Early Human Ancestors May Have Walked AND Climbed for a Living

The results of recently conducted field studies on modern human groups in the Philippines and Africa are suggesting that humans, among the primates, are not so unique to walking upright as previously thought. The findings have implications for some of our earliest possible ancestors, including the 3.5+ million-year-old species Australopithecus afarensis, thought by many scientists to be the first known possible human predecessor to have forsaken arboreal life in the trees and live a life walking upright (bipedalism) on the ground.

Associate professor of anthropology Nathaniel Dominy of Dartmouth College, along with colleagues Vivek Venkataraman and Thomas Kraft, compared African Twa hunter-gatherers to agriculturalists living nearby, the Bakiga, in Uganda. In the Philippines, they compared the Agta hunter-gatherers to the Manobo agriculturalists. They found that the Twa and the Agta hunter-gatherers regularly climbed trees to gather honey, an important element in their diets. More specifically, they observed that the climbers “walked” up small trees by applying the soles of their feet directly to the trunk and progressing upward, with arms and legs advancing alternately. To do this successfully, they said, required extremedorsiflexion, or bending the foot upward toward the shin to a degree not normally possible among most modern humans. “We hypothesized that a soft-tissue mechanism might enable such extreme dorsiflexion,” wrote the authors in their study report. 

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A Twa hunter-gatherer in Uganda climbing a tree to gather honey.  Credit: Nathaniel Dominy

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They tested their hypothesis by conducting ultrasound imaging of the fibers of the large calf muscles of individuals in all four groups. The results showed that the Agta and Twa tree-climbers had significantly longer muscle fibers than those of their agricultural counterparts and other “industrialized” modern humans. 

“These results suggest that habitual climbing by Twa and Agta men changes the muscle architecture associated with ankle dorsiflexion,” wrote the authors of the study. It demonstrated that a foot and ankle bone structure adapted primarily for walking upright on land does not necessarily exclude climbing as a behaviorally habitual means of mobility for survival. 

The implications for our possible early human ancestors, such as the species Australopithecus afarensis, are significant. ”Australopithecus afarensis possessed a rigid ankle and an arched, nongrasping foot,” wrote Dominy and his co-authors in the report published in the 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.” But now, the research shows that bone structure alone is not an indisputable indicator that an ancient hominid was exclusively terrestrial. 

Australopithecus afarensis is an extinct hominid that lived between 3.9 and 2.9 million years ago. It was first discovered by Donald Johanson and colleagues in the Afar region of Ethiopia with the recovery of the partial skeleton of a 3.2 million-year-old specimen they named “Lucy”. The find has represented a possible benchmark in human evolution for decades. Along with being among the earliest possible bipedal primates, it has also been thought to be closely related to the genus Homo (which includes the modern human species Homo sapiens), either as a direct ancestor or indirectly through an unknown earlier ancestor. 

The detailed study report can be found in the publication, Proceedings of the National Academy of Sciences (PNAS). 

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