More evidence has emerged that Australopithecus afarensis - generally known as Lucy - lived at least partially in the trees.
The fossil record shows that our ancestors were largely arboreal until Lucy arrived on the scene about 3.5 million years ago. But what happened then is less clear.
"Australopithecus afarensis possessed a rigid ankle and an arched, nongrasping foot," write associate professor of anthropology at Dartmouth Nathaniel Dominy and his co-authors. "These traits are widely interpreted as being functionally incompatible with climbing and thus definitive markers of terrestriality."
But, found the researchers, modern humans also have feet adapted to terrestrial bipedalism, but can still function as effective treeclimbers. They compared Twa hunter-gatherers in Uganda to their agriculturalist neighbors, the Bakiga; and, in the Philippines, Agta hunter-gatherers to Manobo agriculturalists.
Both the Twa and the Agta, they found, habitually climb trees in pursuit of honey. They do this by 'walking' up small-diameter trees, with the soles of their feet applied directly to the trunk and their arms and legs advancing alternately.
Among the climbers, Dominy and his team documented extreme dorsiflexion, with the foot able to bend towards the shin much more than that of 'industrialized' humans. This was made possible, they found, by significantly longer muscle fibers.
"These results suggest that habitual climbing by Twa and Agta men changes the muscle architecture associated with ankle dorsiflexion," write the team.
Their findings bear out the results of recent research from Midwestern University, which indicated that while Australopithecus afarensis was mainly bipedal, the species spent a lot of time in the trees.