Fundamentally, what's the story? There may be one, big, true story that no one knows, but each tribe is going to have it's own stories and legends. Some will be true, some will be subjectively true, some will be dangerously wrong. Many will blend truth, myth and falsehood, depending on where they came from, who was telling them, why they were telling them and how garbled they got in the retelling. The world arises from the interaction of and conflicts between those stories.
Your game, as it plays out in the world, is another story, one that interacts with several of the older stories and, hopefully, influences many of the current stories.