Every time we slice into a steak or cut into some chicken, we’re taking part in a technological heritage that stretches back at least 3.4 million years. Back then, the only cutting implements around were sharp pieces of stone and there were no true humans around to wield them. But there were still butchers– one of our ancestral species, Australopithecus afarensis, was already using stone tools to flay meat off bones, leaving small nicks with every cut. Such marked bones have been found and they push back the earliest estimates of tool use among human ancestors by 800,000 years.

In January 2009, a team led by Shannon McPherron from the Max Planck Institute found bones which had clearly been worked over with stone tools. The bones, uncovered in Dikika, Ethiopia, include the rib of a cow-sized animal and the thighbone of a goat-sized one. Both bore cuts and scratches inflicted by sharp objects and dents produced by crushing hammers.

By peering at the marks under powerful microscopes and analysing their chemical composition, McPherron confirmed that they were made by stone rather than teeth, and they were created before the bones fossilised. These were not accidental scratches, but the remnants of strikes used to carve off the meat and break into the marrow.

Based on the surrounding rock layers, which have been very accurately dated, McPherron calculated that the bones are at least 3.39 million years old. These relics push back both the history of butchery and the use of stone tools by human ancestors, by almost a million years. Until now, the oldest evidence for the manufacture of stone toolscomes from finds in Gona, Ethiopia that are just 2.6 million years old, while the oldest cut-marked bones were found in nearby Bouri and dated to around 2.5 million years ago.

The Dikika site has been thoroughly studied by a team led by Zeresenay Alemseged (photo below), who also had a hand in the latest discovery. In fact, the new bones were found just 200 metres away from Alemseged’s most famous find – the bones of a three-year-old Australopithecus afarensis girl, known as Selam. No other hominin (a term for members of the human lineage) lived in the same area. This provides strong evidence that A.afarensis , such as the famous Lucy, used stone tools and ate meat. Selam may even have watched or helped as her family members carved up the carcass of a large animal.

In a way, this isn’t surprising. Recent discoveries have done much to strip A.afarensis of its early reputation as a primitive hominin and even other primates like chimpanzees use stone tools. McPherron says, “A. afarensis had a similar sized brain and perhaps somewhat better hands for the job, at some level it is not surprising that A. afarensis should use stone tools. However, we can’t assume that simply because chimps use stone tools and we use tools that the behaviour is as old as our common ancestor.”

Nonetheless, both tool use and meat-eating are critically important events in human evolution. “Some have argued that meat consumption is what set us down the path towards the large brained, behaviorally complex species that we are today,” says McPherron. “It has been said that meat made us human. It provides a more nutrient rich diet that made possible a larger brain.”

The use of tools also gave our ancestors access to rich sources of meat, namely the carcasses of large, dead animals. Most other primates would turn their noses up at such foods but it’s clear that A.afarensis did not. Indeed, the costs of eating such carcasses, such as competition with predators, may have driven the use of more sophisticated tools and close teamwork.

For now, McPherron hasn’t actually found any of the actual cutting tools or, in fact, any sharp-edged stones nearby. That’s to be expected – the area where the bones were found used to be part of the floodplain of a river and probably didn’t contain any stones larger than fine gravel. The nearest suitable materials were around 6 kilometres away. “If the stone tool had been made elsewhere and carried to this spot, as it almost certainly was, the odds of us finding it would be small even if they dropped it there,” says McPherron.

There is, of course, another explanation: McPherron’s team could be wrong. Sileshi Senaw, who discovered the Gona tools, certainly thinks so and says that the data just aren’t strong enough to support their conclusions. The Dikika researchers are making a huge claim based on very meager data,” he says. “Researchers who study bone surface modifications from archeological sites have shown that fresh bones trampled by animals can create marks that mimic stone tool cut marks… I am not convinced of the new discovery.”

But McPherron stands by his interpretation and has other explanations: the butchers might just have picked up naturally sharp rocks from their surroundings; they could have made them so infrequently that they’ll be hard to find; or, simply, no one has looked hard enough. “I favor a combination of the last two,” he says.

Alison Brooks from George Washington University agrees. She thinks the sudden appearance of stone tools in the archaeological record, some 2.6 million years ago, doesn’t represent the point where early humans started using them, but the point where they started making them at concentrated sites where they’re more likely to be found. There was a long time window before that when stone tools were used in a more scattered way, a window that McPherron’s team have been lucky enough to look through.

McPherron plans to return to Dikika in January 2011 for a more intensive search. “There’s a location nearby where raw materials for stone tool production may have been available 3.4 [million years ago], and I hope to target this area to see if we can find evidence of stone tool manufacture.”

“On the other and, stone tool marks, according to some of key taphonomists and archaeologists are almost always ambiguous, or at least difficult to identify positively (see Tappen and Harris 1995 and Dominguez-Rodrigo et al 2005). A stone tool is a rock. It can scratch a bone. There are other rocks. They can scratch bones too. Telling the difference between a rock scratching a bone and a rock scratching a bone is inherently difficult. One way to do this is to look at a zillion cut marks with stone tools and characterize them metrically and qualitatively (done) and look at a bunch of trample marks where bones are ground and stomped into stony or pebbly ground and characterize those marks (done, but not as much because it is less fun) and see if you can get none overlapping sets of characteristics.

You don’t get non-overlapping characteristics, but you do get different looking patterns. So, you can then look at unknown marks and see what pattern they fit. One aspect of the pattern, by the way, is where on the bone the mark is; …

I’ve seen a lot of cutmarks. [long list of experience] …

And when I look at the pictures of the marks on the bones shown in this paper, I see stone tool cutmarks.

But given that stone tool cutmarks are marks made by rocks on bones …. we just can’t be sure. But it is cool nonetheless.”

I.e. in principle you could do a PCA (or some bona fide pattern recognition) and place likelihoods.

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Phenomena is a gathering of spirited science writers who take delight in the new, the strange, the beautiful and awe-inspiring details of our world. Phenomena is hosted by National Geographic magazine, which invites you to join the conversation. Follow on Twitter at @natgeoscience.

Ed Yong is an award-winning British science writer. Not Exactly Rocket Science is his hub for talking about the awe-inspiring, beautiful and quirky world of science to as many people as possible.
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