Almost two 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. If you are what you eat, the new research and other recent studies suggest there was more diversity in the diets of early prehumans, both within and between species, than previously understood. And this could in part account for the recently recognized 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 northwest of Johannesburg, was unexpected for several reasons. It contrasted sharply with available 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 from animal fossils and sediments in the area indicated the presence at the time of vast grasslands in the vicinity. Yet these hominins, their skeletons adapted for tree climbing as well as upright walking, chose to feed themselves 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 some 4.4 million years ago.
An international team of scientists led by Amanda G. Henry of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, reported on Wednesday the research that supported their findings. Their paper was published online by the journal Nature and will appear later in a printed edition.
“If these individuals are representative of the species,” the scientists wrote, “Au. sediba had a diet that was different from those of most early African hominins studied so far.”
They also concluded that the “inferred consumption” of woodland products “increased the known variety of early hominin foods.” But there is still much that is unknown or unclear about the newfound species: how or if it is related to modern humans and just where it fits on the hominin family tree.
The discovery, by Lee Berger of the University of Witwatersrand in Johannesburg, of two partial skeletons — one an adult female, the other a juvenile male — was the basis for the announcement two years ago of the new hominin species. These and at least one other adult specimen indicate that these hominins stood little more than four feet tall and had small brains and a mix of primitive and more modern anatomies. Dr. Berger was an author of the new journal report.
Few other paleoanthropologists agree with Dr. Berger’s contention that the new species is the most plausible known ancestor of archaic and modern humans. Dr. Henry’s group said that studies of additional fossils from the Malapa caves “will provide a better understanding of the dietary ecology of Au. sediba.”
Ian Tattersall, a paleoanthropologist at the American Museum of Natural History in New York, who was not involved in the research, called the findings “intriguing” and the research “an imaginative and multisided approach that makes you want to know more about this morphologically unusual species.”
“Fortunately,” Dr. Tattersall added, “rumor has it that more specimens are on the way.”
Dr. Henry’s team followed three lines of research. One was an analysis of carbon isotopes extracted by laser from tooth enamel, one of the most durable and least contaminated body parts, and one that preserves chemical signatures of what was eaten in one’s youth. The type and amount of isotopes left from a diet of tree leaves, fruit and bark were well outside the range of those seen in all previously tested hominins — at least 95 percent forest food.
A second approach was an examination of dental microwear, which can reveal pits, scratches and cracks left by hard foods consumed shortly before death. Dr. Tattersall said that this “doesn’t help much to clarify the situation, since it appears to differ significantly between the two individuals.”
Finally, microscopic plant particles, called phytoliths, were recovered from dental tartar for the first time from a very ancient hominin (but from only one of the two individuals). Scientists said this apparently confirmed the carbon isotopic evidence for woodland diets.
Benjamin H. Passey, a geochemist at Johns Hopkins University, who conducted the tests determining the high ratio of carbon isotopes indicating a diet mostly of forest foods, explained why the research was important to an understanding of human evolution.
“One thing people probably don’t realize is that humans are basically grass eaters,” Dr. Passey said in a statement. “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? At what point in our evolutionary history did we start making use of grasses? We are simply trying to find out where in the human chain that begins.”