Cutmarks under the microscope

I'm trying to figure out why Science this week has a "perspective" piece on the identification of cutmarks on archaeological bone. It's a nice brief but lacking in context -- why are cutmarks a pressing issue right now?

The essay, by Jackson Njau [1], refers extensively to the dispute over evidence from Dikika, Ethiopia. Shannon McPherron and colleagues [2] claimed that A. afarensis had made cutmarks on bones from bovids dating to 3.3 million years ago; later Manuel Dominguez-Rodrigo and colleagues disputed this interpretation [3]. I wrote about both papers ("Australopithecus afarensis used stone tools", "Cutmarked bones from Dikika critiqued"). Njau argues in favor of standardization:

Standardizing the contextual approach may reduce disagreements regarding the identification of surface marks and assemblage interpretations. For example, the recent interpretation of bone modifications on 3.4-million-year-old fossil fragments from Dikika, Ethiopia (3), as stone tool butchery marks has been challenged (4) by critics questioning the location of stone percussion marks on unexpected anatomical parts, particularly in the absence of broadly contemporaneous lithic-bearing sites in Africa. Critics contend that Dikika specimens were modified by trampling rather than by stone tools (4).

This disagreement illustrates the necessity for emphasizing the full array of contextual criteria in bone modification studies. For example, the presence of stone artifacts, carnivore feeding traces, or hoof-induced bioturbation in the depositional context may indicate that any or all of these agents were responsible for bone modification. This approach helps to eliminate mimicking processes and serves to more fully test interpretations of hominid carnivore.

This reads like a call for peer-reviewers to reject short manuscripts. An examination of purported cutmark evidence that simply says, "no evidence of hoof-induced bioturbation was found in the depositional context", surely wouldn't meet this condition. If Njau's suggestions were implemented, cutmark evidence by itself would rarely be sufficient to document early hominin behavior; other evidence would be necessary to accept conclusions based on cutmarks. Maybe that's as it should be.

Will other archaeologists heed Njau's call for standardized cutmark criteria? I think most would argue they are already using standardized criteria. It's not the cutmarks themselves but their configuration and representation on bones that are the difficult problems. There is some overlap between human-made cutmarks and natural scratches of various kinds, and that overlap must cause false positives when human cutmarks are sufficiently rare. Standardized criteria for depositional contexts will require extensive experimental work, and may not apply readily beyond the archaeological sites used to devise the experiments.

Njau's essay is also notable for this detail:

Given the scarcity of butchered bones from the Pliocene (5.3 to 2.6 million years ago) and Early Pleistocene (2.6 to 0.76 million years ago), even a single misidentification can have profound effects on the interpretation of early hominid behavior.

I'm probably wrong, but this is the first instance I can think of where an archaeologist writing in Science has used the revised chronology for the Pliocene-Pleistocene boundary, which places the beginning of the Pleistocene much earlier than the traditional 1.75 million years ago. I wrote about that decision by the International Commission on Stratigraphy in 2009 ("The Pleistocene 'land grab'"). Ever since then, a broad coalition of archaeologists and anthropologists have been trying to organize resistance to the change.

That resistance is probably futile. It's very hard to fight the conclusions of scientific political committees empowered to name things.

References