Manuel Dominguez-Rodrigo, writing with my University of Wisconsin colleagues Travis Pickering and Henry Bunn, has challenged the interpretation that two bovid bones from Dikika bear cutmarks made by hominins
Our taphonomic configurational approach to assess the claims of A. afarensis butchery at Dikika suggests the claims of unexpectedly early butchering at the site are not warranted. The Dikika research group focused its analysis on the morphology of the marks in question but failed to demonstrate, through recovery of similarly marked in situ fossils, the exact provenience of the pub- lished fossils, and failed to note occurrences of random striae on the cortices of the published fossils (incurred through incidental move- ment of the defleshed specimens across and/or within their abrasive encasing sediments). The occurrence of such random striae (some- times called collectively trampling damage) on the two fossils provide the configurational context for rejection of the claimed butchery marks. The earliest best evidence for hominin butchery thus remains at 2.6 to 2.5 Ma, presumably associated with more derived species than A. afarensis.
These authors are experts on cutmarks, both from their work on Oldowan faunal assemblages and from experimental work where they have controlled the actual circumstances of cutmarking, trampling and weathering. Their critique of the two Dikika bones takes two main paths:
The surfaces of the bones themselves are relatively poorly preserved, with evidence of "trampling" modification and subadult status for one specimen and evidence of "moderate weathering" on the other. The matrix containing the bones was highly abrasive, making spurious marks more likely. This would make it difficult to get clear results even in an experimental context.
The purported cutmarks themselves are similar to marks that occur in bones subject to trampling damage. Dominguez-Rodrigo and colleagues argue that some of these marks are more diagnostic of trampling than of cutting or hammerstone damage.
The authors do not say they have disproven the hypothesis that A. afarensis cut on these bones with naturally-occurring stones, but they clearly question whether such a hypothesis is credible:
The Dikika butchery mark evidence does not, however, withstand peer scrutiny undertaken from an actualistic perspective and with a configurational approach. Our approach in assessing the Dikika claims was intentionally conservative: the claims are extraordinary because of their singularity and because of the inferred age of the fossils. Thus, natural processes of bone modification need to be eliminated before precluding nonanthropogenic origin(s) for the surficial marks on DIK-552 and DIK-553. High probability trampling damage on both specimens does not allow for this elimination and, again, taking our contextualized, maximally conservative position, forces us to reject even marks A1 and A2, the two morphologically strongest claims of cutmarks on DIK-552.
Their discussion emphasizes that, in their view, a hypothesis that an unusual tool type was responsible for cutmarks should be accompanied by experimental or actualistic evidence concerning the effects of that tool type. I think that for discoveries as potentially important as this, it is very reasonable for reviewers to expect such evidence will be provided. Also, a full statistical workup of other faunal bones from the site would be worthwhile. If the matrix really is abrasive and readily gives rise to trampling scratches, these should be evident in a wider distribution of bone from the site.
But for the moment, it looks like we should continue to treat cautiously claims of very early stone tool use. Possibly further comparisons will back up the hypothesis of cutmarks with more evidence. Since it took only three months from the initial publication of the Dikika evidence to this response, maybe we won't have to wait long for more comparisons!