About 3.5 million years ago, our early human ancestors stepped out of the forest, put down their tasty fruits and leaves, and — for whatever reason — started noshing on blades of grass. While that moment might not sound like history in the making, it played a crucial role in shaping our forebears' evolutionary path forward.

By changing the way they snacked, hominins like Australopithecus afarensis and Kenyanthropus platyops essentially eschewed the forest-based diet that gorillas and chimps still eat today, which may have give them the freedom to start wandering through Africa's open savannahs.

Evidence of this dietary shift — which also included sedges and possibly animals that ate those same plants — is the central focus of four new studies published in the Proceedings of the National Academy of Sciences. They take a close look at the fossil records of 11 different species of early humans and other primates in East Africa to hone in on when exactly this shift happened.

"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," says Zeresenay Alemseged, author of two of the papers and senior curator and chair of anthropology at the California Academy of Sciences. Discovery News explains:

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. [Discovery News]

It's unclear if these proto-humans were eating protein-rich meat yet, or if they were still largely vegetarians. "We know that many early hominins lived in areas that would not have readily supported chimpanzees with their strong preference for forest fruits," Dr. Matt Sponheimer, lead author of one of the other papers, tells BBC News. "It could also be argued that this dietary expansion was a key element in hominin diversification."

Nevertheless, this seemingly inconsequential lifestyle change seems to have set the evolutionary stage for a fearsome new species of endlessly curious, bipedal apes to roam the planet. Today, we call said creatures Homo sapiens.