Actually, Aralith, Sephy's words there are rather vague, and are honestly only definitive on the point that the Cetra are migratory.

It could go either way, though. Them having migrated onto Gaia- from Spira, most likely- would explain why they and the Spirans are capable of interbreeding (The chances of two seperate planets growing lifeforms so genetically similar as to be capable of interbreeding is so ridiculous it only happens on Star Trek and other fantasy shows), though that would only account for one instance of interplanetary travel, and it would mean the Cetra philosophy would have to have developed on Spira, or that the majority of Spirans were themselves also falen Cetra.