NEW YORK – Two ancient animal bones from Ethiopia show signs of butchering by human ancestors, moving back the earliest evidence for the use of stone tools by about 800,000 years, researchers say.

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

“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.

The study 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. One bone is a rib from a creature the size of a cow, and the other a leg bone from something the size of a goat. No stone tools were found at the site.

Some experts were unconvinced.

“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, he said.

What’s more, judging from photos in the Nature paper, the bone markings differ from the marks typically left by stone tools, he said. That raises questions about whether they were actually caused by trampling or animal bites, Toth said.