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The Cutting Edge

Can stone-tool marks on fossils be distinguished from tooth marks?

Pat Shipman

Tools of Opportunity

So, how can there be cut marks prior to the existence of stone tools for cutting? There are a few possibilities, some more likely than others.

2010-11MargShipmanFC.jpgClick to Enlarge ImageFirst, maybe there are stone tools going back to 3.4 million years ago, but they have been missed. This is incredibly unlikely. This explanation would mean that all of the researchers on all of the expeditions that have combed the badlands of Ethiopia, Kenya and Tanzania for the past 40 years looking for fossils and early stone tools have been too incompetent to recognize stone tools in layers older than 2.6 million years. The timing of the invention of stone tools is a critical issue in paleoanthropology, and the oldest-known type of tools has been extensively studied. Furthermore, stone tools are sturdy objects not easily destroyed by most natural processes (unlike bones), so they don’t just vanish. It is completely unbelievable that Oldowan tools are scattered over the 3.4-million-year-old landscape and have been overlooked.

Second, the tools might be present but unrecognized because they are not deliberately flaked like the Oldowan tools, but naturally broken sharp stone fragments, or “expediency tools.” Marean favors this hypothesis.

“These will not look like Oldowan stone tools,” he says, “so the search image brought to the field will be inappropriate for finding this material.”

Alemseged also leans toward this possibility, but he cautions that if naturally occurring sharp objects were used as tools, without further modification, they will be very nearly invisible archaeologically. The only hope of identifying such an expediency tool would be if it had been excavated from fine-grained sediments and if it was of a rock type that does not naturally occur in that area (a so-called manuport). Manuports have not yet been recorded in any of the fossil sites older than 2.6 million years.

Third, the marks might be misidentified. This is the view of Tim White of the University of California at Berkeley—the leader of another team that works in an adjacent area of Ethiopia in this time period—who says that too much is being made of too little. “Extraordinary claims require extraordinary evidence,” he observes. “This is not perfectly good evidence that is being dismissed because its implications are startling. It is perfectly ambiguous evidence. This claim is based on about a dozen gouges and scratches on two small bone fragments with no associated context, from the surface of a site that is loaded with crocodiles.“

Because crocodiles chomp on their prey, leaving lots of marks with varied characteristics on the animals’ bones, White thinks crocodiles are probably responsible for the marked Dikika bones. The marks from Dikika are described as being unusually heavy and deep compared to most butchery marks, indicaing that whatever made them used a lot of force, as crocodiles do.

2010-11MargShipmanFD.jpgClick to Enlarge ImageJackson Njau, now a postdoctoral fellow at the University of California at Berkeley, did the most extensive study of crocodile marks on bones as part of his Ph.D. thesis. He is not only a lab analyst but is also the coleader of the Tanzanian International Paleoanthropological Research Project. Njau hasn’t yet seen the original Dikika specimens. But judging from the published photographs of the marked bones, he says:

The random patterning of the marks on those two specimens match those produced by crocodile action (under experimental conditions) or other unknown processes. Crocodile feeding techniques include twirling, twisting and hitting bones on the ground or rocks. Combined with the animal’s dental morphology, these techniques inflict marks of many different types, shapes and varying intensities, some of which look like cut or percussion marks, or like scoring marks that are curved similarly to ones documented on one of the Dikika specimens. One of these curved scoring marks is labeled J {see image at right}, which the authors consider to have been made by an unidentifiable carnivore.

In response to these challenges, Marean stands firm. He has examined the crocodile toothmarks from Njau’s controlled experiments as well as many other types of marks, and is still confident that his identification is correct.

“Most of the marks have features that indicate without doubt that they were inflicted by stone tools,” Marean asserts. “The range of actions that created the marks includes cutting and scraping for the removal of flesh, and percussion on the femur for breaking it to access marrow.” Crocodiles are not the only explanation for deep and heavy marks; hominins clumsily wielding heavy stone tools might also produce such damage.

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