Washington - Some 3 million years ago, the ancestors of modern humans already were climbing down from the trees and gathering food on nearby grasslands, perhaps even catching and eating small animals, a new study shows.
In a report in the journal Science, researchers say that the chemistry of tooth enamel from the remains of hominids call Australophithecus africanus shows that the ape-like creatures were eating a variety of foods.
This means that “africanus was not just a tree dweller,” said Matt Sponheimer, a researcher at Rutgers University.
“Our study shows that this early human ancestor found its food both in the forest and in the open areas,” said Sponheimer, co-author of the study in Science. He said the africanus may have been hunting small animals even before the development of Stone Age weapons and tools.
Earlier studies, based on bones in the jaw, arms, hands and feet, had suggested the species subsisted only on leaves and fruits plucked from forests - - rather like the diet of present-day chimpanzees, the nearest animal relative to modern humans.
But Sponheimer said the tooth chemistry study proves the species also hunted and gathered in the open areas where grass grew. For some specimens, up to 25 percent of the diet came from grass or grass-eating animals, said Sponheimer. “For one specimen, it was 50 percent.”
The study makes clear the early hominids were eating something different from what researchers had assumed, said John Kingston, a Yale University expert on early human-like species.
“Among the things they could have been eating is meat, which is a fascinating possibility,” Kingston said.
But the study did not determine conclusively if the species ate meat from grass-eating animals or merely the seeds and roots of grassland plants, which would give the same tooth-enamel signature, Kingston said.
Fossils of the africanus have been found only in South Africa. It lived about 3 million years ago and resembles Lucy, the famed, nearly complete hominid skeleton found in Ethiopia in North Africa. Lucy, however, is slightly more ape- like and is identified as A. afarensis, an earlier human ancestral species.
Africanus was not much bigger than a chimpanzee, at about 4 feet and 90 pounds, but had a slightly larger brain. It walked on its hind legs, but some specimens suggest an opposing big toe rather like a tree-climbing ape, said Sponheimer.
This feature, along with teeth that resemble those of apes, have led other researchers to conclude the species lived in the trees and ate only plants.
In their study, Sponheimer and co-author Julia A. Lee-Thorp of the University of Cape Town in South Africa extracted tiny bits of tooth enamel from the specimens and then analyzed the carbon isotope ratios.
Isotopes are atoms of the same element that have a different number of neutrons. For instance, trees and bushes contain more carbon-13 than do grasses that grow in open areas. The isotopic ratios in the enamel reflect which of these plants the animals ate. Meat-eaters acquire the isotopic signatures of their prey.
“In effect,” said Sponheimer, “you are what you eat.”
For this reason, if the species ate animals that fed on grasses, their tooth enamel would show the carbon-13 signature of grassland plants, he said.
The specimens analyzed had carbon-13 signatures between that of forest animals and plains-dwelling grass-eaters.
That indicates the species ate foods from both the forests and the plains. It also suggests they fed on high-protein animal foods, perhaps insects or small mammals, that lived on grasses.
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