Quote Originally Posted by jesseleewelch View Post
According to the known lore, we know the following...

"A pantheon of deities revered by the majority of Eorzea's populace, they are said to have ruled the continent and its surrounding islands until the arrival of wandering tribes. Impressed by the resilience of the primitive settlers, each of the Twelve mercifully saw fit to ensure their welfare."

http://ffxiv.gamerescape.com/wiki/Categoryeities

If the 12 existed and ruled Eorzea since before any of the five races ever immigrated there, it would imply they could not have been mortals who ascended to godhood as they existed in Eorzea before mortals went there. It would also rule out the idea that they mated with mortals for the same reason. At least, that is the way I am looking at this right now.
The problem is that just because we are told the Twelve existed before the races came to Eorzea, it doesn't mean it's true. As Fernehalwes said:
Quote Originally Posted by Fernehalwes View Post
4. Do the Twelve exist? Are the Archons from the Circle of Knowing actually the Twelve? Are the primals and the Twelve one in the same (er, twelve in the same)?

More info on the gods of Eorzea and how both the Twelve and the primal fit into the whole celestial scheme of things will be revealed in ARR, but even then things will remain hazy. I mean, it’s religion. It’s hazy by design. In one age, a guy who single-handedly kills a microchu to save a girl is a good Samaritan. The story gets told by a thousand different people, each time changing just a little, but each time becoming more epic (the microchu becomes an ochu, the ochu becomes ten ochus, the girl becomes a princess, and so-on). In the next age, he’s a hero. In the next, he’s a saint. In the next, he’s a god, smiting ochus with levinbolts from high atop a snowy peak. And what of those people who possess magic or technology that is beyond the comprehension of the commonfolk? Would they not appear as gods? And what about those powerful entities who fancy themselves as gods, and use people gullible enough to follow them to obtain what they desire (power, gold, etc.)? And what of the actual gods, who normally don’t give a hoot about the ants crawling about their realms, but will intervene when it amuses them? Hopefully the story in ARR will help categorize who falls into what slot.

I can mention why primals are referred to as primals, though. It is not because they are underdeveloped, younger deities, but because the beast tribes tend to believe that they were the ones who created them, the world, and everything. They are the primal forms of existence.
Myths and legends change over time. The Twelve (or the older members) could have been around since the creation of the world, or they could just as easily be Allagan survivors that seem godly to the "primitives".