Again, some very good points atomic, I found an interesting article about the evolution of upright walking on the
BBC.
"They suggest the shift made by our ancestors to a terrestrial lifestyle came about as climate change thinned out their forest habitat.
In response, these ancient ape-like creatures, or hominids, may have abandoned the high canopy for the forest floor. Here, they remained bipedal and began eating food from the ground or from smaller trees. "