Study shows human ancestors may have ‘walked up’ trees
A new study challenges the notion that humans’ primate ancestors came down from trees and only gradually began to walk upright. According to Scientific American, new findings in the fossil record indicate that our species’ journey to our modern bipedal posture may have been much more complicated and convoluted that previously thought.
Fossil evidence from early hominins suggests that certain adaptations conducive to tree-climbing like longer arms and fingers coexisted with adaptations necessary for upright walking like the arched foot now found only in Homo sapiens. As human beings became the walkers that we are today, the upper body, tree-climber adaptations vanished. However, new findings suggest that the path to walking upright was not as direct as scientists once believed.
Dartmouth University researchers led by graduate student Vivek Venkataraman have been attempting to study that transition not by studying fossils but by observing modern humans as they climb trees. New research on hunter-gatherer groups shows that some humans ascend slender trees using their two feet to “walk up” the tree, rather than relying on a series of hand-holds to pull themselves up like chimpanzees.
The researchers studied two groups of Ugandan tribespeople, the Twa, a group of hunter-gatherers, and their farming neighbors, the Bakiga people, as well as two groups native Philippine groups, the Agta, who are hunter-gatherers, and the Manobo, who are farmers.
Both of the hunter-gatherer groups consume honey as a staple of their diet. In order to harvest it, they begin climbing trees at a young age. Both groups climb trees by wrapping their arms around the tree at head level, then placing one foot above the other, “walk” up the tree to the honey source.
Tree-based foraging like this makes the ankles, calves and lower legs of the Twa and Manobo people more flexible and adept at this type of climbing than people who do not regularly climb trees as a part of their culture. The adaptation is a matter of training soft tissue in the leg to be stretchier.
Ultrasounds of the lower legs of the climbing hunter-gatherers look no different than ultrasounds of their city-dwelling and farming counterparts, but the climbers are able to flex their ankles an average of 45 degrees further upward toward the shin than non-climbers.
The research team said in a paper published in the Dec. 31 issue of Proceedings of the National Academy of Sciences that our upright-walking ancestors like Lucy (Australopithecus afarensis) could have been quite capable of ascending into the tree canopy as needed, whether for food, hunting or safety.
“Our findings challenge the persistent arboreal-terrestrial dichotomy that has informed behavioral reconstructions of fossil hominins,” wrote the team. These findings, they said, highlight “the value of using modern humans as models for inferring the limits of hominin arboreality.”
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