To see the difference between biped and a quadruped, let's compare the skeletons of the man (biped) and the horse (quadruped):
*The man has the bigger pelvis: he has to bear the weight of the body.*The scapula is smaller.*The bones of the hind leg bigger.*The forearms more mobile.*The man has the spine that attaches under the skull.*And the tail that has completely regressed.
the man has adapted to standing and biped (he walks on 2 feet): his hind legs are reinforced.
