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Twa man climbs a tree, by Vivek Venkataraman

What tree-climbing pygmies tell us about foot evolution

At some point in human history, our ancestors descended from the trees and started walking permanently on two legs. In the process, our feet evolved from the grasping appendages of other apes into sturdy levers. We lost an opposable big toe, our ankles became stiffer, and our bones formed an arch that runs from our ankle to our toes. We sacrificed the ability to grip in return for a springy, shock-absorbent step.

These changes were already in place 3.5 million years ago. One of our ancient relatives, Australopithecus afarensis, had a remarkably human foot and was clearly already walking around on two legs. Some scientists have taken this to mean that hominins such as Lucy (the most famous A.afarensis specimen) necessarily walked on the ground. After all, human feet are supposedly ill-suited for life in the trees.

But try telling that to the gentleman in the video below. He’s one of the Twa pygmies—a group of Ugandan hunter-gatherers who often climb trees in search of food, such as honey and fruit. Like other Twa men, he started from an early age. And he’s clear proof that a human foot is no impediment to walking straight up a trunk.

The footage was shot by Vivek Venkataraman, Thomas Kraft and Nathaniel Dominy from Dartmouth College. The trio originally started studying the Twa to understand the evolution of their short five-foot stature but were awestruck at how adeptly they could climb. “We tried to climb the same trees, but we found it extremely difficult,” says Venkataraman. “The Twa were quicker, more agile, and highly coordinated.”

That’s because their ankles are so extraordinarily flexible that their feet can make up to 45 degree angles with their shins. You can clearly see this in the video. It’s a level of flexibility comparable to wild chimpanzees, which walk up trees in much the same way. They plant their soles flat against the trunk, allowing them to hold their bodies closer to the trees and reducing the energy it takes to climb.

For comparison, most people can only bend their feet by 15 to 20 degrees. If you or I tried to match the flexion of a Twa climber, our ankles would rupture catastrophically and we wouldn’t be walking, much less effortlessly scaling a thick vine.

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From Venkataraman et al, 2013

But the Twa’s secret isn’t in their ankles, which are indistinguishable from those of other people. Instead, the team found that the Twa’s flexibility stems from calf muscles (gastrocnemius) with unusually long fibres—far longer than those of the Bakiga, a group of neighbouring Ugandan farmers who don’t climb trees. The team found the same differences in the Philippines. The tree-climbing Agta hunter-gatherers have far longer gastrocnemius fibres than the non-climbing Manobo farmers.

Venkataraman suspects that their calf muscles aren’t born this way. Instead, their fibres lengthen with practice. “People who frequently wear high heels have short calf muscle fibres, and their ankles are stiffer as a result,” he says. Regular tree-climbing does the opposite for the Twa.

The stark lesson from this hard-won research is that there’s nothing about a human foot that precludes us from the trees. And equally, the fact that A.afarensis had a human-like foot doesn’t mean that it was a bad climber. The muscles, which don’t fossilise, can make a huge difference. With the right calves, Lucy could have scampered up a trunk as well as striding across a savannah.

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Lucy at the Smithsonian Natural History Museum., by Ryan Somma

Venkataraman’s study lobs some much-needed data into a longstanding debate about how A.afarensis actually moved. “Our field has been arguing about tree climbing in A.afarensis for 30 years,” says Jeremy DeSilva from Boston University. “Remarkably, this is the first study to thoroughly investigate the tree-climbing habits of its closest living relative: humans! It is not that no one ever thought to do this; data such as these are just very difficult to obtain.”

The study helps to make sense of some A.afarensis’s contrasting anatomy—it had the feet of a committed biped, but the long arms, curved fingers, and ape-like shoulder blades of a competent tree-climber. “[It adds] to the growing body of evidence that the supposedly unique human foot is not quite as distinct from that of other great apes as we have tended to believe, and that human bipedalism had arboreal origins,” says Robin Crompton from the University of Liverpool.

Of course, Venkataraman’s study doesn’t mean that A.afarensis climbed trees, just that we shouldn’t rule out that possibility on the basis of its bones. “Maybe not having an opposable big toe isn’t as catastrophic for climbing trees as we commonly assume, especially if you change your climbing style,” he says.

“My guess is that the A.afarensis was mostly a terrestrial biped, living and feeding on the ground. At night, I suspect that they may have climbed slowly and carefully into the trees to build night nests,” says DeSilva. “But that scenario is really tough to test without more fossils and a better idea of what anatomies allow an upright walker to safely and effectively climb a tree.”

Update: A last-minute comment. Claude Owen Lovejoy from Kent State University says that the study misses the point. “No one has ever claimed that A.afarensis did not climb trees. I’m sure they did. Modern humans climb trees, but so what? The argument is not about behaviour but about principal adaptation.” And that was for bipedality. “Bipedality was so central to A.afarensis’ adaptation even if it climbed trees regularly,” says Lovejoy.

Reference: Venkataraman, Kraft & Dominy. 2012. Tree climbing and human evolution.

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