In his book The Evolution of God, Robert Wright outlines an argument for a natural progression from polytheism to monotheism. But, it seems that polytheism is a also natural part of many people's view of religion. read more
Perhaps monotheism could not have arisen without polytheism as the background but monotheism does not necessarily eliminate polytheism. Like other examples of evolution, of which religion is one, two entirely different species can exist and thrive due to different environmental factors. read more