Human Ancestors Grazed on Grass

Around 4 million years ago, our ancestors’ diets were about 90 percent fruit and leaves, but suddenly incorporated grasses 500,000 years later

Some of the toothy fossils that clued researchers in on our ancestor’s grass-eating tendencies.
Some of the toothy fossils that clued researchers in on our ancestor’s grass-eating tendencies. National Museums of Kenya, Mike Hettwer

Around 3.5 million years ago, human ancestors became less ape-like in their diet, supplementing leaves and fruit with grass and sedge, according to new research published in four new studies in Proceedings of the National Academy of Sciences. The authors think that this transition helped to eventually turn us onto some of the foods we most enjoy today—grains, vegetables and meat from grazing animals. NPR reports:

What the team looked at specifically were the amounts of certain isotopes of carbon that get taken up from our food and deposited in our teeth. These isotopes reveal what we and our ancestors were eating.

The researchers examined 173 teeth from 11 species of hominins, which include human ancestors and extinct relatives.

What the tale of the teeth reveals is this: About 3.5 million years ago, our ancestors started switching from the ape diet — leaves and fruit — to grasses and grass-like sedges. In the terminology, they switched from C3 plants to C4 plants.

Around 4 million years ago, our ancestors ate about 90 percent fruit and leaves, a diet nearly identical to that of chimps. But 1.4 million years ago, grasses made up around 55 percent of some Homo diets.

This switch may mirror changes that were going on in the local environment. Around 10 million years ago, NPR reports, Africa’s forests began thinning into grassy savannas. Over millions of years, animals that lived there, including hominins, adapted, switching to a diet predominantly composed of grass. Some dietary questions remain, NPR reports:

Now, one thing this carbon isotope technique can’t tell is whether Australopithecus just grazed like a bunch of antelope, or whether they ate the antelope that did the grazing. The carbon signal from the C4 plants gets taken up in animal (or insect) tissue and passed on to whoever eats that tissue (thus, when we eat chicken, we’re pretty much eating corn).

By 10,000 years ago—a blink in evolutionary time—Homo sapiens’ teeth give away a diet split neatly between trees and grasses, and also most likely included tree and grass-eating animals. This 50-50 diet is almost identical to that of modern North Americans, the authors write.

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