Slashed animal bones suggest early hominins were chopping up predator kills earlier than we thought.
Early hominins were using stone tools to butcher meat as long ago as 3.4 million years, about 800,000 years earlier than previous evidence dates to, scientists report in this week's issue of Nature.1
The finding comes from an examination of animal bones found last year in the Lower Awash Valley of Ethiopia. This site is not far from the spot where the same research team, led by palaeoanthropologist Zeresenay Alemseged of the California Academy of Science, San Francisco, had previously discovered a 3.3-million-year-old juvenile Australopithecus afarensis fossil dubbed 'Lucy's Baby'. That find is one of the most complete skeletons of an ancient human ancestor to be discovered so far2.
The animal bones — one from an impala-sized creature, the other from one closer in size to a buffalo — bear cut marks that indicate butchering, says their finder, Shannon McPherron, an archaeologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and a member of Alemseged's team.
This, he says, means that early hominins — presumably Australopithecus afarensis — were not only using tools, but also venturing out of the safety of the forests and onto the plains in search of meat.
However, they probably weren't hunting, McPherron says; it is more likely that they were scavenging predator kills. Still, the search for large-animal meat is an important step in human development. "We've put this important, fundamental behaviour back into Lucy's time," says McPherron, who is lead author of the new study.
The same is true for tool usage. Previously, the earliest known date for tool usage was about 2.5 million years ago — right about the time that humanity's own genus, Homo, was first emerging. Now, it seems that tool usage pre-dates our genus. "We're pushing much deeper into our evolutionary past," McPherron says.
Different for chimps
It's an important find, says David Braun, a Palaeolithic archaeologist at the University of Cape Town in South Africa, because our closest living relatives don't engage in such behaviours. "Chimpanzees do not recognize large animals or carcasses killed by other animals as food," he says. "At some point, hominins did."
Proving the discovery was a two-step process, involving both dating the bones and verifying that the marks on them were inflicted by stone tools rather than by trampling, teeth or post-fossilization damage.
To do that, the team examined the bones both chemically and under a microscope. The chemical tests confirmed that the damage had occurred before the bones were fossilized; the microscopic examination confirmed that it was the result of cutting.
"The results are very clear," McPherron says.
Some of the cuts are V-shaped in cross section, for instance — a shape characteristic of those made by sharp tools — with scratches inside the cuts left by the tool's rough edge. Other marks showed signs of scraping, and still others indicated that the bones had been bashed with blunt rocks — perhaps in an effort to reach the marrow.
Paul Renne directs the Berkeley Geochronology Center in California, and has worked on studies of some of the oldest known cut-marked bones found previously. "It sure looks convincing to me," he says of the new find.
As for dating, McPherron says the scientists were lucky, because the fossils came from a gully cutting through strata that had been well studied in conjunction with other finds, such as Lucy's Baby, which was discovered only a few hundred metres away. In particular, radioisotope studies had dated two important strata, one at the highest levels in the gully and the other near the bottom. On the basis of these, the scientists knew that the bones could be no more than 3.42 million and no less than 3.24 million years old.
The pattern of magnetic field reversals — which occur at intervals in Earth's history — in the intervening sediments, and estimates of sedimentation rates, further refined the estimate. "The best estimate is 3.39 million years," McPherron says.
Renne concurs. "I think they have a really good case for 3.2 to 3.4 million years ago," he says. Within that range, he adds, the precise date isn't critical. "The fact that they're older than 3 million is pretty exciting."
However, the discovery doesn't mean that early hominins made tools. They may simply have used convenient rocks for tasks such as butchering. But their efforts still required planning because the nearest source of suitable rocks was about 6 kilometres away from where the bones were found.
Renne and Braun are pleased but not startled. "We were hoping there would be older stuff [than my own findings]," Renne says.
Braun adds that the earliest known tools, dating to about 2.5 million years ago, are very well made, which has prompted scientists to wonder whether our ancestors had somehow instantaneously discovered how to make them, or whether older, cruder tools remain to be found.
"I think many palaeoanthropologists will start looking in this window between 3.2 and 2.5 million years ago for what may be the origins of stone tool production," he says.
McPherron suggests that the best way to do this might be to go to outcrops that could once have been quarry sites. "If we're going to find evidence of tool manufacture in this time period, we're probably going to have to go to where the stones are and look there," he says.
McPherron, S. P. et al. Nature 466, 857-860 (2010).
Alemseged, Z. et al. Nature 443, 296-301 (2006).
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Lovett, R. Butchering dinner 3.4 million years ago. Nature (2010). https://doi.org/10.1038/news.2010.399