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While popular imagination may be fascinated by when our ancestors first began to walk upright, scientific debate has focused on whether these early humans were still skilled climbers. A group of researchers in New Hampshire addressed the issue in a paper recently published in PNAS, gleaning new data from modern humans who climb regularly.

Australopithecus afarnesis, including the famous Lucy, may have been one of the first of our ancestors to walk upright. Scientists know this because the legs of Au. afarnesis fossils have specific modifications suited to an upright posture; unlike earlier hominids and other great apes, they had rigid ankles, arched feet, and a big toe which was incapable of grasping. These changes made it easier for Au. afarnesis to walk on two feet, but how did they affect our ancestors’ ability to climb? An ongoing dispute about this question has centered around how scientists have interpreted the fossil evidence. Some contend that the “modern” ankle and foot of Au. afarnesis were unsuitable for climbing and that these early hominids were mainly bipedal, while others argue that these changes wouldn’t have posed major limitations.

A team of anthropologists from Dartmouth College decided to tackle this question by studying groups of modern humans who habitually climb trees. While humans in industrial societies do climb, we’re quite poor at it compared with our primate cousins. However, humans in many hunter-gatherer populations often climb 20-50m into the treetops to collect honey, which can be an important source of nutrition. Although these people may use tools when climbing thick trees, they regularly climb thinner trees without any assistance. Their climbing style, called changwod in Malaysia, is similar to that used by chimpanzees; the climber places the sole of their foot on the trunk and uses their arms for support as they “walk” up the tree. This technique needs a remarkably flexible ankle in order to be safe and efficient, but modern humans, like Au. afarensis, have relatively rigid ankles. To understand this better, the researchers compared two pygmy populations living near the Bwindi Impenetrable National Park in Uganda, the Twa hunter-gatherers and the non-climbing Bakiga agriculturalists.

The team filmed experienced Twa honey gatherers climbing trees and used still images from the film to measure how far their ankle bent towards their shin while climbing. The findings were striking. Twa climbers can bend their ankles up to 40°; this is within the range of modern chimpanzees and would cause major injury to industrialized humans, whose ankles can bend 15-20°. Could this extreme flexibility be due to skeletal differences? To find out, the researchers visited the University of Geneva and measured the shin-bones of skeletons of male Ituri, another pygmy population who frequently climb trees. After they corrected for differences in size, they found no difference from other modern humans. The team then used ultrasound to compare the calf muscles of the Twa and the Bakiga and found something interesting. Twa men had significantly longer muscle fibers in their calves than did Bakiga men; the women, however, have similar fibers in both groups, since they don’t regularly climb. Similar results were found when comparing the calves of men of the climbing Agta and the agriculturalist Manobo of the Philippines. In other words, the calf muscles of modern humans who are habitual climbers develop in a manner which allows their ankles to bend much further towards their shins, making them safer and more efficient climbers.

This study can’t conclusively tell us whether Lucy and her kin were regular tree climbers, but it does show that their ankles weren’t necessarily a problem. Longer fibers, such as those of the Twa, would have allowed their ankles to bend far enough to be effective climbers. These changes would be hard to detect in the fossil record, since muscle and other soft tissues don’t fossilize. Au. afarensis may have climbed trees to escape from predators, forage for food, or even to rest and sleep. While debate about their lifestyle will doubtless continue, this study shows that our body’s development is flexible enough to allow our bipedal ancestors to have been effective climbers despite their apparently rigid ankles.

Venkataraman, V., Kraft, T., & Dominy, N. (2012). Tree climbing and human evolution Proceedings of the National Academy of Sciences, 110 (4), 1237-1242 DOI: 10.1073/pnas.1208717110