The origins of bipedal stance still remain a baffling subject with some anthropologists and biologist convinced our ancestors learned to walk on two feet whilst living in trees. Others remain convinced. The former accept human style walking began with arboreal apes approximately 17-24 million years ago, rather than the latter school, who estimate humans started to walk between 4-8 million years ago, when the ape-like human ancestors called Australopithecus started to walk upright.
It is unlikely we evolved through 'knuckle walking' to the upright, bipedal gait of people although this hypothesis is not entirely dismissed by many in academe. Tempting as it may to accept tree walking apes transferred to the ground, forced by deforestation there is just enough information available to be anything other than sceptical Arboreal walkers do rely on the use of their hands for balance and ostensibly this represents more of a modified quadripedal motion than actual bipedal stance.
Another thing anthropologists and biologists argue over is what precisely influenced the design of the human frame. Walking and running are two quite different forms of locomotion which involve a stance phase and a swing phase. When walking, the stance phase (ground contact) accounts for 60% of a gait cycle followed by 40% swing (through). Up to middle distance running the cycle remains more or less the same but in running stance phase is reduced markedly as the swing phase increases and the time spent in dual support is reduced.
Different musculature is necessary to run as opposed to walk and now researchers are hypothesising it took a few million more years for the running physique to evolve and it was running, not walking which made the naughty bits so visible to the naked eye.
The human running frame requires longer legs, shorter forearms to counterbalance the upper body and larger vertebral discs to give better shock absorption. Big buttocks are also very important. The common factor in all of the above is the development of the human foot which could provide a platform for lateral weight bearing and leverage for propulsion.