Vegetarianism proved literally hard to swallow for early humans, according to new research that bolsters evidence our ancestors likely veered from this lifestyle around 2.6 million years ago in favor of eating raw red meat and starch-rich plants.

The study, published in the journal Nature, suggests that meat and tools, and not the later advent of cooking, freed early humans to evolve smaller chewing-related features, such as smaller teeth and smaller, shorter faces. These, in turn, might have paved the evolutionary way for improved speech, thermoregulation and even the development of a bigger brain.

"No one knows for sure why hominins started to eat more meat around 2.6 million years ago, but there is abundant evidence for this behavior, including stone tools and cut marks on bones," lead author Katherine Zink of Harvard University's Department of Human Evolutionary Biology told Discovery News.

What Did Prehistoric Humans Eat?

"The most common explanation is climate change," she added. "During this period, Africa became more open grassland, with more antelopes and other herbivores."

This early Paleo diet also included zero seafood.

She explained, "The ability to fish appears to have come much later in human evolution. Shellfish consumption also appears to be relatively recent."

The study was prompted, in part, by a paradox that has puzzled researchers for years. By the time of Homo erectus about 2 million years ago, humans had evolved bigger brains, bodies and presumably appetites, but their teeth and gut were smaller than those of earlier ancestors. Zink and co-author Daniel Lieberman suspected that mechanical processing of food, as well as the addition of red meat into the diet, could explain the seeming paradox.

Early Humans Skipped Fruit, Went for Nuts

To test this out, the researchers fed adult test subjects samples of goat meat (one of the chewier red meats) and jewel yams, carrots and beetroots (to stand in for the starchy plant storage organs that early humans ate). They measured the muscular effort required for chewing and how well the food was broken up before swallowing.

The scientists found that pounding the plant material with stone tools and eating a diet composed of one-third sliced red meat reduced the need to chew by 17 percent and lowered needed force by 26 percent.

Zink and Lieberman point out that prior archaeological evidence indicates early humans fabricated stone tools by 3.3 million years ago, but did not learn to control fire until around 1 million years ago. Evidence for cooking on a regular basis dates to at least 500,000 years ago, long after evolutionary selection for smaller human teeth began.

Butchery tools and bones of animals associated with Heidelberg Man suggest that this human hunted -- among other animals --- hippos, rhinos and megaloceros, one of the largest deer that ever lived. Tim Evanson, Wikimedia Commons

As for why there was (and still is, among African foragers) dietary emphasis on underground starchy plants, the researchers explained that humans emerged at a time when the environment in Africa was drying and juicy, sweet fruits were less plentiful.

"During this time," Lieberman said, "many of the forests thinned and transitioned into more open grasslands."

He and Zink added that starchy underground plants are much more plentiful in grasslands than the fruit/tree-based foods of most chimpanzees, and possibly also of the last common ancestor of chimps and humans.

Kennewick Man Ate Mostly Seafood

Anthropologist David Strait of Washington University in St. Louis told Discovery News that the study inspires many follow-up questions, such as how tooth mechanics affect the processing of meat, tubers, bulbs and corms. Strait also wonders if particular tooth shapes, sizes and configurations are better able to resist being fractured when eating such foods.

Matt Sponheimer, an anthropologist at the University of Colorado at Boulder and director of the Nutritional and Isotopic Ecology Lab, believes the new paper "does a nice job broadening the recent debate about changes in the masticatory morphology (anatomy related to chewing) of Homo erectus by moving beyond the effects of cooking, and by taking a deeper look at mechanical processing -- slicing, pounding, etc. -- of foods."

"That mechanical processing of foods can lead to a net energetic benefit should be intuitively appealing to most people, or at least to people who have spent much time in the kitchen!" Sponheimer added. "I'm more of a microwave guy myself, but as we process foods in my lab and in the field all the time, this is pretty obvious."

Paleolithic Hunter-Gatherers Loved Oatmeal Too

So should dieters take their cue from this original paleo diet of red meat and tubers?

The authors caution against the idea that there is an optimal diet now for everyone. Later humans in places like Europe and Asia ate tons of fish, for example, with some living to ripe old ages.