Human Ancestors' Ape-like Diet Changed 3.5 Million Years Ago to Grass

First Posted: Jun 04, 2013 08:25 AM EDT

Feel like eating some grass? Didn't think so--but our ancient ancestors did. About 3.5 million years ago, our human forebears added tropical grasses and sedges to an ape-like diet of leaves and fruits from trees and shrubs. This change, in particular, helped set the stage for our modern diet of grains, grasses and meat and dairy from grazing animals.

About six to seven million years ago, the grassy savannas and woodlands in East Africa spread out across vast swathes of land. Yet while the abundant grass was a valuable resource to herd animals and other creatures, humans didn't take advantage of the food source. Instead, they continued to stick to their usual, ape-like diet--until about 3.5 million years ago.

In order to learn more about our human ancestors, researchers examined carbon isotopes in fossilized tooth enamel from scores of human forebears and baboons in Africa from about four million to 10,000 years ago. In the end, the scientists found a surprising increase in the consumption of grasses and sedges.

"At last, we have a look at four million years of the dietary evolution of humans and their ancestors," said geochemist Thure Cerling, principal author of two of the four new studies detailing the findings, in a news release. "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."

While the scientists were able to find out that our ancestors consumed grasses, the isotope method couldn't distinguish what parts of the grasses and sedges that humans ate. It could have been the stems, seeds, leaves or even the roots. Yet it does show that humans were entering a crucial period in the evolution of their diet, paving the way to our modern diet of today.

So why do these findings matter so much? The earliest ancestors that consumed substantial amounts of grass foods may signal a major and adaptive divergence from the last common ancestor we shared with African great apes. Changes in diet have been linked to larger brain size and the advent of upright walking, which means that the shift could have signaled the advent of modern humans.

"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," when it was believed human ancestors ate mostly leaves and fruits, said Cerling in a news release.

The findings are published in four different papers in the Proceedings of the National Academy of Sciences. You can find them here, here, here and here.

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