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New research by University of Utah geoscientists could overturn long-held views about an early human relative's dietary choices in the East African savannah.

The teeth of the hominid Paranthropus boisei, first discovered by Mary Leakey in 1959 in Tanzania, are so big and flat, and the jaw so strong, that anthropologists wondered if the creature ate hard foods plucked from trees and nicknamed the species "Nutcracker Man."

That powerful face, however, was more likely built for chewing grass like a cow, according to the U.'s Thure Cerling.

Isotope analysis of fossil teeth proves this species was nourished on tropical grasses and other so-called C4 plants, rather than the trees and shrubs believed to have supported most primate species, according to Cerling's study published Monday online in the Proceedings of the National Academy of Sciences.

"You would see a lot more dental damage if they were really cracking nuts. What caught me was the heavy wear on the teeth from grinding a low quality food. That meant they were eating a lot of it," said Arizona State University anthropologist Donald Johanson, who did not participate in the study but is familiar with Cerling's work.

The findings raise new questions about the evolution, physiology and diets of early humans, said co-author Kevin Uno, a doctoral candidate in geology. The authors and others believe this finding builds a case for conducting isotope studies, which involve grinding minute samples from priceless specimens, on other hominid species that occupied Africa up to 5 million years ago.

"This drill is a tiny thing. It extracts milligrams of material, but it's a big payoff to know about the diets of these early hominids," said Johanson, renowned for his discovery of the earlier hominid called Lucy.

Over the past 20 years, Cerling has perfected ways to determine what ancient animals ate by analyzing ratios of carbon isotopes in tooth enamel, both in terms of effectiveness and of being less destructive to fossil specimens.

"We need to keep looking at what we think we know. We have to try new approaches," said Cerling, a U. professor of both geology and biology. Enamel rich in carbon-13 indicates a diet of so-called C3 plants like trees and shrubs, while the presence of the lighter carbon-12 atom indicates a food source arising from tropical grasses and other C4 plants.

Using this analysis, Cerling and Uno recently published a study that revealed the time frames of when East Africa's various large herbivores switched their diets from leaves to grasses. The new study shows an important human relative developed a dietary niche that put them in competition with four-legged grazers, like ancestors of hippos and horses.

"They were eating at the same table," Cerling said.

P. boisei inhabited East Africa from 2.3 to 1.2 million years ago. Cerling's team examined 24 teeth from 22 individuals, whose ages range between 1.9 and 1.4 million years, recovered from various sites stretching from Tanzania to Ethiopia. They chose specimens, held by the National Museums of Kenya, from broken teeth that are less valuable to science, but would still yield worthy material to study.

The analysis showed their diet was, on average, three-fourths grass, from a low of 61 to a high of 91 percent.

"What surprised me was that over this length of time and space it was eating a restricted diet. It was extremely specialized and for a long time," Cerling said.

Monday's paper had University of Arkansas anthropologist Peter Ungar "grinning ear to ear" because it affirmed his theories about P. boisei based on physical examination of the big occlusal or chewing surfaces of its teeth, which revealed wispy scratches that are hardly consistent with hard foods.

"We weren't sure what that diet was. They answered the question of what are the not-hard foods these guys were eating," Ungar said.

The study's authors included Emma Mbua, Francis Kirera and Fredrick Manthi of National Museums of Kenya; Frederick Grine of Stony Brook University; and anthropologists Matt Sponheimer of the University of Colorado and Meave Leakey.