As early as 3.4 million years ago, some individuals with a taste for meat and marrow — presumably members of the species best known for the skeleton called Lucy — apparently butchered with sharp and heavy stones two large animals on the shore of a shallow lake in what is now Ethiopia.

Scientists who made the discovery could not have been more surprised. They said the cut marks on a fossilized rib and thighbone were unambiguous evidence that human ancestors were using stone tools and sometimes consuming meat at least 800,000 years earlier than previously established. The oldest confirmed stone tools are less than 2.6 million years old, perhaps from only a little before the emergence of the genus Homo.

Some prominent researchers of early human evolution were skeptical, saying the reported evidence did not support such claims.

If true, though, the new find reveals unsuspected behavior and dietary habits of the Lucy species, Australopithecus afarensis. Though no hominid fossils were found near the butchered bones, A. afarensis is thought to be the only species living in this region at the time. The species’ large teeth with thick enamel indicated it subsisted mainly on tubers and other vegetation.

So the international team of paleoanthropologists, archaeologists and geologists concluded that it had found the first evidence that kin of the 3.2-million-year-old Lucy had used some form of stone tools and would not pass up a chance to feast on a cut of meat and nutritious bone marrow.

Pending new discoveries, the team wrote in a report being published Thursday in the journal Nature, A. afarensis is the only hominid group “to which we can associate the tool use.” Whether these individuals made the tools or only selected naturally sharpened pieces of stone, the scientists added, was not yet determined. Nor is it known whether they were hunters or, more likely, scavengers of a lion’s leftovers.

In any case, the scientists concluded, the butchery evidence “offers a first insight into an early phase of stone tool use” by human ancestors, and it should “improve our understanding of how this type of behavior originated and developed into later, well-recognized stone tool production technologies.”

The leader of the research project, at Dikika, Ethiopia, was Zeresenay Alemseged, an Ethiopian paleoanthropologist at the California Academy of Sciences in San Francisco. The lead author of the Nature paper was Shannon McPherron, an archaeologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

“Now, when we imagine Lucy walking around the East African landscape looking for food, we can for the first time imagine her with a stone tool in hand and looking for meat,” McPherron said in a statement issued by the Leipzig institute.

“With stone tools in hand to quickly pull off flesh and break open bones, animal carcasses would have become a more attractive source for food,” he said. “This type of behavior sent us down a path that later would lead to two of the defining features of our species — carnivory and tool manufacture and use.”

Alemseged said in a telephone interview before he returned last week to Ethiopia, “Our future work will be to find those stone tools that have shifted the framework for such an important transition in the behavior of our ancestors.”

The site at Dikika is in the heart of hunting grounds for fossils and artifacts related to human origins. The desiccated country is in the Lower Awash Valley, bordered on the north by Gona (the source of the oldest known stone tools) and Hadar (Lucy’s home). Four years ago, Alemseged announced the discovery at Dikika of the 3.3-million-year-old skeleton of a girl who died at the age of 3 — and was dubbed Lucy’s baby.

In January 2009, fossil hunters concentrated their search over a surface about 300 yards from where Lucy’s baby was found. Four bones bore intriguing cut marks. Scientists said that microscopic and chemical tests proved that two of the bones, a right rib from a cow-size ungulate and the femur, or thigh, of a goat-size antelope, were evidence of butchery.

The marks must have been made by stone tools, said Curtis Marean, a member of the team from the Institute of Human Origins at Arizona State University. “The range of actions includes cutting and scraping for the removal of flesh, and percussion on the femur for breaking it to access marrow,” he said.

Geologists on the team determined the age of the sediments in which the fossils were found to be 3.2 million to more than 4 million years old, based on dating of volcanic ash. The nearest stone that could have been usable for butchering seemed to be several miles from the site, which the scientists said suggested some premeditation on the part of the hominids. They had perhaps arrived for dinner with their utensils in hand.