The Cutting Edge
Can stone-tool marks on fossils be distinguished from tooth marks?
Is there enough evidence to support the stunning claim by the Dikika team? Obviously the team thinks so, and so do the scientists who reviewed their paper for Nature.
Sileshi Semaw of the Stone Age Institute in Indiana doesn’t think so. He is the leader of the ongoing research team that found the oldest known Oldowan tools and bones with cut marks at Gona. When his team published their discovery, they had excavated thousands of stone artifacts and many fossilized bone fragments from well-dated, fine-grained sediments, as well as collected surface samples.
Semaw asks, “What is the Dikika evidence? Two modified bones with possible cut marks from the surface, and not a single stone flake nor a single hominin-modified stone! Such a major leap in ancestral hominin tool-use behavior should have been supported by strong evidence, but the Dikika researchers are making a huge claim based on very meager data.”
Others challenge the interpretation of the Dikika bones because of the lack of context. The bones could not be linked by an adhering matrix (in other words, stone) to the particular rock layer from which they must have originated. Because it is the rocks that are dated, not the bones, the antiquity of the bones depends on their being matched to either a rock layer or, through excavation, to specimens still in place in the ground.
Speaking for the team, Alemseged responds to this objection: “We do not have any doubt that these bones are older than 3.24 million years and very likely 3.39 million years old. The juvenile skeleton Selam was discovered only 222 meters away, so intensive attention was paid to documenting the geologic history of that particular area. Almost all fossils from the Lucy site, Hadar and Dikika were dated using the same methods.” Of course, Alemseged adds, excavating additional pieces that confirm these finds would be great, and he hopes to find such material in his next field season.
But he doesn’t have the evidence yet, his critics retort.
The lines are clearly drawn.
The Dikika team has conducted all of the studies they can to support the identification and the antiquity of the marks. They chose to make a prominent announcement of their findings because they are confident that their work has been done correctly and because the implications of finding pre-Oldowan cut marks are huge.
If their interpretation is correct, it reshapes our understanding of the life of Australopithecus afarensis. This species may well have had more meat and fat in their diet than we supposed, but the addition of these important foods did not cause a rapid physiological change in Australopithecus afarensis that can be seen in the fossil record. Were the first tools used too rarely to have a dramatic impact on the species right away? Possibly.
Also, the odd pairing of bone with cut marks and an utter lack of recognizable tools suggests how tool use might have begun: opportunistically and infrequently with expediency tools. Maybe simply picking up handy objects showed australopithecines that what they really needed was a cutting edge. If so, then it took these early hominins 800,000 years to figure out how to make and use tools regularly.
In contrast, the skeptics feel the announcement was premature. They argue that the team should have waited for more, better and less ambiguous evidence before making such an enormous claim.
If stone tools were invented 2.6 million years ago and not much earlier, their first appearance coincides with the presence of Australopithecus garhi and is close in time to the oldest known occurrence of A. garhi’s possible descendent, an early species of the genus Homo. In this reading of the record, the invention of stone tools provoked a dietary change—an increase in carnivory—that led to a rapid increase in body and brain size seen in Homo.
Is the identification of marks on bone surfaces so reliable that a few cut marks can be taken as proof of stone tool use, even in the absence of recognizable tools?
Do we know enough about taphonomic agents that cause marks that closely mimic stone-tool cut marks and how to distinguish among them?
How long did it take to invent stone tools? Did using naturally occurring sharp stones give our ancestors the idea that they could make sharp objects out of stones?
And how long would a dietary and ecological shift—from herbivore to carnivore, from prey to predator—take to produce dramatic changes in the biology of our ancestors?
In this intriguing case, the truth is balanced precariously on a knife’s edge.