Final answer:
Habitual bipedalism is unique to humans among living primates. Early human ancestors were also bipedal, but other great apes are mainly quadrupedal and only occasionally move on two legs.
Step-by-step explanation:
Habitual bipedalism, or walking on two legs, is a defining characteristic of the hominin clade within the primate family. The only current members of the primate order that are habitually bipedal are humans. Early human ancestors, such as Australopithecus and Homo erectus, also exhibited this trait, but among the living primates, only humans have retained this mode of locomotion.
While other members of the family Hominidae, which includes the great apes—chimpanzees, gorillas, and orangutans—and humans, are mainly quadrupedal, they can occasionally move bipedally. However, such movements are not habitual but rather situational, such as when gathering food or materials. In contrast, humans have evolved anatomical adaptations that facilitate efficient bipedal locomotion, including a broader pelvis, longer legs, and an arched foot, allowing for an upright stance and a bipedal gait.