claim it’s the only time he’s been impressed with my scientific acumen,” Zuk jokes.
As she continued to visit Kauai over the years, it seemed clear that the parasites were winning. By 2001, Zuk heard only one calling male in her routine field visits and found just a few crickets remaining. So when she arrived on that night in 2003, she nearly decided not to even look. It was just so quiet. But she had come a long way, and so she got out of the car. “In my headlamp, I started seeing all these crickets,” Zuk remembers. “We ended up finding way more crickets than we had imagined. But they weren’t calling. That was just bizarre.”
Male crickets use their distinctive chirps to attract mates, and the discovery of silent crickets was puzzling, even to a researcher who had studied them for 20-plus years. Even more surprising was Zuk’s subsequent discovery that the Hawaiian crickets hadn’t just learned to stop singing to escape detection by parasites; they had actually lost the parts on their wings that allow them to make noise. The change came from a single gene mutation that spread through the population in just 20 generations over five years—the equivalent of a few centuries in human years. According to a traditional understanding of evolution, such significant genetic changes should take hundreds or thousands of human-equivalent years, making crickets one of the faster cases of evolution ever observed in the wild.
Zuk’s realization that evolution can happen so quickly in crickets led her to collect case studies of rapid evolution in fish, birds, snakes and, yes, people. From there, she developed a bold theory that inadvertently pissed off a lot of people.
Veins bulge out of Alicia Kleppinger’s lean, steely arms as she lifts the floppy menu at Brasa, a go-to destination for the Twin Cities Paleo community for its locally soured, high-quality meat. After starting an internship at CrossFit St. Paul two years ago, Kleppinger, 32, joined one of the gym’s 30-day challenges to eat Paleo. It was hard at first to resist donuts and other sweets, she says, and in the beginning, she found herself irritable, foggy, and hungrier than usual. But after a week and a half, she started to feel better than ever. Her energy held steady throughout the day. She stopped having monthly mood swings. Her seasonal allergies cleared up. Though Kleppinger was not trying to drop weight, she lost a few pounds.
As she became used to a new way of eating, the switch became permanent. Her typical breakfast might be eggs with sauerkraut and bacon. Lunch is salad or leftovers. And dinner consists of some kind of meat with vegetables, sometimes followed by berries or homemade coconut-milk ice cream. Kleppinger and her husband grow herbs and lettuce in a small garden and they compost kitchen scraps. Meat is so central to their diets that they recently bought a quarter of a cow through a share organized by a CrossFit coach and stocked their freezer with nearly 100 pounds of vacuum-sealed, grass-fed beef.
Like many people who join CrossFit and try the caveman diet, Kleppinger’s reasons for going Paleo aren’t necessarily evolution-related. “I don’t know if we’re designed to eat grains,” she says, digging into a lunch of rotisserie chicken, salad without cheese, and guacamole on chips—a small cheat since the chips are made from corn. “It’s less about trying to recreate a diet from so long ago than trying to stick as close to nature as possible,” she says. “What do you have to lose from cutting out processed food that comes out of a box, versus [eating] something that was raised locally? I always ask, ‘Would my great-grandmother recognize that?’ That’s a good way to describe this lifestyle.” (So is Kleppinger’s T-shirt, which displays phrases such as “farm-to-table,” “fair trade,” and “locally grown” clustered into the shape of Minnesota.)
Kleppinger’s goal is to be healthier, not necessarily to live like a cavewoman. After all, she doesn’t even like camping. And yet, at its root, the Paleo lifestyle is based on the belief that our modern lives are out of sync with our genes. On the website cavemanforum.com, Paleo proponents share recipes, workout journals, and tips for wild-food foraging. They also grapple with difficult questions: “Can a cookie really be Paleo?” Members also discuss parenting. One post, captioned “Cave girl in the making,” shows a preschool-age child eating a drumstick nearly as big as her head.
Cordain, who is based at the department of Health and Exercise Science at Colorado State University in Fort Collins, has published dozens of studies, both ethnographic and molecular, over the past several decades that illustrate how much healthier people are when they act as if they were living a long, long time ago. In a 2011 paper called “Exercise Like a Hunter-Gatherer: A Prescription for Organic Physical Fitness,” he explains that natural selection shaped our genes to fit a physically vigorous way of life. Because the bulk of our genes have remained the same for at least 40,000 years, many of today’s chronic diseases likely emerged from an inactive, indoor lifestyle. “Literally,” he writes in the Paleo Diet, “we are Stone Agers living in the Space Age.”
The same, Cordain argues, goes for food. In a 2002 study, he and colleagues analyzed the diets of 229 hunter-gatherer societies, which are often used as a proxy for people who lived eons ago. Though the details varied, results showed that animal foods made up about 65 percent of all of the diets. “If you look at the diets of wild animals, their genes have been hardwired by the selective pressures of their ecological niches, and their nutritional needs are hardwired into the genome,” Cordain says, intermittently fuming about Zuk and her lack of qualifications for criticizing his theories.
