Human skeletal changes due to bipedalism
[6] The human foot evolved as a platform to support the entire weight of the body, rather than acting as a grasping structure (like hands), as it did in early hominids.
The stiffening of the arch would be required of an upward gait, all considered that modern bipedalism does not include grasping of tree branches, which also explains the hallux evolving to line up with the rest of the toes.
[8] Human knee joints are enlarged for the same reason as the hip – to better support an increased amount of body weight.
[7] As a consequence, since the human forelimbs are not needed for locomotion, they are instead optimized for carrying, holding, and manipulating objects with great precision.
[11] Having long hind limbs and short forelimbs allows humans to walk upright, while orangutans and gibbons had the adaptation of longer arms to swing on branches.
This adaptation lets humans lock their knees and stand up straight for long periods of time without much effort from muscles.
[14] Modern human hip joints are larger than in quadrupedal ancestral species to better support the greater amount of body weight passing through them.
This alteration in shape brought the vertebral column closer to the hip joint, providing a stable base for support of the trunk while walking upright.
[1][14] The ilium changed from a long and narrow shape to a short and broad one and the walls of the pelvis modernized to face laterally.
These combined changes provide increased area for the gluteus muscles to attach; this helps to stabilize the torso while standing on one leg.
To increase surface for ligament attachment to help support the abdominal viscera during erect posture, the ischial spines became more prominent and shifted towards the middle of the body.
Without the lumbar curve, the vertebral column would always lean forward, a position that requires much more muscular effort for bipedal animals.
It combines both the pelvic tilt and sacral slope to determine approximately how much lordosis is required for the upright gait to eliminate strain and fatigue on the torso.
If the inward curvature of the spine is not enough, the center of balance would be offset causing the body to essentially tip forward, which is why some apes that have the ability to be bipedal require large amounts of energy to stand up.
Because of this, the erect position of the head is possible without the prominent supraorbital ridges and the strong muscular attachments found in, for example, apes.
Only after the more demanding posture was enabled by changes in the nervous system, could advantages of bipedal over quadrupedal locomotion be utilized, including better scanning of the environment, carrying food and infants, simultaneous upper extremity movements and observation of the environment, limitless manipulation of objects with upper extremities, and less space for rotating around the Z-axis.
