Well first we would have to define what materia is, which Sephiroth above me did a nice job of doing. To me, it's the condensation of knowledge from the life stream into a tangible form. Assuming the ancients knew magic, their knowledge of various spells are able to go back to the life stream after they pass and eventually come back in a physical form. Maybe attributes of people who were really strong or courageous come back in the form of "Cover" and "HP Plus."

So who knew Meteor? I do like the theory that it's tied to Jenova, that maybe someone accidentally called it from space, or it's the results of Jenova being in the planet/life stream for so long.

But Jenova, to me, had done this whole planetary thing multiple times before, it's what it does, I don't think it's specifically tied to Gaea. I think the Ancients just had really great mages, and one of them was able to attain a level where he/she could call something that could even destroy the planet. Knowing that kind of power, they sealed it as the Temple of the Ancients.