WASHINGTON, June 4 (Xinhua) -- Human ancestors expanded their menu 3.5 million years ago to include tropical grasses and sedges, setting the stage for our modern diet of grains, grasses, and meat and dairy from grazing animals, researchers reported in the journal Proceedings of the National Academy of Sciences Tuesday.
In four new studies of carbon isotopes in fossilized tooth enamel from scores of human ancestors and baboons in Africa from 4 million to 10,000 years ago, a team of two dozen researchers found a surprise increase in the consumption of grasses and sedges, plants that resemble grasses and rushes but have stems and triangular cross sections.
Prior to about 4 million years ago, Africa's hominids, or early humans, were eating essentially chimpanzee style, likely dining on fruits and some leaves, said Professor Matt Sponheimer from the University of Colorado who led one of the studies.
He said that despite the fact grasses and sedges were readily available back then, the hominids seem to have ignored them for an extended period.
"We don't know exactly what happened," said Sponheimer in a statement. "But we do know that after about 3.5 million years ago, some of these hominids started to eat things that they did not eat before, and it is quite possible that these changes in diet were an important step in becoming human."
The isotope method cannot distinguish what parts of grasses and sedges human ancestors ate -- leaves, stems, seeds and-or underground storage organs such as roots or rhizomes.
The method also can't determine when human ancestors began getting much of their grass by eating grass-eating insects or meat from grazing animals. Direct evidence of human ancestors scavenging meat doesn't appear until 2.5 million years ago, and definitive evidence of hunting dates to only about 500,000 years ago.
Researchers said the earliest human ancestor that consumed substantial amounts of grassy foods may signal a major and ecological and adaptive divergence from the last common ancestor we shared with African great apes. Changes in diet have been linked to both larger brain size and the advent of upright walking in human ancestors, they noted.
"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 Thure Cerling from the University of Utah who co-wrote two of the studies.
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