I've had the privilge of seeing a cast of those famous footprints in Tanzania. Now careful analysis of these 3.5 million year old prints confirm that fully upright walking was involved. Ape walking involves bent knee and bent thigh. To walk upright involves changes in the leg bones, the pelvis, the spine, the head and neck, a beter balance mechanism, and perhaps vision changes. Human walking or hominim walking is more energy efficient.- http://www.plosone.org/article/info:doi/10.1371/journal.pone.0009769