The bones were found last year in Dikika, located in the Lower Awash Valley in Ethiopia, and belonged to one impala-sized creature, and another buffalo-sized animal, this week's edition of the journal Nature reported.
They were discovered by a team of researchers from the California Academy of Science in San Francisco. Their finder, team member and archaeologist at the Leipzig-based Max Planck Institute for Evolutionary Anthropology, Shannon McPherron, told the magazine that they bear the unmistakable cut marks of stone tools, presumably used for butchering.
McPherron said that this means early hominims were both using tools and searching for meat. This would apply to the 3.3 million-year-old Australopithecus afarensis species, associated with the discovery of the juvenile specimen known as “Lucy's Baby” in the same region in 2000.
"We've put this important, fundamental behaviour back into Lucy's time," McPherron, the study author, told the magazine.
However Lucy's relatives were unlikely to have been hunting, but were likely scavenging kills from other predators, he added.
Previous findings had placed such tool use at 2.5 million years ago, but this discovery pushes that back by some 800,000 years, predating even the emergence of the human genus “Homo.”
"We're pushing much deeper into our evolutionary past," McPherron told the magazine.
The team used a two-step process to prove the markings in the bones were indeed from stone tools, first dating the bones and then using chemical tests to insure that the damage had happened prior to fossilisation. Microscopic exams then made sure the damage was the result of cutting, and not teeth or trampling, the magazine said.
The scientists were fortunate to find the fossils in a gully that cut through strata they had already studied extensively with other discoveries – including Lucy's Baby, found just a few hundred metres away, McPherron said. This made it easier to date the bones at around 3.39 million-years-old, he explained.
Though the tools used to cut the bones may have just been what the magazine called “convenient rocks,” this would have still required a bit of forethought, because suitable rocks could only be found some 6 kilometres away.
Experts told Nature that they had been hoping to find tools older than the earliest known tools, which date back 2.5 million years. These tools were already well-made, leaving scientists to wonder whether more rudimentary versions were yet to be found.
These slashed bones are now likely to send paleoanthropologists out hunting for tool origins between 3.2 and 3.5 million years old, they said.
McPherron suggested potential quarry sites would be a good place to start.