He had the powerful jaws and big chompers to crack the toughest of shells, but a new study has shown that the ancient human relative known as "Nutcracker Man" actually preferred to munch on grass.
"It most likely was eating grass, and most definitely was not cracking nuts," said University of Utah geochemist Thure Cerling, lead author of the study published in the Proceedings of the National Academy of Sciences.
With molars about triple the size of a modern man's, the human ancestor, Paranthropus boisei, is believed to have roamed the Earth between 1.2 million and 2.3 million years ago.
A skull found in Tanzania in 1959 quickly drew the nickname "Nutcracker Man" because of its giant teeth, but US and Kenyan researchers now say the species grazed on the same wild fields as the ancestors of zebras, pigs and hippos.
"They were competing with them," said Cerling. "They were eating at the same table."
Researchers used a drill to pulverize tooth enamel, taken from already broken tooth samples from 22 individuals who lived in that period, and examined carbon isotope ratios that revealed what kind of food they were eating.
They could see that the specimens ate tropical grasses and herbs that use C4 photosynthesis, and not leaves, nuts and fruits that use C3 photosynthesis, the study said.
Only one similar diet has been seen in an extinct species of grass-eating baboon, the researchers said.
"The high proportion of C4 vegetation in the diet of Paranthropus boisei makes it different from any other hominin to date," said co-author Kevin Uno of the University of Utah.
"The results make an excellent case for isotope analysis of teeth from other members of our family tree, especially from East Africa."