Final answer:
Bipedalism refers to upright two-legged walking and is the key feature that differentiates hominins from apes. It is characterized by anatomical changes such as a broader pelvis, longer legs, and arched feet, which are evident in species like Homo erectus.
Step-by-step explanation:
The term that refers to upright two-legged locomotion is bipedalism. This mode of locomotion is considered a key feature differentiating early hominins from the apes. Several key adaptations facilitated the developments of bipedalism in early hominins, such as a broader pelvis, longer legs, arched feet, and a valgus angle in the femur that positions the knees and feet under the center of gravity. These anatomical changes, along with other traits like increased brain size, smaller jaws and teeth relative to other primates, and more dexterous hands set the stage for the unique human mode of walking. Homo erectus is a notable example of an early human ancestor that exhibited these characteristics and was very efficient at bipedal locomotion, similar to modern humans.