The debate revolves around when and how the marks were made. Shannon McPherron, who led the team that discovered the bones, thinks that they're the handiwork of stone tools, wielded by prehistoric butchers at least 3.39 million years ago. That predates the evolution of modern humans, meaning that the tool-users probably belonged to one of our ancestral species, Australopithecus afarensis (such as the famous 'Lucy'). The timing also pushed back the earliest estimates of tool use in our family tree by 800,000 years.
McPherron's game-changing conclusions were published earlier this year in the journal Nature and unsurprisingly, they stirred controversy. Now, another team led by Manuel Dominguez-Rodrigo has hit back with a rebuttal paper published in PNAS.
They argue that similar cuts can be produced when bones are gnawed by animals, trampled into rough ground, or even eroded by plants and fungi. Their conclusion: the marks on the Dikika bones were probably created by trampling and their age is uncertain. To them, the best evidence for butchery by human ancestors comes from stone tools recovered in Gona, Ethiopia, which are just 2.6 million years old.
Last year, Dominguez-Rodrigo showed that marks produced by genuine cutting tools can be easily distinguished from those produced by trampling. His experiments revealed that the former have a V-shape in cross-section, with steep walls that meet at a pointed base. Trampling marks, however, have a flatter base (\_/). They also tend to be curved or S-shaped, rather than the straight slashes you'd expect of a tool. He argues that the Dikika marks - flat-bottomed and curvy - are much more likely to be the result of trampling.
Other researchers share his scepticism, including Sileshi Semaw, who discovered the Gona tools. When I wrote about McPherron's work last year, Semaw said that he thought the marks were caused by trampling and he says, "I am now totally convinced". He adds, "Such a huge claim needs to be substantiated by overwhelming evidence, and before rushing for publication, the Dikika researchers should have eliminated every other possible bone-modifying agents, such as trampling."
But McPherron stands by his work. He says that he did rule out alternative explanations such as trampling in his original paper and that Dominguez-Rodrigo hasn't actually examined the original specimens. His criticism is based solely on the photos in the Nature paper. "Out of a very large sample of experimentally trampled specimens, they succeed at finding a small sub-set of the trampled sample that superficially resembles a small sub-set of the Dikika marks," he argues. "This of course means that they failed to find any trampling damage that resembled the many other stone tool inflicted marks on Dikika."
McPherron also points out that two of the most compelling marks - known as A1 and A2 - are clearly not caused by trampling. Dominguez-Rodrigo actually agrees. He writes that the marks are "compelling in their similarity to verified cut marks created by stone tools". He also says that "in a less contentious context, the marks would likely be accepted as genuine cutmarks," before rejecting that conclusion because of the doubts raised by the other marks. McPherron puts it like this. "Their massive sample of trampling damage could not produce a set of marks that overlapped in [shape] with some of the key Dikika marks… This is our point exactly."
Controversy over Dikka Bones
Dominguez-Rodrigo also objects to the origin of the Dikika bones. The specimens were all found on the surface, which makes it hard to know about where exactly they came from. Such locations are important - they're the only way of reliably telling how old such specimens are. Dominguez-Rodrigo asserts that the Dikika team should have looked for other fossils from the same location to confirm the age of the bones.
McPherron counters that this would solve nothing, as it would be "logically impossible" to show that such fossils came from the same layers as their bones. Based on where the bones were found, and the fact that they had very little dirt sticking to them, McPherron thinks that they came from a nearby sand bed. The age of those sediments helped him to work out how old the bones are, giving him an interval of 3.24 to 3.42 million years ago. "This is [an] uncertainty of about 40 thousand years in an age of about 3.4 million years," he says.
If the bones did come from the sandy layer, they might indeed have been eroded by the surrounding sediment, as animals walked overhead. When bones are trampled, they show two types of obvious marks: shallow, randomly spaced furrows; and even shallower furrows that intersect deeper grooves. McPherron ruled out trampling because he didn't find any of these distinctive marks the Dikika bones. But Dominguez-Rodrigo argues that some of the Dikika 'cuts' do match the signs of trampling, and the Dikika bones aren't well-preserved enough for more of these shallow furrows to show up.
In the end, this is a matter of interpreting uncertain evidence and all the quirks that go into that. The path to greater understanding is riddled with debates such as these - that's how science works.
Dominguez-Rodrigo admits that he has been "intentionally conservative" because of the "extraordinary" claims at stake. He thinks that the evidence from Dikika doesn't justify McPherron's lofty conclusions (although one could argue that the evidence for trampling is equally uncertain).
McPherron "respectfully disagrees" and is unimpressed with the rebuttal. He says, "Their approach… fails to meet a basic premise of the scientific method - independent tests of diagnosis." In his study, "three highly trained analysts with wide backgrounds… independently identified marks of the original specimens as stone tool-inflicted. In this PNAS paper, three analysts who regularly work together (two of which are holding their prior analysis up as the earliest stone took marks, and thus have a clear bias), reach a conclusion as a group." He describes their conclusions as "group-think applied to picture-matching."
Dominguez-Rodrigo accepts that many of his objections come from comparing the Dikika marks to those made by sharp stones. However, McPherron says that the marks were probably made by naturally occurring stones with broader and blunter edges.
That's a big hole in both sets of arguments. On the one hand, Semaw says, "The Dikika researchers have not even found a single stone tool any where nearby, and their suggestion [that unmodified stones] created those marks makes no sense." On the other hand, McPherron argues that at the dawn of stone tool use, they would have been used infrequently and their remains would be hard to find. Furthermore, no one has ever tested unflaked stone tools in butchery experiments so no one really knows what type of marks they would produce.
Dominguez-Rodrigo says that "the onus to fill that gap falls on anyone who contends such a possibility" and indeed, McPherron is on the case. He says, "Recent experiments reported in a paper now under review show that the Dikika marks are a tight fit to marks produced by unflaked stone." This story is not over yet…
By Ed Yong
Reprinted with permission from Discover