Did Lucy walk, climb, or both?

Much has been made of our ancestors "coming down out of the trees," and many researchers view terrestrial bipedalism as the hallmark of "humanness." After all, most of our living primate relatives -- the great apes, specifically -- still spend their time in the trees. Humans are the only member of the family devoted to the ground, living terrestrial rather than arboreal lives, but that wasn't always the case.

Representation of Lucy [Credit: WikiCommons]

The fossil record shows that our predecessors were arboreal habitués, that is, until Lucy arrived on the scene. About 3.5 million years ago in Africa, this new creature, Australopithecus afarensis, appeared; Lucy was the first specimen discovered. Anthropologists agree that A. afarensis was bipedal, but had Lucy and her legions totally forsaken the trees? The question is at the root of a controversy that still rages.

"Australopithecus afarensis possessed a rigid ankle and an arched, nongrasping foot," write Nathaniel Dominy and his co-authors in Proceedings of the National Academy of Sciences (PNAS). "These traits are widely interpreted as being functionally incompatible with climbing and thus definitive markers of terrestriality," says Dominy, an associate professor of anthropology at Dartmouth.

The famous “Lucy” skeleton [Credit: WikiCommons]

But not so fast; this interpretation may be a rush to judgment in light of new evidence brought to light by Dominy and his colleagues. They did what anthropologists do. They went out and looked at modern humans who, like Lucy, have feet adapted to terrestrial bipedalism, and found these people can still function as effective treeclimbers.

Co-authors Vivek Venkataraman and Thomas Kraft collaborated with Dominy on field studies in the Philippines and Africa that inform their PNAS paper. Venkataraman and Kraft are Dartmouth graduate students in the Ecology and Evolutionary Biology PhD program in the Department of Biological Sciences, and are supported by National Science Foundation graduate research fellowships.

Men in Twa society from Uganda regularly climb trees to gather honey [Credit: Nathaniel Dominy]

The studies in Uganda compared Twa hunter-gatherers to their agriculturalist neighbors, the Bakiga. In the Philippines, the researchers studied Agta hunter-gatherers and Manobo agriculturalists. Both the Twa and the Agta habitually climb trees in pursuit of honey, a highly nutritious component of their diets. They climb in a fashion that has been described as "walking" up small-diameter trees. The climbers apply the soles of their feet directly to the trunk and "walk" upward, with their arms and legs advancing alternately.

Among the climbers, Dominy and his team documented extreme dorsiflexion -- bending the foot upward toward the shin to an extraordinary degree -- beyond the range of modern "industrialized" humans. Assuming their leg bones and ankle joints were normal, "we hypothesized that a soft-tissue mechanism might enable such extreme dorsiflexion," the authors write.