Teenage chimps with spears and hammers
Two related papers this week: â€śSavanna chimpanzees, Pan troglodytes verus, hunt with toolsâ€? by Jill Preutz and Paco Bertolani, published in Current Biology (17:1-6), and â€ś4,300-year-old chimpanzee sites and the origins of percussive stone technologyâ€? by Julio Mercader et al., published in Proceedings of the National Academy of Science (104:3043-3048).
Preutz and Bertolani report field observations of chimpanzees in Senegal making simple wooden spears and using them to kill bushbabies (sleeping in hollow trees) for food. Several individuals were seen making and using the spears, although apparently they only saw one successful â€śhunt.â€?
Some press reports have said that most of the chimps making and using spears were female, but what the abstract of the paper actually says is â€śfemales and immature chimpanzees exhibited this behavior more frequently than adult males.â€? Table 1 shows one adult male, one adult female, and eight immatures (four male and four female) making and/or using spears. So it might have been more informative to say that â€śimmature chimpanzees exhibited this behavior more frequently than adults.â€? But apparently they wanted to contrast hunting with spears with â€śchimpanzee hunting in general, a predominantly [adult?] male activity.â€?
What can we conclude about tool use in human ancestors? Both humans and chimps are separated from our last common ancestor by a few million years of genetic and cultural evolution. Because other chimps have not been seen making spears, maybe â€śspear cultureâ€? arose in this particular chimp lineage some time after it diverged from our own. Is it possible that the first chimp to make a spear was copying a human spear-maker?
Mercado et al. found stones at three nearby sites in Cote dâ€™Ivoire that had apparently been used by chimps to crack nuts thousands of years ago. Experts classifying the stones by their appearance as having been used for pounding. To avoid bias, these tests were done "blind" (experts weren't told which stones were which), with natural stones included as controls, including some with â€śnatural modification that gives them the appearance of artifacts.â€? (The Far Side cartoon â€śCow Toolsâ€? illustrates the problem.) Starch grains on some of the stones were identified as coming from nuts eaten mainly or exclusively by chimps and not by (modern?) humans. Carbon-14 dating of charcoal from forest fires showed that the layer where the stones were found was 4300 years old. This â€śpredates the advent of settled farming villages in this part of the African rainforestâ€?, making it less likely (though still possible) that the first chimps to use stones as â€śhammersâ€? were imitating humans. Maybe the last common ancestor of humans and chimps used stone hammers and passed this cultural trait on to both lineages, or maybe the two species invented hammers independently.
Preutz and Bertolani endorse an earlier suggestion that â€śthe earliest tool technology likely consisted of pounding or throwing rocks and hitting and jabbing sticks at about 6 million years ago.â€? Both of these studies, and other reports of chimps using sticks to collect termites, etc., show that animals with chimp-size brains can make and use rudimentary tools. If the last common ancestor of chimps and humans was using sharpened sticks as spears and stones as hammers several million years ago, humans have certainly made a lot more technological progress than chimps have. Maybe technologically innovative adolescent chimp nerds had more trouble finding mates?