Living Links: Near and Far

An article published December 4th reported the first observations of chimpanzees using tools to dig up tubers for consumption. Digging for food is a remarkable behavior on many levels, but primarily I believe it captures our imagination because we immediately connect this behavior with its ultimate application by humans: agriculture. The discovery by Hernandez-Aguilar, Moore, and Pickering was made by observing tell-tale indirect evidence of the behavior: sticks with dirt on one end, holes, wadges of chewed tubers, and chimpanzee knuckle-prints, feces, and nests. Think CSI: Tanzania.

These chimpanzees live in a region that is a mixture of forest and grassland with highly seasonal rainfall. The habitat is believed to be both the limit of what chimpanzees can tolerate, and similar to what early hominins at one point occupied. The thought process is that since our early human ancestors made the transition from forests to grasslands, discoveries like this one offer a glimpse into what behaviors these ancestors may have been capable of, and thus how this transition may have begun. As fascinating as this discovery is, many of the conclusions and implications for human evolution were already known from a different source, un-cited by the authors, several thousand miles and many millions of years of evolutionary time away.

Three years ago scientists in Brazil directly observed and videotaped capuchin monkeys digging for tubers using stones. De A. Moura and Lee were the first to observe an animal, any animal, using tools to dig up food outside of humans. What can the behavior of a monkey, with whom we diverged roughly 35-40 million years ago, tell us about human evolution? Well, quite a bit, actually. The capuchin monkeys observed digging live in a dry forest with highly seasonal rainfall at the limit of what this species is believed to tolerate (sound familiar?). The authors of the chimpanzee study conclude that digging for tubers “need not have required a technology that preserved in the archaeological record,” This is in reference to the stick tools used by chimpanzees, but the unmodified stone tools used by capuchin monkeys already demonstrated that. Hernandez-Aguilar and co-workers also say “the discovery that savanna chimpanzees use tools to obtain USOs [underground storage organs, jargon for tubers] shows that such consumption was within the grasp of chimpanzee-like hominins.” Again, this was implied by the capuchin monkeys doing the same, as it demonstrated that the behavior could evolve without modern human-sized brains.

Most importantly, the authors “argue that chimpanzees adapting to such extreme (for them) conditions can be used as models for investigating particular aspects of early hominin behavioral ecology.” It seems appropriate to add capuchin monkeys to that statement. Why? Analogous (or convergent) evolution, when unrelated species independently evolve similar adaptations, can be just as informative as homologous evolution, when adaptations share a common origin. The conditions under which the separate behaviors evolved may be similar, and thus informative, especially when examples from close living relatives are not known or do not exist. In our desire to fully understand human origins, we must not focus only on our closest relatives for answers. The Living Links to our past are all around us; we only need look for them.

–Matthew Campbell

Video of a capuchin digging for tubers

Full references to the articles mentioned:

de A. Moura, A. C., & Lee, P. C. (2004). Capuchin Stone Tool Use in Caatinga Dry Forest. Science, 306(5703), 1909.

Hernandez-Aguilar, R. A., Moore, J., & Pickering, T. R. (2007). Savanna chimpanzees use tools to harvest the underground storage organs of plants. Proceedings of the National Academy of Sciences, 104(49), 19210-19

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