Major deserts in Africa are the Sahara and Kalahari. The native tsetse fly is known for two very important things: Protecting the rainforest from traditional development using livestock and also for warding off European invaders. Animism is the beliefs that spirits in people, animals, plants and other natural forces play an important role in everyday life. Griots were the African storytellers who kept alive and passed down the unwritten histories of their people. During the 11th century, a lot of African leaders adopted Islam. This religion was spread by trade and the settling of Muslim merchants and teachers in the states south of the Sahara desert. Gold and salt were the most important trade items for the Soninke people in Ghana.