Evolution - T.Rex with it's upright stance evolved out of earlier animals that walked on four legs. The small size of the arms of most (but not all) species in

Evolution - T.Rex with it's upright stance evolved out of earlier animals that walked on four legs. The small size of the arms of most (but not all) species in this family of animals tells you that the arms were not normally used. Some snakes (also having evolved from animals with legs) still have vestigial legs - all but disappeared since they too serve no purpose.