No idea. White wolf never knew, and never said.
Werewolf Mythos: the wyld, the Wyrm, and the Weaver. Mokole have this wierd ancestral memory thing. They remember when the wyrm went crazy, because they were there. they remember how gaia was made by the wyld, the wyrm, and the weaver, and the remember how the weaver took the critters of the wyld and gave one, humans, technology, giving them a leg up over humans. In "The Silver Record", we learn that the weaver took it's most beloved human, and made him immortal (this is caine). The wyrm saw this and went "OH SHI-" and ate caine to keep the weaver from having an unfair advantage. caine lived on in the wyrm, and got real hungry, so he bit the wyrms stomach. This unhealing injury made the wyrm crazy, and the blood tainted caine, turning him into the first vampire.
Kindred Mythos: Caine is cain of the bible. god cursed him to live forever. there are many hints and fragments of documents proving this.
Demon Mythos: The christian god made everything. he made the world. The demons were there, they saw it. they remember the truth. (Just like the mokole?)
The conflicts in the fluff hurt so much.