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The real Paleolithic diet, though, wasn’t all meat and marrow. It’s true that hunter-gatherers around the world crave meat more than any other food and usually get around 30 percent of their annual calories from animals. But most also endure lean times when they eat less than a handful of meat each week. New studies suggest that more than a reliance on meat in ancient human diets fueled the brain’s expansion.

Year-round observations confirm that hunter-gatherers often have dismal success as hunters. The Hadza and Kung bushmen of Africa, for example, fail to get meat more than half the time when they venture forth with bows and arrows. This suggests it was even harder for our ancestors who didn’t have these weapons. “Everybody thinks you wander out into the savanna and there are antelopes everywhere, just waiting for you to bonk them on the head,” says paleoanthropologist Alison Brooks of George Washington University, an expert on the Dobe Kung of Botswana. No one eats meat all that often, except in the Arctic, where Inuit and other groups traditionally got as much as 99 percent of their calories from seals, narwhals, and fish.

So how do hunter-gatherers get energy when there’s no meat? It turns out that “man the hunter” is backed up by “woman the forager,” who, with some help from children, provides more calories during difficult times. When meat, fruit, or honey is scarce, foragers depend on “fallback foods,” says Brooks. The Hadza get almost 70 percent of their calories from plants. The Kung traditionally rely on tubers and mongongo nuts, the Aka and Baka Pygmies of the Congo River Basin on yams, the Tsimane and Yanomami Indians of the Amazon on plantains and manioc, the Australian Aboriginals on nut grass and water chestnuts.

“There’s been a consistent story about hunting defining us and that meat made us human,” says Amanda Henry, a paleobiologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig. “Frankly, I think that misses half of the story. They want meat, sure. But what they actually live on is plant foods.” What’s more, she found starch granules from plants on fossil teeth and stone tools, which suggests humans may have been eating grains, as well as tubers, for at least 100,000 years—long enough to have evolved the ability to tolerate them.

The notion that we stopped evolving in the Paleolithic period simply isn’t true. Our teeth, jaws, and faces have gotten smaller, and our DNA has changed since the invention of agriculture. “Are humans still evolving? Yes!” says geneticist Sarah Tishkoff of the University of Pennsylvania.

One striking piece of evidence is lactose tolerance. All humans digest mother’s milk as infants, but until cattle began being domesticated 10,000 years ago, weaned children no longer needed to digest milk. As a result, they stopped making the enzyme lactase, which breaks down the lactose into simple sugars. After humans began herding cattle, it became tremendously advantageous to digest milk, and lactose tolerance evolved independently among cattle herders in Europe, the Middle East, and Africa. Groups not dependent on cattle, such as the Chinese and Thai, the Pima Indians of the American Southwest, and the Bantu of West Africa, remain lactose intolerant.

Humans also vary in their ability to extract sugars from starchy foods as they chew them, depending on how many copies of a certain gene they inherit. Populations that traditionally ate more starchy foods, such as the Hadza, have more copies of the gene than the Yakut meat-eaters of Siberia, and their saliva helps break down starches before the food reaches their stomachs.