Ahn Dun
The Myth of Ahn Dun is, essentially, a creation myth that manages to appeal to both the religious and the rationalists. In the myth, Ahn Dun is a giant world traveler of unknown intent. It travels to many planets, seeding life (human or otherwise), dramatically altering the development of each world, before moving on.
As a religious myth, Ahn Dun is viewed as a much-removed god-like figure, not necessarily benevolent of malicious, but nonetheless deserving of praise and thanks.
As a scientific theory, Ahn Dun is viewed with some skepticism, but many rationalists hold that it is quite possibly that the earliest materials for life could have been transferred throughout the worlds and galaxies by, say, a giant asteroid or something similar.