Our ancestors used to dine almost exclusively on leaves and fruits from trees, shrubs and herbs until 3.5 million years ago when a major shift occurred, according to four new simultaneously published studies.

During this shift, early human species like Australopithecus afarensis and Kenyanthropus platyops began to also feast on grasses, sedges and succulent plants — or on animals that ate those plants — the studies, published in the latest Proceedings of the National Academy of Sciences, conclude.

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“What we have is chemical information on what our ancestors ate, which in simpler terms is like a piece of food item stuck between their teeth and preserved for millions of years,” said Zeresenay Alemseged, senior curator and chair of anthropology at the California Academy of Sciences and a co-author on two of the papers, was quoted as saying in a press release.

Alemseged and the other researchers found the “chemical information” in ancient teeth from our early human ancestors.

They explained that teeth contain isotopes that lock in information about what the individual ate. Here’s how that works: Plants can be divided into three categories based on their method of photosynthesis: C3, C4 and CAM. C3 plants (trees, shrubs, and herbs) can be chemically distinguished from C4/CAM plants (grasses, sedges, and succulents) because the latter incorporate higher amounts of the heavier isotope carbon-13 into their tissues. When the plants are eaten, the isotopes become incorporated into the consumer’s  tissues. These include the enamel of developing teeth.

Demonstrating the sturdiness of well-preserved teeth in the fossil record, the relative amounts of carbon-13 in such teeth can be read by scientists millions of years after the individual’s demise. Your veggie lifestyle, or not, is therefore locked into your teeth seemingly forever.

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The 4 new papers, Alemseged said, “present the most exhaustive isotope-based studies on early human diets to date. Because feeding is the most important factor determining an organism’s physiology, behavior and its interaction with the environment, these finds will give us new insight into the evolutionary mechanisms that shaped our evolution.”

The findings raise some interesting questions:

Were our ancestors broadening their vegetarian diet 3.5 million years ago, or were they becoming carnivorous?

What caused the shift?

An intriguing clue goes back to an earlier paper Alemseged worked on. He and his team found tools for meat consumption dating back to 3.4 million years ago. My guess is that improved technology and perhaps environmental changes led to our becoming more omnivorous then.

We can’t, however, rule out a less hunting-based change, given that possible C4/CAM-derived meals include grass seeds and roots, sedge underground stems, termites, succulents or even scavenged carcasses.

No matter how the nutrients were acquired, these same sources of energy are important to our diets today. People who enjoy mashed potatoes with corn are practicing a 3.5 million-year-old habit.

Image: tarale, Flickr