Ethnographers working with the Klamath and Modoc Native American groups in northern California and southern Oregon during the early 20th century encountered a yearly celebration known as the Shaman's Dance. The celebration was held during the winter, and people would gather at the home of the village shaman for a several-day long event in which the shaman, as well as others who held mystical ability, would demonstrate their powers. The show was part entertainment, part community gathering, and part reinforcement of the Shaman's position as a person of power within the community. It also served as a time for people to gather together and discuss community business, an important part of which was sorting out the stories.
Gathered together in the Shaman's home, one person would start telling one of the stories - a story of the culture heroes, a story of the spirits, a story of the group's origins, etc. The teller would continue until somebody objected to some aspect of the story being told, at which point all gathered would debate the matter until a consensus could be reached and the story continued or completed. This would, obviously, result in the stories that served as the base of the religion being different after the Shaman's Dance than they had been before it. This confused many of the ethnographers, one of whom wrote in his final monograph that he had asked several people present whether the new or the old version was the true version, and he received the answer "the story used to go one way, now it goes another."
As ethnographers covered more ground over the course of the 20th century, it became clear both that the mythic stories of pre-literate societies changed over time, and that the people of these societies were generally aware that they changed and were not perturbed by this fact. The way in which the changes were acknowledged and incorporated varied across time and space. Amongst the Klamath and the Modoc, a frank admission that the stories had changed was noted, and these people saw nothing at all wrong with this. By contrast, amongst some northern Australian Aborigine groups, there existed a position known as the Law Man, and the Law man's job was to tell the Dream Time stories, and to interpret them, often changing them (or creating new ones) in the process.
This phenomenon of changing stories provides a fascinating insight into the origins of religion. It is no secret to anyone that the stories that make up the mythic base of any religion provide the basic rules as to how to live as a member of society. When one looks at the myths of many groups, they also provide information regarding the locations of resources, allowing a "mythological map" to be held by anyone who knows the myths (I have often wondered if our very idea of a "sacred space" may be descended from, or at least related to, this use of myth). Those of us who live in post-literate societies with religions that either are (or at one point were) closely tied to the government (even within the U.S., we are descended from nations that had official churches) tend to think of myth as being unchanging, it's even common for people to talk about the "eternal truths" of their particular religion's stories.
And yet, conditions change. Social alliances shift, resource locations move, and behaviors that were once advantageous may become counter-productive (and vice-versa). And if you are basing your strategies for dealing with the world on myth, as most humans have for most of our history, then myths need to change to allow the people who make use of them to adapt to a changed world. Having a mechanism to create these changes allows the traditions, culture, and (importantly) people to better navigate a world that is not static. However, an acknowledgement of that change, which would have made perfect sense to the people who carried and lived with these myths, seems bizarre and counter-intuitive to those of us who live in literate societies where the myths have been written down and usually have been declared "eternal and inerrant."
But if religion has an evolutionary advantage, it likely is in allowing us to find a way of navigating the world - physically, socially, and psychologically. The shifting myths, with no "true" version, encountered by early ethnographers are probably much closer to religion when it served as a strong tool for our ancestors.
Indeed, for all of the talk of the "big three" modern monotheisms (Christianity, Judaism, Islam), these religions are also open to change. The Bible (inclusive of the Torah or Old Testament and the New Testament) and the Koran are both large, sprawling, and internally inconsistent. Churches and mosques choose to ignore certain passages, emphasize others, and downplay yet others. Religious authorities and interested writers of all of these religions produce volumes explaining why some religious passages allegedly means something very different from what they actually say. This is really an attempt to make a more rigid written mythology flex in the way that earlier mythologies did, even if people deny that this is what they are doing.
When we look at the Pope of the Catholic Church or the Mormon Prophet, we see roles that are large-scale adaptations of something like the Law Man of the Aborigines. They even have their own elder's councils. And other religions have similar individuals are groups who modify the doctrine to be in better keeping with the changing world. There are, of course, differences (many of them dictated by the sheer scale of modern world-spanning churches), but the same basic seed is present.
But even if the modern religions are able to adapt their written myths to the modern world, they suffer the basic problem that everyone can read the text and determine whether or not what the leadership says is in keeping. Without an open acknowledgement that the stories must change, schism is probably made inevitable.