Human evolution from its first separation from the last common ancestor of humans and chimpanzees is characterized by a number of changes. The most significant of these are bipedalism, increased brain size, lengthened gestation and infancy, and decreased sexual dimorphism. Bipedalism is the basic adaptation of the hominin and is considered the main cause behind skeletal changes shared by all bipedal hominins. Anatomically, the evolution of bipedalism was accompanied by a large number of skeletal changes, not just to the legs and pelvis, but also to the vertebral column, feet and ankles, and skull.
The images above all illustrate the presence of a vestigial structure. What is this structure and why is it no longer used for its original function?