The evolution of human bipedalism from our quadrupedal ancestors was a complex process. We now think that our earliest bipedal ancestors did not walk bipedally at all times. By 4.5 million years ago, skeletal evidence of hominin pelves and lower limbs show that human ancestors were clearly adapted for bipedal locomotion. However, the hominin skeleton also retained features for maneuvering within the trees such as a grasping big toe. It is likely that fossil species from this time, such as Ardipithecus ramidus, walked bipedally in some contexts and quadrupedally in others. Other fossil species, such as Australopithecus anamensis, show more signs of regular or habitual bipedalism. The Kanapoi tibia (shin bone), a fossil discovered in Kenya that dates to 4.2 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.

Human walking with a heel-to-toe stride is a form of locomotion that is unique to our species in the modern world, and there were a variety of forms of bipedal locomotion among hominin species. Bipedality is an energetically efficient mode of locomotion for both long distance travel and for running. More open landscapes and wider home ranges would have favored greater degrees of bipedal locomotion in hominin species.

Image credit https://www.earth.com/news/early-hominids-walk-upright/