Human Ancestors Opted For a Grassier Diet 3.5 Million Years Ago

As human beings we have one of the most varied and exotic diets in all of the animal kingdom, and scientists may have finally narrowed in on when our meals began resembling the dishes they do today. According to four new studies, it was around 3.5 million years ago when our ancestors added grass to their diet, leading to the multitude of grains, meat, and dairy we see on our grocery store shelves today.

Early human ancestors generally ate leaves and fruits from trees and shrubs - a diet similar to the ones apes prefer. Evidence of hominins scavenging for meat doesn't appear until 2.5 million years ago, and it was only 500,000 years ago that humans have been definitively spotted hunting.

"At last, we have a look at 4 million years of the dietary evolution of humans and their ancestors," says University of Utah geochemist Thure Cerling, principal author of two of the four new studies

"For a long time, primates stuck by the old restaurants - leaves and fruits - and by 3.5 million years ago, they started exploring new diet possibilities - tropical grasses and sedges - that grazing animals discovered a long time before, about 10 million years ago" Cerling said. "Tropical grasses provided a new set of restaurants. We see an increasing reliance on this new resource by human ancestors that most primates still don't use today."

The four studies tested carbon isotopes from 173 teeth of 11 species of hominins to determine their diets. According to Matt Sponheimer, a University of Colorado, Boulder anthropologist, the technique is only around 20 years old and really pushes the idea of "you are what you eat." By looking at the carbon isotopes present in the teeth, the researchers could figure out what class of plant they were eating, but not exactly what plants were eaten, leaving the exact diet still a mystery.

The change in diet also correlates with a shifting human morphology.

"If diet has anything to do with the evolution of larger brain size and intelligence, then we are considering a diet that is very different than we were thinking about 15 years ago," Cerling explained.

While humans consume more grasses than other primates, they aren't the only ones who prefer them. Cerling goes on to show in one of his studies that two extinct Kenyan baboons also preferred grasses to trees and shrubs.

You can read the four published studies in the Proceedings of the National Academy of Sciences here, here, here, and here.