Final answer:
Bipedalism is characterized by the inward valgus angle of the femur for balance, the development of spinal curves for weight distribution, and the evolution of the foot's arch and toe alignment for stability and weight transfer.
Step-by-step explanation:
Three characteristics of bipedalism involve key anatomical changes that occurred in hominins through evolution. First, the femur (upper leg bone) angles inward at what is known as a valgus angle, positioning the knees and feet beneath the center of the pelvis. This is crucial for maintaining balance during the stance phase of walking. Furthermore, bipedal hominins have developed spinal curves that allow for the effective balancing of upper body weight over the hips. Lastly, the evolution of the arch in the foot and the alignment of the big toe parallel to the other toes enhances stability and weight transfer during the step phase.
These adaptations were significant in the shift from quadrupedal locomotion to upright, two-legged walking. This change is evidenced by the Laetoli footprints discovered by Mary Leakey, which indicate a modern striding gait from about 3.5 million years ago. Additionally, early Homo species like Homo erectus had a morphology conducive to efficient bipedal movement, featuring broader pelvises, longer legs, and arched feet, in comparison to their quadrupedal counterparts.