Tibia & Talus (Ankle)

The distal end of the tibia articulates with the talus at the ankle. In humans, the tibia’s articular surface for the talus is situated relatively more inferior when compared to the anteroinferior orientation in quadrupeds. In addition, the shape of the distal tibia in apes is relatively trapezoid when compared to the square shape of modern humans. This is because the anterior aspect is relatively wider mediolaterally in African apes34

The talar superior articular surface which articulates with the distal tibia sits almost directly superior, or nearly parallel with the talar body in humans. Plantar articulation surfaces on the talus (i.e., the calcaneal articular surface and the navicular articular surface) are also less angled than typically seen quadrupeds. Instead, these surfaces trend downward, forming a plantarly oriented foot when standing. Both the ankle and the subtalar joint are situated directly at the end of the tibia’s long axis, which helps to transmit stress loads from the legs through the foot33,34. Additionally, the relatively inflexible midfoot and horizontal orientation of the ankle joint encourage a straighter foot path during walking. The talus is also relatively robust in humans, which helps absorb stress during foot strike.

In comparison with humans, corresponding articular surfaces in chimpanzees appear more angled. For example, the superior articular surface on the chimpanzee talus is relatively more medially angled than in humans33. This medial orientation of the talar superior articular surface may be associated with the inverted position of the foot used during vertical climbing33. Likewise, the calcaneal articular surface is more rounded suggesting relatively agile mid-foot flexibililty typical of arboreal primates. 

The A. afarensis distal tibia articular surface is oriented relatively inferior, as seen in modern humans. The talus is relatively derived in that it is positioned at the end of the tibia’s long axis, and the articular surfaces sit relatively parallel with the talar body34. The fossil material suggests that A. afarensis had an inflexible midfoot, although perhaps not as restrictive as seen in modern humans34.