The Arboreal Ancestry
Recent research has proved to support the hypothesis that one of the hallmarks of human evolution, bipedalism, arose from an arboreal ancestry. As implied, the tree-living ancestor was to have a benefited from a bipedal gait through aided mobility throughout the tree network (Thorpe et. al., 2007). In addition, the use of the upper body in the arboreal ancestor would be helpful in foraging (Stanford 2006). The importance of the arboreal ancestor hypotheses is their relation to the lower body development towards a bipedal posture and gait.
Orangutan Navigation: An Arboreal Hypothesis
As proposed by Thorpe et. al. the importance of unlimited mobility within the canopies provides a selective pressure for the evolution of bipedalism (2007). The basis of the study not only addresses increased mobility, but also increased stability on less stable branches (Thorpe et. al. 2007). Thorpe et. al. proposes that the increased stability on less stable branches is positively correlated to increased mobility (2007). Orangutans exhibit this correlation through their ability to access a wide variety of branches with the aid of the upper limbs (Thorpe et. al. 2007). The study found that 75% of all biped activity individuals would use hands for additional support (Thorpe et. al. 2007). Furthermore, Thorpe et. al. found that 90% of bipedal bouts occurred while supporting with extended hind-limb posture (Thorpe et. al. 2007). Thorpe et. al. evidence proves bipedal posture is widely used as a mechanism of support (2007). A mechanism of support in this case would act as a selective pressure for stability, thus making the bipedal trait a form of stabilizing selection (Thorpe et. al. 2007). Additionally Thorpe et. al. states that bipedal stature and mobility was more common when multiple supports were available (Table 1) (2007).
When addressing locomotion safety Thorpe et. al. found that orangutans, on smaller, flexible branches, would access peripheral branches (2007). The reaching out for peripheral braches placed the orangutan promoted an extended hind limb, which is a prominent mechanism of bipedalism (Thorpe et. al. 2007). “Hand-assisted locomotor bipedality adopted under these strong selective pressures, seems the most likely evolutionary precursor of straight-limbed human walking.” (Thorpe et. al. 2007). The selective pressure was made further evident with forest fragmentation which could limit navigation (Thorpe et. al. 2007).
The study places a large importance on selective pressure that would favor bipedalism (Thorpe et. al. 2007). The first selective pressure involves increased safety through navigation; this pressure provides sound support for an arboreal ancestor (Thorpe et. al. 2007). Increased safety would decrease the morbidity of the individuals who would use upper limbs for support. That being said, arboreal bipedalism in Orangutans is not age specific, which draws the conclusion that the trait is heritable. Although no...