Early human ancestors may have been grass-fed, competing directly with grazing animals such as the forebears of zebras and hippos. The findings, from a research team at the University of Utah, could mean a rethink of early human diets.

The study was done on the teeth of a creature called Paranthropus boisei, a primate that lived between 2.3 million and 1.2 million years ago in Africa. For decades, scientists thought that the large, heavy teeth the primates had were used in cracking open hard foods such as nuts. The common name for Paranthropus was Nutcracker Man for this very reason.

A group at the University of Utah took a new look at the enamel on Nutcracker Man's teeth. They analyzed the ratios of carbon isotopes in the enamel. Those ratios tell what kind of photosynthesis was happening in the plants the primates ate. One type, called C3 photosynthesis, is done by trees, shrubs and herbs. It leaves its carbon signature in the fruits and nuts trees produce. The other is called C4, and is the type that grasses and sedges do.

Analysis of the tooth enamel was a surprise: instead of the signature of C3 photosynthesis, the researchers found evidence of C4 photosynthesis. That means it is more likely that Paranthropus boisei was eating grasses and sedges -- about 77 percent of their diet was C4 plants such as grasses, ranging from a low of 61 percent to a high of 91 percent.

A similar experiment was done in 2008, but it only used two teeth. This study used teeth from 22 individuals, making the findings much more robust.

Lead study author Thure Cerling, a professor of geology, geophysics and biology, notes that there is no modern primate that eats primarily C4 plants. Though there is a baboon species that eats them on occasion, it eats C3 grasses. No modern great ape uses sedges (or grasses) as its primary food source.

Kevin Uno, a doctoral student in geology and a co-author, said the findings make a good case for doing similar studies on the teeth of other human ancestors. Among Paranthropus boisei's close relatives are the Australopithecines, which also gave rise to humans. An interesting question is when did this occur, Uno said. One thing we're really interested in doing is moving back up the lineage.

Uno notes that it is possible that this branch of the human family tree -- which ended in extinction -- might have been one that filled a particular niche -- feeding on certain types of plants. When the plants disappeared as the climate changed, Parathropus boisei may have gone with them. A real question is why this trait did not persist in other primate species. Modern humans eat C4 plants, but many of them are domesticated (corn is one example). Wild C4-eaters don't seem to be around.

It will take studying more teeth to reach any firm conclusions. The next step is getting them, and that may prove no easy task. You go to the [Natural History Museum] of Kenya and the teeth are in a vault, Uno said. It's the only air-conditioned room in the museum. The reason is that the teeth of primates are rare and hold very valuable information.

The work was published in the May 2 online edition of the journal Proceedings of the National Academy of Sciences.