Final answer:
The femur in bipeds is angled inward, known as the valgus angle, which helps position the knees and feet under the pelvis for efficient walking. This contrasts with the straight femur of quadrupeds. Bipedalism has led to significant evolutionary changes in hominin skeletal structure for better balance and mobility.
Step-by-step explanation:
While the femur of a quadruped is straight, the femur of a biped is angled inward, an adaptation known as the valgus angle. This inward angle positions the knees and feet directly under the center of the pelvis, aiding in bipedal locomotion. Homo erectus, an early human ancestor, showcased a nearly as efficient bipedalism as modern humans, with a broader pelvis, longer legs, and arched feet. However, their upper body morphology, including a sloping forehead and small brain, differed significantly from ours.
Bipedalism in hominins has also led to the evolution of spinal curves to balance the weight of the upper body and an alignment of the big toe with other toes to transmit weight during the step phase of walking. Changes in skeleton and joint anatomy, like the femur, pelvis, and knee joints, have been critical in evolving from quadrupedalism to bipedalism. These anatomical changes facilitate a more efficient stride while moving over land and help stabilize the body when only one foot is on the ground.