This is a great question and one we should all be asking. Given that our closest relatives the great apes are all quadrupedal, our ancestors at one point must have also been quadrupedal. In fact we as humans are mainly quadrupedal between our first nine to fifteen months of postnatal life. As adults we still engage in quadrupedal behaviors from time to time. For instance, humans often copulate, climb steep hills and walk through thick underbrush on all fours. Because all great apes also engage in bipedal behaviors the difference between us and them is one of frequency. As adults we are much more likely to be bipedal while great apes are much more likely to be on all fours. Because changes in behavior precede changes in structure, to diagnose fossil behaviors we use characters that change during the individual's lifetime in response to practiced behaviors (i.e if we used characters that are mainly inherited to diagnose behaviors we would not know if it is the fossil in question that practiced these behaviors or an ancestor of the fossil that did). In the case of <PERSON> we can use the x-sectional area of the arm bone vs the x-sectional area of the lumbar vertebra. 249
Bone x-sections change during our lifetime in response to the loads placed on them. Because bipeds place all their upper body weight on their lower back vertebrae and none on their arms, while quadrupeds distribute their upper body weight on their arms, it would be expected that a biped have a much larger lumbar and smaller arm x-sectional area than a quadruped. This expectation is confirmed in comparisons of humans to great apes, or to baboons and other large monkeys. Notably, <PERSON> shows ratios that are most similar to baboons (a small lower back and large arms) suggesting it wasn't mainly a biped but often practiced quadrupedal behaviors. Lucy does not preserve a foramen magnum and shows no pelvic characters that are exclusive to bipeds. As you note it only preserves a thigh bone, a leg bone, a single foot bone and two toe bones, none of which show exclusively bipedal characters. The foot bones of the first family fossils which are ~200,000 yrs older than <PERSON> and have been assumed to be the same as Lucy(but may not be) show a gorilla-like foot.
Unfortunately, paleontologist would not gain as much fame or money if they found a possible ape ancestor. As such to be able to claim it is human they claim these fossils are bipeds using whatever human-like characters they exhibit to associate the fossil to bipedality. More than likely <PERSON> represents an African great ape that diverged from the common human African ape group around the time of the chimpanzees/gorilla/human split. <PERSON> is thus a human ancestor and a biped for convenience sake and not based on facts. 249