The ‘‘tension’’ pelvis of arboreals evolved toward a ‘‘pressure’’ pelvis with a shorter distance between the
sacroiliac and coxofemoral joints and a major widening of the sacrum. These morphological rearrangements of the chidpelvis had to face the compromise between the birth and bipedalism constraints. The straightening up of
the trunk was combined with the acquisition of four spine curves, the lumbar curve being the result of a reverse incurvation of the sacrum. In the hip region, the development of the gluteus maximus contributed to the straightening up of the spine. Primates’ prehensile foot was converted into a weight-bearing and propulsion foot with reduction of the
distance between the first and second rays, the appearance of two arches of the foot, the internal longitudinal arch and
the anterior transverse arch, and the horizontality of the
tibio-talar joint surface. In the knee region, the permanent
knee flexion used in non-human arboreal and terrestrial primates
was converted into a complete weight-bearing knee
flexion and extension specific to humans.