Origine et évolution de la bipédie

During around five millions years our ancestors, who were arboreal primates, became progressively occasional, frequent and permanent terrestrial bipeds. Thus, they restricted their locomotor repertoire from polyvalence to specialization. We show that the adaptation to bipedalism is the result of two different processes, the result of natural selection and the result of learning to walk. The characters of our skeleton, heritage of natural selection, affected first pelvis and vertebral column and then cranial base, body proportions and feet. Since reaching bipedal balance is a very demanding constraint in the face of gravity, we describe the “epigenetic” characters which develop in association with learning to walk. Important femoral angular modifications result in the adduction of lower limbs. The angle of pelvic incidence, which defines the sagittal pelvic morphology, increases with gait acquisition in tight association with the formation of lumbar curvature. A link can be established between these genetic modifications and these epigenetic acquisitions. As an illustration we show how the pelvis and spine became a functional unit during the transition from facultative to permanent bipedalism, probably during the evolution of the large group of Homo erectus.