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The Cutting Edge

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

Pat Shipman

Choosing to Publish

An international team found themselves in this sort of situation, with new evidence contradicting established theories, when surveying for fossils in the Lower Awash Valley of the Dikika region of Ethiopia—and they opted for publication. The Dikika Research Project team is led by Zeresenay Alemseged, curator of anthropology at the California Academy of Science. The team includes geologists, experts in radiometric dating, archaeologists and paleontologists, as well as experts in an obscure specialty called taphonomy, which is one of my own areas of expertise. Taphonomists strive to reconstruct what has happened to a fossil, from the time it was a living organism until it was found, and to deduce what that history tells us about the ancient environment and adaptive niche of the animal, including how it died.

The team has been working in the Lower Awash Valley for 11 years and has made impressive finds. The most spectacular has been the partial skeleton of a young Australopithecus afarensis—the same species as the Lucy skeleton—that has been nicknamed “Selam” or “the Dikika baby.” Selam is 3.3 million years old by geological standards and was about 3 years old at death by human standards.

2010-11MargShipmanFB.jpgClick to Enlarge ImageIn 2009 paleontologist Denis Geraads of the French Centre National de la Recherche Scientifique was collecting bones from the surface of one area and noticed, on one calcaneum (a foot bone) from an antelope, a set of obvious, unusual marks. Based on field inspection, he thought these might be cut marks made by a stone tool, so he brought the bone back to camp and showed it to the rest of the team. They decided to return to that area and several others to inspect every fragment of fossilized bone for similar marks. Eventually they collected four specimens with intriguing marks on their surfaces for further study. There were no identifiable stone tools or sharp stone fragments found in those areas.

Back in the United States, taphonomist Curtis Marean of Arizona State University examined the marks microscopically and compared them with his large collection of bones with known taphonomic histories. He became convinced that the marks on two of the specimens (a rib and a femur, both probably from antelopes) were unambiguous cut marks, some of which graded into percussion marks made by striking the bone with a hammerstone rather than slicing. Two other bones had marks that were too ambiguous to identify. His judgments were confirmed by an advanced graduate student and a former student in taphonomy, who made independent assessments of the marks.

Unfortunately, the two marked specimens were found on the surface and had very little rock adhering to them, so they could not be linked definitely to any particular stratigraphic layer. However, the surface on which they were found is bracketed by two well-dated deposits of volcanic tuff, the higher one at 3.24 million years and the lower one at 3.42 million years. Thus the surface is estimated to be 3.39 million years old.

On August 12 of this year, the team published the find as the cover story in the journal Nature, with the title “Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia.”

The applause from those who accepted their interpretation, and the criticism from those who didn’t, began almost immediately.

Why? The bones the team identified as having cut marks were more than 800,000 years older than the oldest-known type of stone tools, called the Oldowan industry, which come from nearby Gona, Ethiopia, and are 2.6 million years old. There is not a single confirmed stone tool anywhere that is older than the Oldowan tools at the Gona site—and there is not even one possible stone tool from this region of Dikika.

If the team’s interpretation is correct, their discovery “dramatically shifts the known timeframe of a game-changing behavior for our ancestors,” says Alemseged. “We are putting stone tools in the hands of Lucy and her kind.”

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