The arms and forearms shortened relative to the legs making it easier to run.
The foramen magnum migrated under the skull and more anterior.
The femur evolved into a slightly more angular position to move the center of gravity toward the geometric center of the body.
The knee and ankle joints became increasingly robust to better support increased weight.
Humans and bonobos are the only apes in which the female is fertile year round and in which no special signals of fertility are produced by the body (such as genital swelling during estrus).
Nonetheless, humans retain a degree of sexual dimorphism in the distribution of body hair and subcutaneous fat, and in the overall size, males being around 15% larger than females.
It is possible that bipedalism was favored because it freed the hands for reaching and carrying food, saved energy during locomotion, enabled long distance running and hunting, provided an enhanced field of vision, and helped avoid hyperthermia by reducing the surface area exposed to direct sun; features all advantageous for thriving in the new savanna and woodland environment created as a result of the East African Rift Valley uplift versus the previous closed forest habitat.
This change in gait saw a lengthening of the legs proportionately when compared to the length of the arms, which were shortened through the removal of the need for brachiation. Recent studies suggest that Australopithecines still lived part of the time in trees as a result of maintaining a grasping big toe. Anatomically, the evolution of bipedalism has been 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.
There are several theories of the adaptation value of bipedalism.(73–116 cu in), larger even than modern Homo sapiens.This brain increase manifested during postnatal brain growth, far exceeding that of other apes (heterochrony).The most significant of these adaptations are bipedalism, increased brain size, lengthened ontogeny (gestation and infancy), and decreased sexual dimorphism.The relationship between these changes is the subject of ongoing debate.