Study: Stone tools used 3.4 million years ago

The bones appear to have been cut and smashed 3.4 million years ago, the first evidence of stone tool use by Australopithecus afarensis, the species best known for the fossil dubbed "Lucy," says researcher Zeresenay Alemseged.

"We are putting stone tools in their hands," said Alemseged, of the California Academy of Sciences, who reports the finding with colleagues in today's issue of the journal Nature.

Some experts urged caution about the study's conclusions.

The authors said the bones indicate the human ancestor used sharp stones to carve meat from the carcasses of large animals and other stones to smash bones to get at the marrow. No stone tools were found at the site.

The researchers also called the finding the earliest evidence for meat-eating among hominins, an evolutionary group that includes people.

The study authors attributed the tool use to afarensis because no other hominin is known from that time in the area where the bones were found.

The Lucy fossil, which dates to 3.2 million years ago, was found in the same general area in 1974.

Alemseged said afarensis probably scavenged carcasses rather than hunting live animals, and ate the meat raw. The researchers said it's not clear whether the stone tools were made or were simply stones that were used as tools.

Some experts were unconvinced by the Nature paper's arguments.

"I'm very cautious about the conclusions," said Nicholas Toth of Indiana University, a paleoanthropologist who studies early stone tools.

The bones were found on the surface rather than being excavated, he said. That means nobody knows exactly what layers of earth they came from, which is key to knowing their age and associating them with other bones and materials to give them context, he said.

The markings look like the work of crocodiles, said Tim White of the University of California-Berkeley.

And they don't appear in the places on the bones that one would expect from a butchering, he said.