What characteristic distinguishes bipedalism from quadrupedalism?

Bipedalism is distinguished from quadrupedalism mainly by the skeletal modifications that support upright walking on two legs. One of the key features associated with bipedalism is the short, bowl-like, broad pelvis. This pelvis is adapted for human locomotion, allowing for a stable base of support for the body when standing and walking.

In bipedal species, the shape of the pelvis supports the internal organs, aligns the legs under the torso, and aids in balance and weight distribution, which contributes to the efficiency of bipedal gait. The broad and bowl-like structure of the pelvis also facilitates the attachment of muscles necessary for walking upright.

This anatomical change is a significant adaptation that differs from quadrupedal organisms, which often have longer and narrower pelvises suited for their different locomotion patterns. Other choices, like walking on knuckles, using long limbs for swinging, or employing a tail for balance, do not represent defining characteristics of bipedalism compared to quadrupedalism. These features are more relevant to specific adaptations or forms of locomotion seen in some quadrupedal animals or primate behaviors, rather than distinguishing forms of locomotion between bipeds and quadrupeds as a whole.