This paper examines shamans and other "magico-religious" healing practitioners through a cross-cultural study that empirically determines types of magico-religious healers and their characteristics. It is argued that shamanism is found throughout the world because it derives from an ecological adaptation of hunting and gathering societies to biologically based altered-state-of-consciousness potentials. Agriculture and political integration are shown to cause the transformation of the shaman into other types of magico-religious healing practitioners, labeled in this study as shamanlhealers, healers, and possession - trance mediums.