Almost 2 million years after their last meals, two members of a prehuman species in southern Africa left traces in their teeth of what they had eaten then, as well as over a lifetime of foraging. Scientists were surprised to find that these hominins apparently lived almost exclusively on a diet of leaves, fruits, wood and bark.

The research and recent studies suggest there was more diversity in the diets of early prehumans than previously understood. And this could in part account for the physical diversity among the long intermediate line of hominins belonging to the genus Australopithecus.

The dietary pattern of the enigmatic species, Australopithecus sediba, discovered four years ago in the Malapa caves of Johannesburg, was unexpected. It contrasted with data for other hominins in the region and elsewhere in Africa; they mainly consumed grasses and sedges from the savanna.

The Au. sediba diet also appeared to be a matter of choice, not necessity. Other evidence indicated the presence at the time of vast grasslands. Yet these hominins, their skeletons adapted for tree climbing as well as upright walking, chose to eat in adjacent woodlands.

In this, scientists said, their behavior was more like that of modern chimpanzees, which tend to ignore savanna grasses, or perhaps the more apelike hominin Ardipithecus ramidus, which lived largely on hard foods 4.4 million years ago.

A team of scientists led by Amanda Henry of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, reported the research online in the journal Nature, which will later print the findings.

Benjamin Passey, a geochemist at Johns Hopkins University, explained why the research was important to an understanding of human evolution. He said, "We eat grass in the form of the grains we use to make breads, noodles, cereals and beers, and we eat animals that eat grass. So when did our addiction to grass begin? ... We are simply trying to find out where in the human chain that begins."