The chimpanzee is sometimes referred as walking on "4 legs", their arms representing 2 of their legs. Four legs are far more stable then two legs; usually the person who walks bipedal will be slower and get tired faster than a four legged animal. Chimpanzees don't use their arms as legs, they use it more like a walker. .
Reasons for Bipedalism.
Humans did not walk bipedal from the start, nor did they just stand up and start doing it. We use to knuckle walk like apes. There are many theories out there for why we first started becoming bipedal. Such are to free our hands for carrying things, or we could see further. Some propose that it was to help reach for food in the branches of trees; others say that it was to discourage violence through upright posturing. Some claim that that it enabled them to travel long distances more efficiently, others say that it helped them do slow-speed foraging. One even suggests that the earliest bipeds, in order to gain an advantage over their rivals in foraging time, moved from the shade of trees out onto the open grassland, like mad dogs or Englishmen, into the mid-day, equatorial sun. Once there, the theory claims that they would have moved in an upright manner, by some strange logic, in order to keep cooler. These are some theories why we first became bipedal. .
Advantages of Walking Upright.
.
The Anatomy of Bipedalism (Comparison of the bipedal and knuckle-walkers).
.
Head - The foramen magnum is the hole through which the spinal cord exits the skull. In apes the foramen magnum is angled backward rather than beneath the skull, as in humans. In humans the foramen magnum is located beneath the skull to balance the head and hold it upright.
.
Spine - The spine has several curves to maintain balance. Because apes have no lumbar curve to pull the upper body back over their pelvis, their weight pulls them forward, forcing them to knuckle walk. Humans, however, have an additional lumbar curve that positions the body inward, over the pelvis.
