To biped or not to biped?

The obturator externes groove is the groove on the back end of the femur neck. It allows for the connection of the obturator externes muscle, which is important for bipedal locomotion. Simply put, it serves to steady the hip joint and allow us bipeds to flex and rotate our thigh. By bridging the leg and hip, it allows the hip to support the body’s wight and allow upright walking. In the Orrorin bones, this groove is clearly visible in two of the specimens. The presence of the groove reveals that the species often hyper-extended their femora and, thus, used frequent bipedalism. It is interesting that chimpanzees, who occasionally walk upright, do not have obturator externes grooves. What this points to is that Orrorin and other species with these grooves used bipedal locomotion frequently enough to require these grooves to support their bodyweight. 

The existence of these grooves in Orrorin, is indicative of frequent upright posture and bipedal locomotion. However, this doesn’t mean they walked exactly like modern human species or that bipedal locomotion was their only means of locomotion. They definitely were able to climb trees, but were also capable of some bipedal ground walking. One hypothesis of early bipedalism is that early apes walked atop branches and used their arms to balance. This could be one of the ways Orrorin and similar early species moved. 

Sources:

Bipedalism in Orrorin tugenensis revealed by its femora by Martin Pickford, Brigitte Senut, Dominique Gommery, Jacques Treil 

https://humanorigins.si.edu/evidence/human-fossils/species/orrorin-tugenensis