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Lessons > Step by Step: The Evolution of Bipedalism
 

Anatomical Adaptations for Bipedalism: Tibia & Talus (Ankle)

Articular facets of the medial condyles are more elliptical in bipeds and more round in quadrupeds. The size of the lateral condyle is larger and more concave in bipeds.

As the angle of the femur brings the knees inward, the tibia is now almost directly parallel with the center of gravity, creating a right angle between the tibial shaft and its proximal surface. One consequence is that the human medial and lateral proximal articular condyles (i.e., the flattened surfaces on the top of the tibia that articulate with the femur) are more similar in size and are elongated anteroposteriorly (i.e., longer front to back) compared to quadrupeds. The increased size in the lateral proximal condyle is an adaptation to increased weight transfer from the femur to the foot. Modern human condyles also are more concave and elliptical in shape to match the more elliptical femoral distal condyles. Quadrupeds tend to be more spherical in shape and more convex. The elliptical helps lock the knee in place and create a straighter forward leg movement.9

The distal end of the tibia, which along with the talus forms the ankle, is another adaptation to bipedalism. The articular surface for the talus is oriented more inferiorly than the anteroinferior orientation seen in quadrupeds. This inferior orientation results in a straighter foot path during walking. In addition, the talus is more robust which compensates for greater weight transmission during the foot strike.

Chimpanzees retain smaller lateral proximal condyles and the shaft lies at more of an angle to the proximal surface of the tibia. The australopithecine tibia has a nearly right angle between the tibia's shaft and proximal surface, and the distal end is oriented more inferiorly, as seen in modern humans.9