Stephen Le was finishing his Ph.D. at the University of California in Los Angeles when he got devastating news from home in Canada. His mother’s breast cancer, diagnosed years earlier—she had had mastectomies and chemotherapy—had metastasized to her lungs. Le rushed through his dissertation and returned to Ottawa, the city where his parents settled in the 1960s (originally from Vietnam, they met in school, in Montreal) and where they raised three boys. Le’s mother died three months later, at age 66—just two years after her own mother died, at the ripe old age of 92.

That fact—a catastrophic drop in lifespan of nearly three decades, from one generation in his family to the next—made a deep impression on Le, a biological anthropologist. His way of coping, he says, was to start researching ancestral diets and food-related illness. “Some of my preliminary readings,” he recalls, “showed that Asians who migrate to North America and Europe see elevated rates of breast cancer as well as prostate cancer.” He wanted to understand the risk factors for such diseases, which are prevalent in the West: Do we consume too many calories? Are diets to blame, or inactivity, or both?

The surprising answers he uncovered in his research and his travels around the world, from rural China to Australia, southern India to remote Papua New Guinea, form the basis of his provocative new book: 100 Million Years of Food: What Our Ancestors Ate and Why It Matters Today. What Le found, looking back at evolution through the prism of what and how we eat, is that humans, on average, don’t consume more calories than we used to millions of years ago. Despite our sedentary lifestyles we also—stand back!—expend about as much energy as our ancestors did, thanks to our faster metabolism rates. So fewer calories aren’t the answer, he argues, nor is mere exercise.

Le’s deceptively simple prescription (one of several in the book): Eat according to your ancestral diet. He argues that the adaptations humans have made to their environments in the 12,000 years since human settlement began means that the food that is healthiest for you may be tied to your particular cultural and genetic history. He also argues that some of the more serious health issues of our times, from cardiac disease and obesity to cancer, may be tied to an approach to eating that goes against human evolutionary history. His book is a rallying cry for traditional diets—whether meat and potatoes or noodle soups or lentils and vegetables with rice—and for common sense over newfangled “nutrition” and faddish eating. In other words, put away that kale salad and pineapple-banana soy chia smoothie. The path to longevity and good health may lie in eating like your grandparents—or better yet, your great-great-great-great grandparents.

It’s a startlingly intuitive approach in an era of admonishments: Eat vegetables of different colours! Cut back on salt, and sugar! It’s also a radical directive for our times. We are a society of culinary cherry pickers, unfettered by cultural boundaries. We still love fusion, 25 years later. Our food trick du jour: the lunch bowl, in which Japanese miso, Indian curry, avocados, South American quinoa and Greek yogourt all cohabit, constrained only by a cook’s imagination. Some of us eat little but meat, eschewing wheat and dairy. Others elevate vegetables, Le suggests, to religion. (What is a cold-press juice if not ritual cleansing?) But is this in fact the way we were meant to eat?

Le’s advice to reclaim ancestral diets subverts an old tradition of thought. Early thinking about ancestral diets came from patronizing and often racist views of how “simpler” pre-contact cultures were closer to nature and must return to that state, notes Adrienne Rose Johnson, a doctoral candidate at Stanford University, who analyzed literary and mythical themes in diet books for her thesis, “Diet and the disease of civilization, 1975-2008.” A more modern view has gained momentum since the 1990s, applied by anthropologists to communities such as the Pima Indians in the American Southwest, and Pacific Islanders, whose diets changed dramatically with exposure to American culture. “The researchers often provided the same recommendations, that the best answer is to return to some fabled, mythical past,” says Johnson. More recently there’s the discredited Paleo movement and the raw-food diet, whose advocates believe they are returning to an idealized, and more natural, way of eating.

As a bioanthropologist, Le isn’t trying to hit on a dieting formula or tap the nostalgia vein (at one point he tells us that, in a pinch, hominids ate their own children). Rather, he traces the history of eating from our predecessors’ insect-chowing days of 100 million years ago—we once had abundant enzymes to digest the hard exoskeletons of insects—to a primate fruit diet of 30 to 60 million years ago, through the emergence of dairy and alcohol and on, to see how we got to the present day. “Trying to understand human nutrition and health without understanding evolution,” he argues, “is like trying to eavesdrop on a snippet of conversation without knowing the context. It … can be very misleading.”

