Applying a new method of analyzing the chemistry of tooth enamel, scientists have examined molars of prehuman ancestors who lived in Africa 3 million years ago and determined that they had more varied diets than had been supposed.
They dined not only on fruits and leaves, as modern chimpanzees do, but might also have been meat eaters.
In a study reported in today's issue of the journal Science, anthropologists said their tests of the carbon content of teeth of Australopithecus africanus indicate that they ate large quantities of food rich in carbon 13 -- such as grasses and sedges -- or animals that ate these plants, or both.
Specialists in human evolution said the findings could modify assumptions about the behavior and diet of human ancestral species, hominids, before the emergence about 2.5 million years ago of the lineage that led to Homo sapiens.
The research indicated that the australopithecines, which walked upright but also climbed trees, were venturing out of their usual forest habitat to forage in open grasslands. It also suggested that hominids were consuming high-protein animal foods before the development of stone tools for butchering.
The journal, in a separate article commenting on the research, noted that many theories of human origins invoke a switch to a meat-rich diet to explain the sudden expansion of brain size with the first Homo species. No stone tools have been found associated with pre-Homo hominids.
The tooth analysis was conducted by Matt Sponheimer, a graduate student at Rutgers University, working with Dr. Julia A. Lee-Thorp, a paleoanthropologist at the University of Cape Town in South Africa. Lee-Thorp was a pioneer in developing techniques for sampling tooth enamel for carbon clues to diets of fossil species.
Sponheimer said it isn't certain that the australopithecines were eating meat. But if they were getting the telltale carbon only from eating grasses, he said, the teeth should have scratches caused by the wear from silica associated with wild grasses, and they do not.
If they were eating meat, it probably came from small animals that could be caught without tools or the scavenged remains of meals left by large predators. Another source could have been grass-eating insects such as grasshoppers, which are a favorite of today's baboons.
"Whatever they were doing, they were doing a lot of it," Sponheimer said in a telephone interview, referring to the australopithecines.
One of the four molars tested, he said, contained evidence that half of the hominid's diet consisted of grasses or meat from grass-eating animals, or a combination of the two. Two others indicated a 25 percent grass-based diet.
"So this is not a case of an occasional taste of a blade of grass," Sponheimer said. "This is habitual behavior."
Chimpanzees sometimes vary their fruit-and-leaf diets by eating monkeys, but monkey meat would not have left the same carbon signature in the teeth.
Sponheimer said the dietary findings could be clues to a critical transition in the lives of hominids 3 million years ago.
Environmental pressures might have started them moving out of the trees and forests and more and more into open land. In their survival strategies, they seem to have been adapting to the behavior of less-specialized eaters.
"Maybe their hearts and homes were still in the trees," Sponheimer said, "but their bellies were tied to the open areas."
Pub Date: 1/15/99