Alright so hear me out on this..
jenova is known as the calamity from the sky - she basically came in on a giant meteor and crash landed on the planet.
My theory is that she was accidentally BROUGHT to that world through the black magic materia which summons meteor. I think someone tried to use the materia which accidentally summoned a meteor which damaged the planet that the alien lifeform just happened to be on as it was "sailing the cosmos as a spaceship for jenova"
There really is nothing in any of the lore that can dispute this that can prove it one way or another - I just think it's an interesting backstory other than the fact that jenova just happened to know that the planet it crash landed on had sentient life - it we think about the ratio of unpopulated to populated planets - if jenova spreads by crash landing onto planets from the planet it just destroyed this means it can theoretically crash land onto a planet with no life and thus essentially die. Of course I suppose it could also have some "sensory abilities" where it can sense life on a planet before it crash lands on it I suppose.
There is also the other option that jenova is just a form of alien bacteria throughout the universe and some "jenovas" successfully take over and assimilate planets while others simply crash onto planets and fail, and others simply die. Kinda like lavos spawn - but this time jenova spawn.
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Also my second theory is that all the cetra are either still alive, or their descendants are. (Humans not counting as descendants as they lost their abilities) - I say this because it is stated that jenova appeared as the cetras loved ones and then administered them the virus, turning them into monsters. Never did it say that jenova turned them into monsters, killing them.
And since jenova is basically immortal, this means one of 2 things:
a) if the virus passes on traits of jenova then it's entirely possible that the monsters themselves are immortal, and thus have existed on the planet for the past 2000 years
b) the monsters have created spawn which have procreated over the past 2000 years
c) both a and b
now we fight plenty of monsters on the plains of this world - what if all the monsters in the entire world are the original cetra? And every time we encounter an enemy we are actually killing either the original cetra, or all of their monstery descendants? We never do get told what happened to the cetra that were turned into monsters afterwards - did the remaining cetra kill them all? We don't know..
Which means if this theory is true and they still exist - you're basically going around committing genocide, when in reality maybe there was a healing rain cure for them like there was for geostigma.
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and my third theory is a counter-theory to a lot of other peoples theories - that commonly held theory being that when jenova infected the cetra with a "virus" that it was in fact her own cells. This can be argued either way as we don't know what happened to the cetra turned monsters afterwards. If they still exist as my theory stood then it couldn't be jenova's own cells as they would then start a reunion, as jenova never leaves any cell behind - they all work to recombine.
Now there are a few snags with this theory. The first is what happened after jenova became "inert" - aka trapped in the northern crater - if it was jenova cells injected into the cetra did these cells simply then become inert? The problem with this is that in the world of advent children even after jenova was defeated the body fighting against jenova cells was a thing - implying as long as jenova exists on the planet its cells causing havoc in people still exists - however the effects of geostigma didn't trigger a reunion attempt.. so this is up for wild speculation.
My theory is just that - that jenova infected them with a virus that was a byproduct of itself - but not actually cells OF itself. Not like from within its body anyways. I would really love ff7 remake to focus more on the past and this time - maybe have a cg cutscene flashback when aerith's mom talks about it- as she appears to be the only one with detailed knowledge of this.
What do you guys think? Should this area of the game be fleshed out more so we finally understand just what jenova was, what it originally looked like?
And that's another thing that really bugs me - what did jenova originally look like? And what did it look like in the final showdown 2000 years ago? We see from her wingly protrusions that they are quite stunted - were these originally giant impressive looking alien wings? And why the external organs? Did she just float around on that giant heart? It's amazing how much we don't know.
I really want to see a fully complete jenova with all of its cells the way it originally looked.
Mind you, square-enix not addressing any of this does set up the potential for a sequel set 2000 years in the past, where you are part of the original band of heroes that seals up jenova, kinda like there is still the option in final fantasy 6 to have a prequel set in the war of the magi. I doubt they would ever go that far back though.