The use of stone tools by wild-living chimpanzees and earliest hominids

Abstract

o (1) At Bossou in Guinea, and at Beatty's site in Liberia, chimpanzees use stone hammers and anvils to crack cultivated oil palm nuts in order to eat the kernel. They do so in the human manner and do not crack wild fruit. Elsewhere in the geographical range of the species, in most areas where oil palms commonly occur, chimpanzees swallow the whole fruit, digest the outer layer, and defaecate the nuts undamaged. Evidence from Bossou suggests that the habit of cracking oil palm nuts by means of stone tools has been copied by the chimpanzee community from the local human population as a result of observational learning under pressure of habitat deterioration. This community and Beatty's may represent the first identifiable cases of direct cultural transmission of technology from man to animal in the wild. (ii) At several sites in a region that ranges from the SE tip of Sierra Leone to the SW part of the Ivory Coast, chimpanzees use much heavier hammer stones in more varied ways than at Bossou to crack the very hard nuts of certain wild fruit species, while they do swallow oil palm fruit but do not crack these nuts. At two sites (at least) they also use wooden clubs as hammers, and roots and branches of trees as anvils. The manipulation techniques require a degree of arboreal competence, manual dexterity and integration of skills that far exceeds that currently observed among chimpanzees elsewhere in Africa. A possible explanation might be that this region is covered by the most humid type of evergreen lowland rain forest, and has been the most important lowland rain forest refuge of West Africa during dry epochs in the Pleistocene. Consequently the apes in this area may have been genetically selected for, and culturally adapted to, a highly specialized way of life in such habitats for hundreds of thousands, or perhaps millions, of years. (iii) With chimpanzees, the direction of the blow of the hammer on the nut follows the perpendicular to a flat surface of the hammer through its gravity centre. No similar use of stone tools has been inferred from the earliest hominid artefacts. On the contrary, nearly all early hominid stone tools were used as cutting, cleaving and chopping devices over the entire lengths of their edges. A cause of this difference might be that chimpanzees (in the trees) often have only one hand free both for positioning the nut and for subsequently wielding the hammer, while hominids (on the ground) could simultaneously use one hand to hold the object and the other hand to manipulate the tool. Moreover, “Nutcracker Man” may have cracked edible nuts with his teeth while Homo habilis may have developed the use of stone tools primarily for butchering prey or carrion. In conclusion, the use of stone tools by chimpanzees and earliest hominids shows no indications of homology, functional equivalence, similarity of motor patterns or identity of motivation.