“Characteristics of the Western diet exceed by multiple standard deviations anything that our species ever consumed,” he says. “In a typical Western diet, we obtain 70 percent of our calories from refined sugar, refined grains, refined vegetable oils, and dairy. None of those foods existed until fairly recent times. When we talk about going back to the types of foods our ancestors ate, if you take those out of the diet, what’s left is real food: lean meats, fruits, vegetables, nuts, seeds, and living foods.”
Despite Zuk’s claims of dietary indifference, Paleo proponents are unhappy about a book that even hints that they might be living a fantasy. “I’ve spent the bulk of my career doing human research. I’ve co-authored papers with anthropologists and archaeologists and ethnologists, people who have been in the trenches with this data,” says Cordain. “I feel a little miffed that this person who is outside the field, who has never published a paper in this discipline, comes in and criticizes not just me but the entire discipline. I would never dream to critique insect biology. I would never dream to critique her papers. She is essentially uninformed.”
For Zuk, food is worth discussing mainly as a window into new discoveries in human genetics and evolution that show how our genes have changed and the speed at which those changes have occurred. A mutation within the past 10,000 years, for example, gave adults the ability to digest the lactose sugar in milk; today, about a third of the population possesses that gene variant, raising doubts about the Paleo diet’s stance against dairy. “There are people who argue from a Paleo diet perspective that we shouldn’t drink cow’s milk because we didn’t evolve to drink milk from cows,” says Michael Wilson, an evolutionary anthropologist at the U of M. “That’s a fair argument to make for cats. You shouldn’t give a cat a lot of milk to drink because they haven’t evolved to drink milk as adults. But people in populations that have had dairy animals for many generations have evolved to drink dairy from their animals. It’s one of the examples of human evolution still going on.”
Strict Paleo eaters also eschew grains and sometimes even potatoes, but some evidence suggests that tubers were an important source of carbohydrates as far back as the time of Australopithecus, the genus that lived before the Homo line emerged, Wilson says. According to archaeological remains, Zuk writes in her book, early people in Europe appear to have mixed water with the flour of ground-up plants to make a bread-like product. And recent gene shifts have given people who live in carb-reliant cultures the ability to digest starches more quickly. Compared to hunter-gatherer groups who rely mostly on meat, people in places like rice-eating Japan have more copies of a gene that helps them break down starches.
Even if you wanted to eat like people did in the Stone Age, Zuk argues, it would be impossible. The Paleolithic period lasted nearly two million years. During that time, humans lived in many different climates and terrains, and their diets varied widely. Depending on where they lived, some groups did a lot of hunting. Some did more gathering. Some ate a lot of fish. Some had more access to fruit. That variety makes it hard to pick one ancestral diet most worth emulating. Meanwhile, the animals and plants that we eat have themselves changed drastically through breeding and natural selection. “The ancestral potato, for example, was a bitter, lumpy root a fraction the size of the average Idaho baker,” Zuk writes. “What we now know as corn started out 9,000 years ago as a Mexican grass called teosinte, with a shape and size more reminiscent of a stalk of rice than of the fat yellow kernels on a cob.” Many of the foods our ancestors ate, in other words, no longer exist as they knew them.
Often overlooked by vehement Paleo proponents are Zuk’s chapters on high-altitude adaptations, cancer in ancient people, barefoot running, variations in the stickiness of earwax, the development of resistance to malaria, and a discussion of the tolerance different cultures have for letting their babies cry. Everywhere you look, there are examples like these, Zuk writes, of humans adapting to new challenges—sometimes through changes in behavior, sometimes through changes in genes, and sometimes through a combination of the two.
Evolution is not simple, adds John Hawks, a paleoanthropologist at the University of Wisconsin, Madison, who agrees with Zuk that the process is far from over. “When I was a student, most professors assumed that humans had really stopped changing 100,000 years ago,” he says. (Other experts have made similar arguments on scales ranging from 10,000 to a million years.) “Now, we know we’ve been changing much faster in that period than we ever did before. We’ve never been perfect. We’ve never stopped changing. We are absolutely still evolving.”
New genetic tools are revealing so many new examples of evolution happening at a rapid clip that Zuk had trouble letting go of the Paleofantasy manuscript; new studies kept coming out that she wanted to include. But even as our genes change, Hawks says, they don’t necessarily work as a blueprint for determining how our lives will go. That’s where arguments that claim we are designed to live a certain way fall flat. “Humans are behaviorally flexible and culturally flexible, and the choices we make are often determined by what people around us are doing,” he says. “If there’s one thing about humans, it’s that we surpass our genetics. We do things you wouldn’t predict from genetics. To say today that we should do something because that’s the way it was done in the past doesn’t make sense.”
Yet neither should we necessarily do something—drink milk, for example—just because we now have the genes to do it. Paleo-eating might suit some people regardless of their genetics; there is unlikely to be a single diet that works best for everyone. “For a long time, there was a common notion that evolution happens slowly, that evolution is progressive, proceeding toward a particular goal, that anything that’s more recently evolved is necessarily better adapted in some way, and that humans are the pinnacle of evolution,” Zuk says. “In a lot of ways, those are wrong ideas.”
Emily Sohn writes about health and science for the Los Angeles Times, Science News, and other national publications. She lives in Minneapolis.