The transition from a facultative biped into a habitual biped is relatively short. As it became more advantageous for this facultative biped to walk upright, the behavior would increase in frequency and influence the anatomical adaptations suited to it. The Kanapoi tibia (shin bone), a fossil discovered in Kenya that dates to 4.1 million years ago, shows an expanded and concave surface at the knee joint as well as a robust ankle joint, indicating that this tibia bore weight regularly.

Image credit David L. Brill. From Lucy to Language, Simon & Schuster, 2006.