Today we know that chimpanzees use tools such as modified sticks and rocks to assist in their feeding. Rocks are used to crack open shells of nuts or seeds. Today fossil remains in Tai National Park have been identified as chimpanzee tools. Physical characteristics that distinguish them as being products of thrusting or percussion separate them from the other rocks in the area. Furthermore, starch found stuck to some rocks are said to prove that the rocks were used in feeding. These fossils predate known human inhabitants in the Tai National Park.
These geofacts found date back 4,300 years. Since humans did not inhabit the area before then, we are left wondering how chimpanzees learned their stone-age technology. This evidence suggests that either chimpanzee stone-age and humans stone-age took place at approximately the same time, or that chimpanzees evolved their tool using abilities independently (without watching or learning from) humans.
The study classified the area the artifacts were found in as “muddy sands” by an Ingram – Wentworth model that shows “pebble, cobble, and boulder sized rock fragments would be naturally lacking” (Mercader). Examiners proved their ability to distinguish between geofacts and other rocks by analysis of rocks collected from Canadian glacier sites mixed amongst rocks from a proven behavioral site. Scientist examiners were in 90% agreement at to which stones were geofacts. Additionally starch residues found on the geofacts are said to be remnants of the food the chimpanzees consumed.
This interesting study based its hypothesis, results, and conclusions on what we know today. Today chimpanzees are observed to use mostly granitoid as well as laterite, diorite, feldspar, and quartz. This information was used to show that the choice of stone in the mid- Holocene atones mirrored modern day choices in the Tai forest.
Additionally the authors admit that the starch residue could be from root growth or anything that came into contact with the stones in the last few thousand years, however they included “all nut and tuber species relevant to modern hominid starch use plus other species present in the study area” for analysis by examiners. 37% of the residues on the rocks matched one of these plant materials. All three examiners only agreed on classification for the starch grains 48% of the time.
Blind testing showed examiner’s ability to distinguish the rocks used for thrusting or percussion, from any other rock, to have accuracy values of 65%, 75%, and 90%. Examiners distinguished between different rocks by logical reduction patterns such as radial cores vs. systematic flaking. “Because chimpanzees have not yet been observed to flake stone intentionally it is unquestionable that human lithic reduction is present…the possibility that humans could be the sole culprit of our stone collections must be carefully examined.” Basically the authors argue that the large sizes of some stones are more comparable in size to the hand of a chimpanzee and therefore some must be from chimpanzee origins.
From reading this article I can conclude that the authors have much more work to do and that this paper is based on one hypothesis. I don’t disagree with their overall conclusions, however their assumptions based on current plants and rocks as well as the marginal agreement between examiners is not entirely convincing.
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