Uncovering the “Chimpanzee Stone Age”

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An adult chimpanzee in Bossou, Guinea uses hammer and anvil stones to crack nuts as younger individuals look on. From Haslam et al., 2009.

Before 1859 the idea that humans lived alongside the mammoths, ground sloths, and saber-toothed cats of the not-too-distant past was almost heretical. Not only was there no irrefutable evidence that our species stretched so far back in time, but the very notion that we could have survived alongside such imposing Pleistocene mammals strained credulity. Contrary to what might be immediately expected, however, it was not Darwin’s famous abstract On the Origin of Species that changed appraisals of human prehistory. Instead it was a collection of stone tools found mingled among the bones of extinct mammals found in deposits on either side of the English Channel.

The discovery of stone tools from places like Brixham Cave in England and France’s Somme Valley confirmed that industry was a very old human enterprise, and so some scholars naturally felt quite comfortable in giving out species the honorary title of “Man the Toolmaker.” The ability of our species to make and use tools clearly separated us from all other organisms, at least until it was discovered that chimpanzees, too, made and used tools. More than that, studies since the 1960’s have confirmed that different populations of chimpanzees have distinctive tool cultures affected by the contingencies of their surroundings, and a recent study published two years ago in PNAS illustrates that these cultures of tool use among non-human primates stretch back at least 4,300 years.

Since September of 1979 primatologists have studied the wild chimpanzees of the Tai National Park in the western African nation of C√¥te d’Ivoire, and in this particular location the chimpanzees use a variety of tools. Among the most common tools are twigs used to get at different kinds of food (be it honey in a tree or the brains of a monkey they have killed), but the Tai chimps also frequently use stone hammers and anvils to crack open nuts. Naturally this process modifies the stones used in the process, and this made some researchers wonder whether chimpanzees might have an archaeological record all their own.

To find out, scientists Julio Mercader, Huw Barton, Jason Gillespie, Jack Harris, Steven Kuhn, Robert Tyler, and Christophe Boesch looked for signs of ancient chimpanzee sites within the Tai forest. They found three, all of which were within about 200 meters of each other in an area of the park still inhabited by chimpanzees and dated to a span of time between 4,300 years ago and 2,200 years ago. From these sites the researchers gathered a large collection of modified stones, most of which (206 pieces) came from a single site, but the question was whether they were truly looking at a chimpanzee-made assemblage or one that had been artificially made by flowing water.

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A collection of stone tools created by chimpanzees and deposited between 4,300 and 2,200 years ago in what is now Tai National Forest. From Mercader et al., 2007.

In their study of the site the team found that the sediments in which the artifacts had been found were of a sort that would not be able to transport many bits of rock larger than a pebble. This suggested that the damaged stones really had been created by chimpanzees, but to make sure they were not being deceived three of the authors participated in a triple blind test in which stone tools were mixed among rocks that might look like tools but had actually been created by geological phenomena. This forced them to identify what characteristics distinguished a tool from a tool-shaped rock, and during the tests the researchers agreed on the identification of the tools 90% of the time. Based upon this test and the details of the stones it was highly likely that the identification of the tools was correct, but the proof that they had been actually used to crack nuts came from a different line of evidence.

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An young male chimpanzee in Bossou, Guinea carries a hammer and an anvil. In this way sites like the one uncovered in the Tai forest begin to be formed. From Haslam et al., 2009.

The stone tools the team found closely resembled those used by Tai chimpanzees to crack nuts today, and to investigate this idea they looked at whether any starch residues left by the nuts remained on the tools. They were in luck. Not only did the Tai tools preserve some of these residues, but many of the preserved residues matched those of nuts eaten by chimpanzees.

When put all together the evidence the team collected strongly supported the hypothesis that they had found a 4,300 year old chimpanzee nut-cracking site. The damage to the tools matched that wear inflicted on similar stones by living chimpanzees, the distribution of tools around the site was consistent with chimpanzee tool assemblages, and the residues on the tools matched nuts that chimpanzees regularly eat. Perhaps more importantly, however, the study showed that the archaeological record of non-human apes could be detected.

How far back might that record extend? It is difficult to say. Many chimpanzee tools, such as the twigs they often use, may never have made it into the fossil record, and the likelihood of identifying such tools is low. Stone tools are sturdier vestiges of ancient activities that bear tell-tale signs which distinguish them from surrounding rocks, and now that scientists know what to look for perhaps more will be discovered. Remnants of what the authors call the “Chimpanzee Stone Age” are probably waiting to be found in other African forests, and I hope that future research will be able to tell us more about prehistoric tool use in among some of our closest relatives.

[Also see this recent review in Naturethis recent review in Nature about primate archaeology.]

Mercader, J., Barton, H., Gillespie, J., Harris, J., Kuhn, S., Tyler, R., & Boesch, C. (2007). 4,300-Year-old chimpanzee sites and the origins of percussive stone technology Proceedings of the National Academy of Sciences, 104 (9), 3043-3048 DOI: 10.1073/pnas.0607909104