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The adoption of habitual bipedal locomotion required a backward shift of the centre of gravity of the body, to a level relative to the supporting surface area of the body and pivotal axis of the hips at which walking with extended knees became practicable. In the morphology of the immediate ancestors to hominids, there were relatively few features whose change could have effectively affected the position of the gravity vertical of the body. Weight could be distributed posteriorly mainly by increasing the mass of the hindlimbs and the kyphotic curve of the vertebral column, by developing buttocks. and by flattening the thorax and abdomen. Estimates indicate that the adoption of a suitable curvature of the vertebral column alone was not sufficient for shifting the centre of gravity behind the ‘threshold of bipedalism’. A considerable increase in the mass of the hindlimbs was also required, and the addition of the mass of the buttocks may have represented the decisive factor for crossing the threshold. A possible physical enviroment in which the change to bipedalism could have taken place was a mountainous terrain with long, steep slopes, transected by gorges and precipices. In such a terrain the increase in the power and mass of both the hindlimbs and the gluteus maximus proprius muscle could have been favoured by selection, leading ultimately to a condition in which the combined effect of heavy hindlimbs. a suitable curvature of the spine and the weight of the buttocks shifted the centre of gravity of the body backwards to a level at which habitual walking with extended knees became practicable. □Hominid evolution. bipedalism, evolutionary thresholds.