The salad days of human evolution saw a dietary shift toward grasses and probably grass-fed animals, analyses of more than 100 fossilized teeth from eight species of ancient hominids indicate.
“These changes in diet have been predicted,” says paleoanthropologist Richard Klein of Stanford University. “But it’s very nice to have some data, and these data support it very strongly.”
Changes in the size and shape of jaws and teeth in both ancient hominids and their ape relatives point to changes in diet. The new study adds to these lines of anatomical evidence chemical analyses that look at different forms of carbon in the fossilized teeth.
The ratio of two types of carbon in tooth enamel reflects diet, says geochemist Thure Cerling of the University of Utah in Salt Lake City, who spent weeks in a vault in the National Museum of Kenya collecting milligram-sized samples of tooth enamel for the analyses.
Grasses, grasslike sedges and many other plants in hot, arid environments have evolved a trick that helps prevent water loss. The metabolic adjustment results in taking up more of a heavier form of carbon, known as carbon-13, than most trees and shrubs do.
The tooth studies, which cover more than 3 million years and include specimens from southern, eastern and central Africa, found greater quantities of this heavier carbon in hominids that are closer to humans on the evolutionary tree. This pattern suggests that, compared with humans’ more ancient relatives, recent ones were eating more grass or more grass-feeding animals, like zebras. The analyses appear June 3 in the Proceedings of the National Academy of Sciences.