1% not chimpanzee

by Matthew Cobb

It’s often said that we share 99% of our DNA with our closest relatives, the chimpanzees. The evolutionary biologist and science writer, Jared Diamond, memorably called us ‘the third chimpanzee’ (the other two kinds are the chimp you probably think of – Pan troglodytes – and the delightfully sensual Bonobo chimpanzee, Pan paniscus).

There are lots of obvious differences which are related directly or indirectly to that 1% difference – physical, behavioural and cognitive. Now Jeremy DeSilva of Worcester State College, writing in the Proceedings of the National Academy of Sciences, has looked at an apparently minor difference that has a major significance: the ankle.

Using video analysis, DeSilva studied the “dorsiflexion” of chimp ankles as the apes climbed – this is the extent to which the ankle rotates when the toes are pointing upwards. If you try it now, you’ll find your foot moves at most 15-20º. See? Not much movement there. But if you were a chimp (and if you are, try it now), you’d find it moved around 45º. One of the reasons the chimps can do this is that their tibia (the larger of the lower leg bones) has a recess that enables the ankle to flex more.

This isn’t just a tedious piece of comparative anatomy – it has a major effect on the chimp’s behavior. They can climb using their feet in a way that is just impossible for us – and not just because they also have an opposable big toe.

Having established that the shape of the end of the tibia was a diagnostic difference between humans and chimps for their climbing ability, DeSilva turned his attention to our ancestors. He studied 29 tibia and ankle-bones from hominin skeletons of 4.12 to 1.53 million years old. None of them had the chimp-like shape, suggesting that their climbing behavior would have been more like our own than that of chimps.

This is important because there have been arguments about whether our ancestors had at least a partly arboreal life-style, like modern chimps. This study suggests that this was not the case. Or rather, as DeSilva puts it in the careful conclusion to the Abstract of his paper: “This study concludes that if hominins included tree climbing as part of their locomotor repertoire, then they were performing this activity in a manner decidedly unlike modern chimpanzees.”

Citation:

J. M. DeSilva (2009) Functional morphology of the ankle and the likelihood of climbing in early hominins. Proceedings of the National Academy of Sciences (USA). Published online before print April 13, 2009.

Abstract available here – you or your institution will need a subscription to read the original article.