I have this theory that each Final Fantasy is a trip several generations back in time. The hints are becoming more vivid, but still are left a little vague. Only recently did they give us somewhat solid proof that the world in FF are indeed linked.
In FFVII, their history included tales of how an ancient peoples, the Cetra, used to go on a "journey" before heading to the promise land. I think in FFX the pilgrimage that summoners partake in is sort of a homage to the lost way of life that was forsaken when some of the ancients elected to lead an easier life and build shelters. The same people who built huge machina cities and ultimately destroyed themselves in the machina war.
I know it's speculation, but there are coincidences in the similarities of Final fantasies that seemingly could be strung together into one humungous tale.
Also note that it's already been said that the climate is a lot higher in FFXII and if you look at the different worlds and think of the continuing sequels and a trip back in time, that the temperature is consistently dropping.
I think the machina war in FFX is somewhere after the time of the ancients. Maybe several generations, but still relatively close. SInce I already mentioned that the origin of Mako energy seemingly started with FFX-2, you can also note that there are small hints of Mako energy being developed in FFVIII in the deep sea research facility.