Vegetarian, vegan and raw diets can be healthy — likely far healthier than the typical American diet. But to continue to call these diets "natural" for humans, in terms of evolution, is a bit of a stretch, according to two recent, independent studies.

Eating meat and cooking food made us human, the studies suggest, enabling the brains of our prehuman ancestors to grow dramatically over a period of a few million years.

Although this isn't the first such assertion from archaeologists and evolutionary biologists, the new studies demonstrate, respectively, that it would have been biologically implausible for humans to evolve such a large brain on a raw, vegan diet and that meat-eating was a crucial element of human evolution at least 1 million years before the dawn of humankind.

Shhh, don't tell the gorillas

At the core of this research is the understanding that the modern human brain consumes 20 percent of the body's energy at rest, twice that of other primates. Meat and cooked foods were needed to provide the necessary calorie boost to feed a growing brain. [10 Things You Didn't Know About the Human Brain]

One study, published last month in the journal Proceedings of the National Academy of Sciences, examined the brain sizes of several primates. For the most part, larger bodies have larger brains across species. Yet human have exceptionally large, neuron-rich brains for our body size, while gorillas — three times more massive than humans — have smaller brains and three times fewer neurons. Why?

The answer, it seems, is the gorillas' raw, vegan diet (devoid of animal protein), which requires hours upon hours of eating only plants to provide enough calories to support their mass.

Researchers from Brazil, led by Suzana Herculano-Houzel, a neuroscientist at the Federal University of Rio de Janeiro in Brazil, calculated that adding neurons to the primate brain comes at a fixed cost of approximately six calories per billion neurons.

For gorillas to evolve a humanlike brain, they would need an additional 733 calories a day, which would require another two hours of feeding, the authors wrote. A gorilla already spends as much as 80 percent of the tropic's 12 hours of daylight eating.

Similarly, early humans eating only raw vegetation would have needed to munch for more than nine hours a day to consume enough calories, the researchers calculated. Thus, a raw, vegan diet would have been unlikely given the danger and other difficulties of gathering so much food.

Cooking makes more foods edible year-round and releases more nutrients and calories from both vegetables and meat, Herculano-Houzel said.

"The bottom line is, it is certainly possible to survive on an exclusively raw diet in our modern day, but it was most likely impossible to survive on an exclusively raw diet when our species appeared," Herculano-Houzel told LiveScience.

The study puts an upper limit on how big a brain is able to grow while on a premodern raw, vegan diet. But the researchers could not determine when daily cooking began. Was it about 250,000 years ago, when humans were nearly fully evolved with big brains, which is supported by archaeological findings; or was it about 800,000 years ago, when prehumans began their most dramatic brain-growth spurt, an era for which there is little archaeological evidence of controlled fires for cooking?

Meet the meat-eater

If cooking wasn't routine in the years before the dawn of modern humans, eating meat certainly was.

The second study, published in October the journal PLoS ONE, examined the remains of a prehuman toddler who died from malnutrition about 1.5 million years ago. Shards of a skull found in modern-day Tanzania reveal that the child had porotic hyperostosis, a type of spongy bone growth associated with low levels of dietary iron and vitamins B9 and B12, the result of diet lacking animal products in a species that requires them. [10 Mysteries of the First Humans]

The child was around the weaning age. So, either the child's mother's breast milk lacked key nutrients, or the child himself did not consume enough nutrients directly from meat or eggs.

Either way, the finding implies that meat must have been an integral, and not sporadic, element of the prehuman diet more than 1 million years ago, said the study's lead author, Manuel Domínguez-Rodrigo, an archaeologist at Complutense University in Madrid.

This supports the theory that meat fueled human brain evolution because meat — from arachnids to zebras — was plentiful on the African savanna, where humans evolved, and is the best package of calories, proteins, fats and vitamins B12 needed for brain growth and maintenance.

"Carnivore animals, whether terrestrial or aquatic, are bigger brained than herbivores," Domínguez-Rodrigo told LiveScience. And he added that "there is no [traditional] society that live as vegans," essentially because it wouldn't be possible to get vitamin B12, which is only available in animal products.

Vegetables still healthy

Both sets of researchers said their conclusion — that cooked food and meat were necessary for human brain development — is not a statement of how the human diet must have been, but rather how it likely was in order to make humans "human."

With supermarkets and refrigeration, humans today can and increasingly do eat a vegetarian or vegan diet year-round. And given the amount of heart-stopping saturated fats in factory-produced animal products, a plant-based diet can be healthier.

Yet both "extreme sides" of the meat argument — the unapologetic meat eater and the raw vegan — should remember that few so-called natural foods today were around as little as a few hundred years ago, from the modern invention called corn-fed beef to genetically altered strains of Queen Anne's lace called the carrot.

From health to the environment, there are many reasons to go vegetarian, go vegan and even go raw, but evolution isn't one of them.

Christopher Wanjek is the author of a new novel, "Hey, Einstein!", a comical nature-versus-nurture tale about raising clones of Albert Einstein in less-than-ideal settings. His column, Bad Medicine, appears regularly on LiveScience.