Final answer:
Humans are classified as bipedal due to evolutionary traits such as the angling of the femur, spinal curves, and arched feet, irrespective of individual differences. Prosthetics may enable locomotion in individuals born without legs, but the species' classification as bipedal remains unchanged due to our ancestral adaptations.
Step-by-step explanation:
Although humans are characterized as bipedal due to our evolutionary adaptation to walk on two legs, individual variations such as a child born without legs do not change the classification for the species. Bipedal locomotion involves one leg being the stance leg while the other, the step leg, moves forward. This distinction is what sets humans apart and contributes to our ability to walk efficiently on two legs.
In the case of humans born without legs, prosthetic technology and adaptive devices broaden the definition of bipedalism by allowing such individuals to participate in bipedal movement. However, anatomically, the defining features of bipedal hominins such as the valgus angle of the femur, spinal curves, and arched feet evidence the extensive anatomical adaptations facilitating our bipedal gait. These aspects, developed over millions of years since our earliest hominin ancestors, are instrumental in bipedal locomotion.
Overall, bipedalism as a human characteristic is defined by these evolutionary traits and not solely on the physical ability of every individual. Also, early Homo species such as Homo erectus were nearly as efficient at bipedal locomotion as modern humans and differences in skeletal structure among various hominids, like H. floresiensis, show varied adaptations for bipedality.