People have been debating the natural human diet for thousands of years, often framed as a question of the morality of eating other animals. The lion has no choice, but we do. Take the ancient Greek philosopher Pythagoras, for example: “Oh, how wrong it is for flesh to be made from flesh!” The argument hasn't changed much for ethical vegetarians in 2,500 years, but today we also have Sarah Palin, who wrote in Going Rogue: An American Life, “If God had not intended for us to eat animals, how come He made them out of meat?” Have a look at Genesis 9:3—“Every moving thing that liveth shall be meat for you.”

While humans don't have the teeth or claws of a mammal evolved to kill and eat other animals, that doesn't mean we aren't “supposed” to eat meat, though. Our early Homo ancestors invented weapons and cutting tools in lieu of sharp carnivorelike teeth. There is no explanation other than meat eating for the fossil animal bones riddled with stone tool cut marks at fossil sites. It also explains our simple guts, which look little like those evolved to process large quantities of fibrous plant foods.

But gluten isn't unnatural either. Despite the pervasive call to cut carbs, there is plenty of evidence that cereal grains were staples, at least for some, long before domestication. People at Ohalo II on the shore of the Sea of Galilee ate wheat and barley during the peak of the last ice age, more than 10,000 years before these grains were domesticated. Paleobotanists have even found starch granules trapped in the tartar on 40,000-year-old Neandertal teeth with the distinctive shapes of barley and other grains and the telltale damage that comes from cooking. There is nothing new about cereal consumption.

This leads us to the so-called Paleolithic Diet. As a paleoanthropologist I'm often asked for my thoughts about it. I'm not really a fan—I like pizza and French fries and ice cream too much. Nevertheless, diet gurus have built a strong case for discordance between what we eat today and what our ancestors evolved to eat. The idea is that our diets have changed too quickly for our genes to keep up, and the result is said to be “metabolic syndrome,” a cluster of conditions that include elevated blood pressure, high blood sugar level, obesity and abnormal cholesterol levels. It's a compelling argument. Think about what might happen if you put diesel in an automobile built for regular gasoline. The wrong fuel can wreak havoc on the system, whether you're filling a car or stuffing your face.

It makes sense, and it's no surprise that Paleolithic diets remain hugely popular. There are many variants on the general theme, but foods rich in protein and omega-3 fatty acids show up again and again. Grass-fed cow meat and fish are good, and carbohydrates should come from nonstarchy fresh fruits and vegetables. On the other hand, cereal grains, legumes, dairy, potatoes, and highly refined and processed foods are out. The idea is to eat like our Stone Age ancestors—you know, spinach salads with avocado, walnuts, diced turkey, and the like.

I am not a dietician and cannot speak with authority about the nutritional costs and benefits of Paleolithic diets, but I can comment on their evolutionary underpinnings. From the standpoint of paleoecology, the Paleolithic diet is a myth. Food choice is as much about what is available to be eaten as it is about what a species evolved to eat. And just as fruits ripen, leaves flush and flowers bloom predictably at different times of the year, foods available to our ancestors varied over deep time as the world changed around them from warm and wet to cool and dry and back again. Those changes are what drove our evolution.

Even if we could reconstruct the precise nutrient composition of foods eaten by a particular hominin species in the past (and we can't), the information would be meaningless for planning a menu based on our ancestral diet. Because our world was ever changing, so, too, was the diet of our ancestors. Focusing on a single point in our evolution would be futile. We're a work in progress. Hominins were spread over space, too, and those living in the forest by the river surely had a different diet from their cousins on the lakeshore or the open savanna.

What was the ancestral human diet? The question itself makes no sense. Consider some of the recent hunter-gatherers who have inspired Paleolithic diet enthusiasts. The Tikiġaġmiut of the north Alaskan coast lived almost entirely on the protein and fat of marine mammals and fish, whereas the Gwi San in Botswana's Central Kalahari took something like 70 percent of their calories from carbohydrate-rich, sugary melons and starchy roots. Traditional human foragers managed to earn a living from the larger community of life that surrounded them in a remarkable variety of habitats, from near-polar latitudes to the tropics. Few other mammalian species can make that claim, and there is little doubt that dietary versatility has been key to the success we've had.

Many paleoanthropologists today believe that increasing climate fluctuation through the Pleistocene sculpted our ancestors—whether their bodies or their wit, or both—for the dietary flexibility that has become a hallmark of humanity. The basic idea is that our ever changing world winnowed out the pickier eaters among us. Nature has made us a versatile species, which is why we can find something to satiate us on nearly all its myriad biospheric buffet tables. It's also why we have been able to change the game, transition from forager to farmer, and really begin to consume our planet.