The long-unresolved and much publicized puzzle of how human ancestors could have ventured upon habitual bipedalism without making themselves highly vulnerable to predation derives from a misunderstanding of the principles upon which predator-prey relations work. Erect stature, regardless of locomotion proficiency, in fact enhances passive, and not simply active, defence against predators. This was the necessary condition, the security ‘cover’, for selection for the elaboration of bipedal behavior. As illustration, I analyse the defence systems ofPan troglodytes and the hunting techniques of their predators, known and potential, in order to hypothesize the defence impact of bipedal selection on the former; the relevance of human experience with felid predators, in particular, is also then explored. Despite known major differences in the late Miocene mammalian predator guild, the predator-prey framework is likely to have been much the same as to-day, making the findings applicable to the situation of proto-hominids. Once it is accepted selection pressure for bipedalism was originally focused on posture and not locomotion, the mystery of its emergence disappears.