Consider the evolutionary case against juicing: Le explains that in our fruit-eating days (we were still swinging from trees at this point) we had a profusion of vitamin C in our diets. We lost the ability to synthesize vitamin C (we had it once, as lemurs still do). A subsequent change brought higher levels of uric acid—which happily provided an antioxidant effect similar to vitamin C’s. But when higher uric acid levels meet high levels of fructose (found in pop, juices, domesticated fruits like apples and pears) or purines (a chemical found in meat, seafood, lentils), the result can be insulin resistance, hypertension, gout and obesity-related disorders. In our fruit-eating days, we actually didn’t consume all that much of either chemical. Now we’re exposed to higher levels of both. Our bodies can no longer handle the “daily jug-loads of fructose” many of us now consume, Le writes.

It’s not just juicing; Le thinks our culture’s veneration of vegetables, too, is misplaced. “People do it out of ideology, almost,” he says. His most shocking argument: eating plant matter, in and of itself, has not been shown to improve health prospects compared with, say, consuming alcohol in moderation, and even being overweight. We remain fixated on thinness, Le says, but moderately overweight people live longer and recover better from chronic diseases. Similarly, research consistently shows that moderate drinking reduces the risk of heart disease and ischemic stroke. But “we went through a period in American history when alcohol was prohibited,” says Le, “so there’s a cultural stigma that surrounded alcohol that extends to this day.”

Le notes that humans resisted vegetables for a long time—they were late arrivals to our diets, long after meat, and even dairy—because they were full of toxins. Lima beans contain cyanide; the phytates in peas, beans, apples and tomatoes can deplete our bodies of essential minerals including magnesium, zinc and iron, Le writes. Our ancestors developed means of neutralizing such problems: boiling, roasting, sautéing and steaming. Cooking, in other words. Tell that to the salad brigade. “For people in Western societies, the salad bar is the epitome of health,” says Le. “If you took people from most parts of the world to a salad bar, they would gasp in horror. They’d say, ‘You can’t digest these things!’ ” In fact, Le says many symptoms associated with gluten sensitivity may also be brought on by exposure to sugars found in everything from apples and peaches to artichokes, sugar snap peas and cauliflower.

It’s not that fruits and vegetables are bad for us. Le, who is not a nutritionist, just quibbles with the impulse to overstate their importance. “We tend to look at vegetables as medicine,” he says. “There’s a veneer of science. But as long as we’re eating an adequately balanced diet, there’s no fear of non-nutrition.” He believes there was wisdom in how our ancestors combined vegetables with meat, lentils, starches—“part of a complete dinner,” to paraphrase the breakfast-cereal marketers.

Taken out of the context of cuisine, a nourishing food may have quite another effect. Take the coconut-oil boom: The benevolent saturated fat in coconut oil, touted by Dr. Oz and embraced by health fanatics, now imbues cookies and brownies with a healthful glow. It’s true that coconut and coconut oil were staples in the otherwise lean traditional diet of Kerala, in south India. But they were almost certainly never combined with a cup of sugar, and refined flour, and chocolate chips.

There’s another pitfall in assuming what works for someone else is good for you, an idea explored by Gary Paul Nabhan, an ethnobotanist and conservation scientist at the University of Arizona. In his 2013 book Food, Genes and Culture: Eating Right For Your Origins, Nabhan argued there are interactions between our genes and the food we eat. Crete’s version of the Mediterranean diet, for instance—local vegetables, whole grains, little meat, and lots of olive oil (more than 25 litres per person a year)—conferred longevity and famously low rates of heart disease on its population. Northern Europeans placed on the Cretan diet for a study couldn’t seem to metabolize that much olive oil in the same way. “After centuries of consuming the largest quantities of olive oil of any peoples in the world, Cretans have evidently developed a genetic adaptation to the oil,” Nabhan writes. Growing research shows that responses to high-fat diets depend on the form of the lipoprotein gene carried by people, Nabhan points out—and “the kind of oil produces different responses among different ethnic populations.” Le notes that higher calcium can be a risk factor for prostate cancer among people whose forebears didn’t eat a diet that included dairy.

