Early Human Ancestors May Have Walked And Climbed Trees
January 1, 2013

Early Human Ancestors Like Lucy May Have Walked And Climbed

April Flowers for redOrbit.com - Your Universe Online

Many researchers believe that one of the pivotal events in becoming human was the development of terrestrial bipedalism, or the ability to walk on two legs. Much has been made of our ancestors "coming down out of the trees." After all, the majority of our living primate relatives — for example, the great apes — still spend a great deal of their time in trees. In the primate family, humans are the only branch devoted to the ground, living terrestrial rather than arboreal lives. However, that wasn't always the case.

It is clear from the fossil records that our ancestors were tree dwellers until Lucy and her relatives arrived on the scene. Australopithecus afarensis appeared in Africa about 3.5 million years ago, and Lucy is the first specimen of this new species ever discovered. Anthropologists agree that A. afarensis was bipedal, but the question remains whether Lucy and her kin had totally forsaken the trees. Controversy over this question still rages today, prompting the new study by Dartmouth College. The findings of this study were recently published in the journal Proceedings of the National Academy of Sciences (PNAS).

"Australopithecus afarensis possessed a rigid ankle and an arched, nongrasping foot," writes Nathaniel Dominy, associate professor of anthropology at Dartmouth College, and his colleagues. "These traits are widely interpreted as being functionally incompatible with climbing and thus definitive markers of terrestriality,"

However, the new study indicates that this might not be the case after all. Based on data gathered during field studies in the Philippines and Africa, Dominy and his colleagues have brought new evidence to light by looking at modern humans who, like Lucy, have feet well adapted to terrestrial bipedalism. The results reveal that these people can still function as effective tree climbers.

In Uganda, the team compared Twa hunter-gatherers to their agriculturalist neighbors, the Bakiga. Likewise, in the Philippines the Agta hunter-gatherers were compared to the Manobo agriculturalists. Honey is a highly nutritious part of the diet of both the Twa and Agta tribes, and they regularly climb trees in pursuit of it. Both use a method of climbing described as "walking" up small-diameter trees; applying the soles of their feet to the trunk of the tree and alternating arm and leg advancements.

Extreme dorsiflexion — bending the foot upward towards the shin to an extraordinary degree —  beyond the range of modern "industrialized" humans was documented by the researchers. The authors "hypothesized that a soft-tissue mechanism might enable such extreme dorsiflexion," assuming that the leg bones and ankle joints were within normal range.

Using ultrasound imaging to measure and compare the lengths of gastrocnemius muscle fibers — the large calf muscles — the team tested their hypothesis. They looked at all four groups and found that among the Twa and Agta, these muscle fibers were significantly longer.

"These results suggest that habitual climbing by Twa and Agta men changes the muscle architecture associated with ankle dorsiflexion," the team says. This demonstrates that the development of foot and ankle adaptations for walking on land does not necessarily mean that Lucy and her kin weren´t still able to use their feet for climbing as well.

The Dartmouth research team says that this is just one of many examples where modern humans serve as useful models for studying anatomical correlates of behavior, both in the present and the distant past of our ancestors.