Reconstructions of human evolution are prone to simple, overly-tidy scenarios. Our ancestors, for example, stood on two legs to look over tall grass, or began to speak because, well, they finally had something to say. Like much of our understanding of early hominid behavior, the imagined diet of our ancestors has also been over-simplified.

Take the trendy Paleo Diet, which draws inspiration from how people lived during the Paleolithic or Stone Age, which ran from roughly 2.6 million to 10,000 years ago. It encourages practitioners to give up the fruits of modern culinary progress—such as dairy, agricultural products, and processed foods—and start living a pseudo-hunter-gatherer lifestyle, something like Lon Chaney Jr. in the film One Million B.C. Adherents recommend a very specific “ancestral” menu, replete with certain percentages of energy from carbohydrates, proteins, and fats, and suggested levels of physical activity. These prescriptions are drawn mainly from observations of modern humans who live at least a partial hunter-gatherer existence.

But from a scientific standpoint, these kinds of simple characterizations of our ancestors' behavior generally don’t add up. Recently, fellow anthropologist C. Owen Lovejoy and I took a close look at this crucial question in human behavioral evolution: the origins of hominid diet. We focused on the earliest phase of hominid evolution from roughly six to 1.6 million years ago, both before and after the first use of modified stone tools. This time frame includes, in order of appearance, the hominids Ardipithecus and Australopithecus, and the earliest members of our own genus, the comparatively brainy Homo. None of these were modern humans, which appeared much later, but rather our distant forerunners.

We examined the fossil, chemical, and archaeological evidence, and also closely considered the foraging behavior of living animals. Why is this crucial? Observing animals in nature for even an hour will provide a ready answer: Almost all of what an organism does on a daily basis is simply related to staying alive; that includes activities such as feeding, avoiding predators, and setting itself up to reproduce. That’s the evolutionary way.

What did our ancestors actually eat? In some cases, researchers can enlist modern technology to examine the question. Researchers study the chemical makeup of fossil dental enamel to figure out relative amounts of foods the hominid ate derived from woody plants (or the animals that ate them) versus open country plants. Other scientists look in ancient tooth tartar for bits of silica from plants that can be identified to type—for example, fruit from a particular plant family. Others examine the small butchering marks made on animal bones by stone tools. Researchers have found, for example, that hominids even 2.6 million years ago were eating the meat and bone marrow of antelopes; whether they were hunted or scavenged is hotly debated.