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.
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"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.
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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.