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Thursday, January 3, 2013

Reinterpreting hominin climbing abilities

It can be dangerous to infer too much about function from the physical traces left by extinct animals. Case in point: the amount of time early hominins spent in trees. Australopithecus afarensis (creatures that lived over three million years ago typified by the specimen known as ‘Lucy’) had the foot and ankle structure required to walk upright. They also lacked the grasping feet possessed by our earlier ancestors and by modern chimpanzees. Can we therefore assume that Lucy and her ilk were poor tree climbers? Not so fast, say Vivek Venkataraman, Thomas Kraft, and Nathaniel Dominy from Dartmouth College. Just look at modern humans who routinely climb trees today.

The main reason modern hunter-gatherers climb trees is to acquire honey. In some communities, during ‘honey season’ men will spend a third of their total foraging time collecting honey. Needless to say, this activity requires much time in trees. Although some people make use of technology (pegs, harnesses, ropes), there are groups of people who climb trees without using any external aids.

The authors compared the agriculturalist Bakiga people with the hunter-gatherer Twa people, all living near Uganda's Bwindi Impenetrable National Park (a place that sounds like it should be featured in The Princess Bride). Twa men routinely climb trees unassisted, whereas Bakiga men rarely climb trees at all.

When chimpanzees climb a tree, they often use the method of grasping the trunk with their hands, planting their feet flat on the bark and ‘walking’ upward. Such ‘vertical climbing’ requires a high degree of ankle flexation that didn’t seem to be permitted by modern human anatomy. It turns out that Twa men are able to flex their ankles to nearly the same degree as chimpanzees and have been documented walking up trees in the same manner. In addition, Twa men had much longer leg muscle fibers than Bakiga men. The combination of longer muscle fibers plus flexible ankles make the Twa extremely accomplished tree climbers despite having modern human anatomy.

Clearly, if modern humans can be expert climbers, so could early hominins like Lucy. This certainly doesn’t prove that A. afarensis spent their days in trees, but it does mean that they could have.

Venkataraman, V., Kraft, T., & Dominy, N. (2012). Tree climbing and human evolution Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1208717110.

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