Human Ancestors' Grassy Diet: Fossilized Teeth Pinpoint Shift 3.5 Million Years Ago
New research shows that the diets of human ancestors expanded 3.5 million years ago to include grasses and possibly meat.
Before their diets expanded, hominins, a human ancestor, had diets similar to gorillas and chimps, eating leaves and fruit.
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Four new papers, published in the journal Proceedings of the National Academy of Sciences, came to this conclusion by analyzing early human teeth from fossils. The researchers studied the tooth enamel from 175 hominins of 11 species, along with other primates found in East Africa.
"Until about 4 million years ago our early hominin ancestors had diets that were, isotopically at least, very similar to chimpanzees'," said Matt Sponheimer, a paleoanthropologist at the University of Colorado at Boulder and co-author of one of the papers. "They started eating new things, using the landscape in new ways around 3.5 million years ago. It's very possible that it was one of the important steps in the diversification of our lineage."
The carbon in the tooth enamel is the key to finding out what early human ancestors ate. One type of carbon shows evidence of a diet of trees and bushes, while another type of carbon -- the carbon researchers found -- shows evidence from grasses and sedges, which are grass-like plants that grow in wet areas.
"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, from the California Academy of Sciences, co-author of two of the PNAS papers.
"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."
Certain carbon isotopes may indicate that the human-like creatures ate animals. But the researchers aren't sure if the isotopes are evidence of the hominins eating certain plants, or if they're evidence of eating the animals who ate those plants. It could also be a combination of the two.
At present, the earliest that early man is known to have eaten meat was 2.5 million years ago, with hunting only appearing some 500,000 years ago.
Another major question that the studies can't quite answer is why hominins started including grasses and possibly meat in their diets.
"All these species who were once in the human lineage, ventured out into this new world of foods 3.5 million years ago," said Thure Cerling from the University of Utah, co-authors of one of the papers, "but we don't yet understand why that is."
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