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The obturator externus groove and bipedality

There are several indicators of bipedality in modern humans and their ancestors, some of which are not easily noticed or known by those who are not experts in the field. 

The obturator externus groove is one of these features. This groove is located on the back of the femoral neck and serves as an attachment area for the obturator externus tendon, which runs from the back of the thigh to the front of the pelvis. The obturator externus serves as a flexor and external rotator of the thigh, shortening the distance between the pelvis and the femur, which helps to steady the hip joint. 
In bipeds, this tendon creates a groove by applying pressure to the back of the femoral neck as a result of frequent full extension of the femur. As pressure is placed onto the neck of the femur, the bone remodels itself to accommodate the tendon. 

Apes are naturally knucklewalkers but sometimes use bipedal locomotion. Even so, there is little to no pressure produced on the femur from this tendon, and no groove is produced. Pickford et al. state that "the limited bipedal activity undertaken by chimpanzees is too transient to lead to remodeling of the outer contour of the bone, so no obturator externus groove occurs in this species." 

Due to research such as those mentioned above, we can use the femora of early hominids to help determine whether or not they would have been habitual bipeds. This is the case with Orrorin tugenesis, a very early hominid. In fossils that we do have from this species, the obturator externus groove is clearly visible. Scientists have used this feature, among many others, to suggest that this species was bipedal, meaning that bipedality evolved in early hominids over, at least, six million years ago. 

Posterior view of the left femur (Orrorin tugenesis)

Pickford, Martin, et al. “Bipedalism in Orrorin Tugenensis Revealed by Its Femora.” Comptes Rendus Palevol, vol. 1, no. 4, 2002, pp. 191–203., doi:10.1016/s1631-0683(02)00028-3.

Comments

  1. Marc KisselJune 6, 2020 at 11:25 AM

    wow, this is such a great post on what is sometimes an overlooked feature. I always was interested in obturator externus groove for some reason (i think cause i liked the name!) so enjoyed reading this. great topic to explore!

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