Final answer:
The most critical physical change for bipedalism's success was the anatomical reconfiguration of the pelvis and lower limbs. The angling of the femur, the development of spinal curves, and structured feet allowed hominins like australopithecines and Homo erectus to walk efficiently on two feet, supporting the savanna hypothesis.
Step-by-step explanation:
The single biggest physical transformation that had to take place for bipedalism to be a successful adaptation was the anatomical reorganization of the pelvis and lower limbs. In particular, one of the most crucial changes was the evolution of the femur at a valgus angle which positioned the knees and feet under the body's center of gravity. This structural adjustment, along with the development of spinal curves, allowed for the balancing of the upper body's weight over the hips, facilitating an efficient bipedal gait. Additionally, the arched feet and the parallel alignment of the big toe played significant roles in transferring weight during walking.
Early hominin fossils, such as australopithecines and Homo erectus, exhibit these traits. The australopithecines were already walking on two feet, leading to further evolutionary changes in early Homo species. These changes included a broader pelvis, longer legs, and the presence of arches in the feet, which were integral for bipedal locomotion.
The transition from arboreal life to a terrestrial one, as seen in our hominin ancestors, required multiple changes conforming to the savanna hypothesis, which suggests that human traits evolved as adaptations to living in a savanna habitat.