One of my graduate professors, Mark Woodward, is the co-author of a widely used textbook titled "Religions of the World." It is now in its 12th edition. The publisher of this textbook added a page explaining why we need this new addition. The first point it lists is that there is a revised section on the prehistorical basic religions emphasizing that it is impossible to know much about the earliest forms of religion. As I will argue below, this presupposes the worldview of "naturalism" which interprets the world in terms of material causes alone and therefore relies on archaeology to explain human origins. By way of contrast, I'll be arguing that religion is part of thinking and interpreting, and therefore has been present with humans from the beginning. Furthermore, naturalism cannot explain the origin of thought because thought does not arise in degrees (one is either a thinker or a non-thinker) and it cannot be reduced to material causes. For a helpful presentation on this you can view Professor Alvin Plantinga's "An Evolutionary Argument Against Naturalism" lectures.
The introduction to this text asks why we study religion, and then works to define religion and give some explanation of the origin of religion.
Although religion is of interest in itself as a subject of study, the text suggests there are also practical reasons for studying religion. Specifically, by doing so we can come to understand how other people think about the world and do our best to avoid conflict. However, there is no suggestion that humans can come to agreement on the answers to religious questions. This is typical of pragmatism, which sets aside questions of truth for "what works" where the latter is defined as people getting what they want.
This pragmatic approach is also found when the text relies upon William James to define religion. James argued that there is not one definition, but a family resemblance of various kinds. These particularly involve the unseen, such as beliefs in God, gods, demons, souls, the afterlife, heaven, as well as the texts, rituals, and morals that support these. Therefore, a dichotomy is establish in the definition of religion between religion and naturalism where the latter is understood to be neutral and therefore able to study religion without bias.
The privileged postion of naturalism is also seen in the theories of the origin of religion that are considered. These are mainly taken from anthropologists influenced by 19th century theories of human development such as Darwinism (humans evolved from no religion, to basic religion, to complex religion), Feuerbach (belief is God is a project of human wish fulfillment), Marx (human society develops through economic tensions and religion is one tool of controlling the ignorance masses), and Freud (religion is infantile illusion).
Others included anthropologists who believed that "primitive" societies were the remnant of past human evolution and therefore could be studied to understand what human society looked like in the past. Of course, another possibility is that these groups were isolated from the rest of human development and so this explains their stage of development.
From here, the chapter categorized religions by geography. Of course there is something to this in that ideas spread from person to person. Francis Schaeffer said that most people get their religion the way children get the measles, by being around others that have it. Most people simply seek to explain and defend what they already believe, rather than going through the process of critical analysis. However, focusing primarily on geography is another aspect of naturalism and pragmatism in that it downplays the role of ideas.
I'd like to suggest that religion involves a set of answers to basic questions which are then used to interpret one's experiences. I believe this unites all of the kinds that the textbook studies. However, it also allows us to include naturalism as one of the religions. Naturalism is not neutral, but instead it also gives answer to basic questions and it cannot be given a privileged status but must defend those answers. The persons mentioned above (Darwin, Feuerback, Marx, Freud) are the "priests" of naturalism who give the naturalistic "myths" or "stories" of origins, authority, and value that then shape how their follows interpret the world.
The naturalist interpretation of the origin of religion relies on philosophical materialism and methodological atheism. It assumes that 1) there was no original special revelation given to humanity; and 2) there is not a clear general revelation about God and the good. Rather than making these assumptions a full consideration of the origin of religion requires exploring what all humans at any time can know about God and the good. Rather than assuming an original polytheism, it may be that polytheism occurs further along in human history and is a corruption of an original monotheism.