The oldest known type of belief system in the world. It is still practiced in a variety of forms in many traditional societies. Animists practice nature worship. They believe that everything in the universe has a spirit. This is exemplified by the practices of the Plains Indians in North America who would praise the spirit of the buffalo that they killed for giving its life to them so that they might survive. Animists also believed that ancestors watch over the living from the spirit world. This belief resulted in ancestor worship as a means of communicating with and showing respect to ancestors.