Here’s the thing: eating ancestral, if you will, isn’t exciting. It’s much less satisfying than alternating between Mexican, Thai, gourmet burger and Italian takeout. In fact, the people who ate these ancestral diets often didn’t care for them. Cretans, for one, “wanted more meat,” Le says. “So as soon as they got a chance, they changed that.” By 2010, Crete’s traditional diet was on the wane, and the average middle-aged man weighed 20 kg more.

Or consider Okinawa—a winner in the ancestral diet sweepstakes. A decade or so, when the Blue Zones project began highlighting the world’s longest-living and healthiest communities, the Okinawans emerged as superstars: the Japanese island boasted an absurd number of centenarians, thanks in part to its diet of vegetables, tofu and miso soup.

When Le travelled to Okinawa, what he found was shocking. In place of the plain fare written about in magazines, there are restaurants and food stalls serving deep-fried fish and onigiri (rice balls) with Spam, introduced by U.S. troops stationed there after the Second World War. According to the Asahi Shimbun newspaper, Okinawa dropped from near the top of Japan’s health rankings in 1995 to 26th place a few years ago. Rates of type 2 diabetes have spiked, and life expectancy has dropped.

Nor do you have to go that far afield. The Toronto Diabetes Atlas, published by the Institute for Clinical Evaluative Sciences and Centre for Research on Inner City Health, shows that the neighbourhoods in the Greater Toronto Area with the highest diabetes rates are ones ringing the downtown core: suburbs populated with visible minorities and recent immigrants. In part this reflects a genetic susceptibility to diabetes among non-European ethnoracial groups. But also, according to the 2013 Diabetes Atlas for Peel Region, which has some of the highest diabetes rates (and immigrant populations) in Ontario, “Migration may influence the risk of diabetes through nutrition transition (i.e., a move from a diet rich in fruits and vegetables to a Western diet rich in fats, meat, processed foods and salt), changes in physical activity levels and stress.”

In other words, if taste is the imperative, following the ways of our ancestors could be difficult. In fact, some argue taste may be a bit of a liability in a modern context. An article by Rutgers University professor Paul Breslin in Current Biology considered taste’s evolutionary role, and noted that at one time, when our hominid ancestors left the forests for the savannah and broadened their diets, taste was a crucial force in helping to choose nutritious foods, and avoid foods with toxins. “The evolved taste abilities of humans are still useful for the one billion humans living with very low food security by helping them identify nutrients,” writes Breslin. “But for those who have easy access to tasty, energy-dense foods, our sensitivities for sugary, salty and fatty foods have also helped cause over-nutrition-related diseases.”

Le doesn’t think an attraction to sweet or salt works against us—if we don’t take it too far. “There are always better-tasting options just around the corner,” he says. “That’s what capitalism is good at providing.” But if health and longevity is what you’re after, you could do worse than to follow your ancestor—or someone’s. In a globalizing world, it’s hard to say what one’s ancestry is, and Le is no fundamentalist. He didn’t skimp on local foods when he travelled—he ate weevil larvae in Thailand, boiled fruit bat in Papua New Guinea, curries off a banana leaf in Kerala. But, for a daily diet, he suggests choosing a traditional cuisine—if not your own, because the idea of returning to boiled meat or copious amounts of cabbage is too awful, then one the family can get behind. And he argues for food systems—sustainable farming, for one—that make it possible to eat in this way.

At their essence, Le’s arguments are rather reasonable: eat meals, not nutrients; avoid foods invented in the last 100 years, particularly processed oil; walk as much as possible. But with all food books, Johnson says, “the stories are sometimes the most persuasive part.” Her extensive study taught her one thing: “The greatest indicator of a diet’s success is the dieter’s adherence to it,” she laughs.

Our ancestors had one invaluable aid in their diets: the absence of choice. Le recalls what he observed in his own life, visiting his grandmother’s apartment. Where his parents, trained in science and fluent in English and French, obeyed every new health directive—cut butter, eat margarine, and the like—his grandmother, unable to read English, stuck with what she knew. Le recalls her apartment, with “just a rocking chair, a few shelves of incomprehensible books, a bottle of fish sauce, a rice cooker, some crumpled old linen towels, and the soft light of day streaming through the window.” It’s a lovely image, just not the kind we see in food magazines.