Human Bipedalism, Evolved from Arboreal Locomotion of Two-arm   Brachiation
  Among all kinds of apes, only gibbons have the slim body as human. Gibbons can move in the forest by cross arm swing, what was the locomotion mode of our arboreal ancestor. Since our ancestor had much heavier body but weaker arms than gibbons, we suppose they had to move with two arm brachiation. Such mode of locomotion can account reasonably for the transition to bipedalism. Firstly, it needed our ancestor to straighten knee and hip joints and flex their lumbar spine. secondly, it evolved the feet of our ancestor with longitudinal arche. And most importantly, it made the ratio of the length of the upper limbs to that of the lower limbs unsuitable for quadruped walking. 
C.Fang, T.Jiang, X.Yuan
q-bio.PE