NEW YORK (AP) — 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 some 3.4 milion 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 Thursday’s issue of the journal Nature.

Some experts urged caution about the study’s conclusions.

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.

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

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 skeleton of a young afarensis female, dubbed ‘‘Selam,’’ had previously been found about 200 yards away from the bone site. The Lucy fossil, which dates to 3.2 million years ago, was discovered 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.

But they plan to look for evidence of tool-making.

Alemseged also said that as some afarensis stripped meat from a carcass, others probably stood guard to ward off predators in return for some of the meat, which would indicate a degree of cooperative behavior.

Until now, the earliest sign of tool use dated to about 2.6 million years ago, also in Ethiopia. It’s not clear who used those 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.

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.

In fact, those 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.

He also said that 30 years of searching has failed to find any stone tools as old as the bones. ‘‘It’s not like people haven’t been looking. People have been looking intensively,’’ he said.

‘‘An extraordinary claim requires extraordinary evidence,’’ White said. ‘‘The evidence is very thin here, and very ambiguous.’’

But Bernard Wood of George Washington University declared, ‘‘I’d be willing to bet a month’s salary that those are cut marks (from stone tools) and not tooth marks.’’

Wood compared the find to the famous 1978 discovery of tracks in Tanzania that showed upright walking 3.6 million years ago, most likely by afarensis.

The bone markings ‘‘are as significant a statement about early hominin behavior as the Laetoli footprints are about hominin locomotion,’’ Wood said. While it’s reasonable to assume that afarensis wielded the tools, he said, Alemseged’s ideas about the butchers being guarded by other afarensis in exchange for meat is ‘‘pushing the envelope a bit far.’’

Wood also said the finding suggests afarensis ate meat but doesn’t prove it, because maybe they cut off animal flesh just to get to the marrow.