Are 4-million-year-old Stone-knife Marks on Bones a Croc?

Observing the marks that crocodile bites leave on bones indicates that ‘evidence’ of stone-tool use in Ethiopian millions of years ago is nothing of the sort

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Linear marks and pits on a 2.5 million-year-old ungulate leg bone from Bouri, Ethiopia. Could be butchery or could be crocodile.
Linear marks and pits on a 2.5 million-year-old ungulate leg bone from Bouri, Ethiopia. Could be butchery, or could be crocodile. Credit: PNAS

Some 4.2 million years ago in Ethiopia, an ancient hominin butchered animals using stone tools. Or maybe a crocodile ate supper. Somebody left linear marks and pits on animal bones in the Plio-Pleistocene era, and now a team of scientists argues that zooarchaeologists have leaned too hard toward the advanced apeman hypothesis.

Natural causes rather than Australopithecus could have caused the marks, the team concludes.

The confusion is due to “equifinality” – when different processes have the same outcome.

Hacking at a giant sloth leg with a flint knife to remove the meat, or being a crocodile and biting the meat off, can have much the same effect on a bone, as demonstrated by Yonatan Sahle and Sireen El Zaatari from the University of Tübingen, Germany, with Tim White of University of California Berkeley. Further complicating the issue, animals trampling carcasses also leave similar marks on bone.

By now, it is clear that linear marks and other damage to bone surfaces previously interpreted as man-made can result from anything from hyenas to crocodilians to plant roots.

Moreover, the earliest categorical evidence for stone tools – found at Gona, Ethiopia – dates to 2.6 million years, Sahle tells Haaretz. “What this means is we need to be extra careful when analyzing fossil assemblages from prior to 2 million years ago,” he says.

Much ado about very little

Meanwhile, claims about stone-tool use in early hominins over 4 million years ago have been based on small numbers of bone samples from a few sites in Ethiopia and Tanzania. After reanalysis, the team suggests reptiles rather than hominins were responsible for the marked remains from 4.2 million and 3.4 million years ago in Ethiopia, including the Australopithecus remains at Asa Issie and bones dating to 2.4 million years ago from Bouri.

File photo: An alligator, thinking about ribs.Credit: Moshe Gilad

For decades, zooarchaeologists were confident that carnivores left U-shaped marks on bones, while stone tools left V-shaped marks, and that certain breaks had to be associated with deliberate marrow extraction.

Scientists were also attracted to seek early tool use after observing chimps using tools to break open nuts, explains the team. (Of course, other animals – from elephants to crows to ants – also use tools, but they aren’t considered our cousins so that wasn’t factored in.)

The controversy turned caustic with the discovery of marks on 3.4-million-year-old bones found at Dikika, Ethiopia.

The finders identified hominin-caused percussion marks in bones of an impala or something like a buffalo, even postulating that the discoveries proved the early hominins, probably Australopithecus, were venturing out of their comfort zone to hunt.

Critics claimed the marks showed the impala or buffalo-like animal had been trampled.

As for the crocodile, scaly bane of ancient hominin, science had looked more at what their digestive systems would do to bones and less at the marks their fangs would leave. Recent experiments, involving feeding crocs and observing the aftermath, changed that.

“The revelation that traces left by crocodile teeth can match those previously thought to be diagnostic of stone-tool butchery is a significant expansion of equifinality that threatens the binary orthodoxy employed by African zooarchaeologists to sort ancient mammalian carnivore traces from marks made by technological hominids,” Sahle and the team write. So there.

The long and short of it is that because of equifinality, ancient stone-tool use needs to be gauged carefully. Consider the arm-bone dating to 3.4 million years ago, found in Assa Issie, Ethiopia, which has both U-shaped and V-shaped marks. Each mark taken alone could be interpreted as evidence of stone-tool use, say the researchers. But based on the fossil’s context and distribution of the surface modifications, they tend to conclude that the cause was crocodile, not Lucy.

MastodonCredit: Damian Dovarganes / AP

Also: “There is a claim for 3.4-million-year-old stone tools from Lomekwi, Kenya,” says Sahle. “Most of us archaeologists think there is not enough evidence to support that claim.”

The same applies to another Ethiopian site, Maka, which had also been associated by its researchers with signs of stone-tool use on bones. Again, no stone tools were found, but signs of crocodile appetite were.

Homo Californicus? Not so fast

As for California, nobody ever thought that was the cradle of modern man. But there is a question about when Homo sapiens reached the Americas.

At Cerutti Mastodon, a site from the early-late Pleistocene era, scientists claimed to have found hammerstones and stone anvils in spatiotemporal association with the remains of a mastodon. They also claimed that the mastodon bones were broken when fresh, presumably to obtain the fatty marrow. No less, the paper concludes that the findings confirm an unidentified human species had been there during the last interglacial period, and it ate a mastodon.

Or not. “The problem with this and other similar exceptional claims that use surface bone modification as evidence, is that they lack the accompanying exceptionality of the evidence,” Sahle told Haaretz by email. “Crocodiles, geological processes and trampling by other animals all produce bone surface modification marks. Some of these mimic cut and percussion marks inflicted by tool using hominid butchers. When all other possible agents of bone modification are not effectively ruled out, then any inference will fall into the equifinality pitfall. The Californian claim is not an exception, as it lacks adequate evidence to securely make such inference.”

Very likely, the fracture patterns found in the mastodon bones at Cerutti resulted from fossilization process, Sahle suggests.

So the jury is out, until many more ancient samples are found and studied. If anything, though, the team is predicting that if experiments are done with even bigger, hungrier crocodiles, the boundaries of equifinality will expand even more. In other words, they think stone-tool use by the precursors of man millions of years ago is a croc.