In four new studies of carbon isotopes in fossilised 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.
"At last, we have a look at 4 million years of the dietary evolution of humans and their ancestors," said University of Utah geochemist Thure Cerling, principal author of two of the four new studies published in the journal Proceedings of the National Academy of Sciences.
Grassy savannas and grassy woodlands in East Africa were widespread by 6 million to 7 million years ago. It is a major question why human ancestors didn't seriously start exploiting savanna grasses until less than 4 million years ago.
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 5,00,000 years ago.
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"Diet has long been implicated as a driving force in human evolution," said Matt Sponheimer, a University of Colorado, Boulder anthropologist, former University of Utah postdoctoral fellow and lead author of the fourth study.
Sponheimer noted that changes in diet have been linked to both larger brain size and the advent of upright walking in human ancestors roughly 4 million years ago.