Apes sometimes move on their hind limbs alone but cannot do so for long. Humans, on the other hand, normally walk on two legs. Around 5 million years ago, our ancestor, early Australopithecus, was also walking on two legs, though probably equally at home climbing trees.
Many parts of our body have developed to make bipedalism - walking on two legs - possible. Our pelvis is short and wide, our leg bones long and slightly angled, with a knee joint that allows the leg to be fully extended in a stride, and we have sacrificed the ability to grasp with our feet in order to stand on them, with an arch to put a spring into our step.
Men are well designed for walking; for women it is harder since their thigh bones are angled more widely by their much larger pelvis, accommodating the conflicting needs of childbearing.
Bipedalism was the first recognizably human trait to develop. What were its advantages? One was the freeing of the hands, allowing things to be carried or brandished. Another was the ability to see further, useful in the environment of woodland savannah that Australopithecus was beginning to utilize.
Perhaps most importantly, bipedalism is much less tiring over longer distances than walking on four limbs, giving our early ancestors the ability to roam widely and for long periods in search of food.
