Animals that walk upright on two legs either some the time or all the time have several advantages over those who do not. This posture raises the head higher up which gives the animals a wider field of view to look for prey and keep an eye out for predators. Being upright also makes it easier to reach up into trees for food (with the hands or the mouth) and it frees up the hands to hold and carry things. Self-defense using the front limbs is also possible with bipedalism. Overall, quadrupeds run faster on four limbs than animals that use bipedal locomotion, but in endurance running, humans can outrun most quadrupeds. Over millions of years, natural selection has repurposed limbs that are not used in bipedalism into structures like wings and hands.
The image above shows the earliest human ancestor Australopithecus afarensis who was fully bipedal. Also known as Lucy, she lived around 3.2 million years ago.
References
Bipedalism. (n.d.). In Wikipedia. Retrieved August 4,2017 from https://en.wikipedia.org/wiki/Bipedalism