Most apes eat leaves and fruits from trees and shrubs¹（灌木）. New studies spearheaded by the University of Utah show that human ancestors expanded their menu 3.5 million years ago, adding tropical grasses and sedges to an ape-like diet and setting the stage for our modern diet of grains, grasses, and meat and dairy from grazing animals. In four new studies of carbon isotopes³ in fossilized tooth enamel⁴ from scores of human ancestors and baboons⁵ 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（莎草） -- plants that resemble grasses and rushes but have stems and triangular⁶ cross sections.

 

"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 published online June 3 by the journal Proceedings⁷ of the National Academy of Sciences. Most funding was from the National Science Foundation.

 

"For a long time, primates⁸ 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" when African savanna⁹（热带草原） began expanding, Cerling says. "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."

 

Grassy¹⁰ savannas¹¹ 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.

 

The isotope² 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 definitive¹² evidence of hunting dates to only about 500,000 years ago.

 

With the new findings, "we know much better what they were eating, but mystery does remain," says Cerling, a distinguished¹³ professor of geology and geophysics, and biology. "We don't know exactly what they ate. We don't know if they were pure herbivores（食草动物） or carnivores（食肉动物）, if they were eating fish [which leave a tooth signal that looks like grass-eating], if they were eating insects or if they were eating mixes of all of these."

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