Well, as Pushing Daisies Man would have said it . . .
The facts are these:
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There is an Earth. Humanity has, at some point, been on it. I saw metal-reinforcement in the ruins and what I believe to be an overturned car. (When Tory approaches Anders) So I’ll posit that this is current-day or later.
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Earth, or at least this part of it, has been subject to nuclear attack. There was a geiger counter there, and it lit up.
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The attack happened some time ago; I’d say decades, easily. Specifically, not the three years of the BSG timeline we’ve been shown. So the Cylons did not strike at Earth at the same time they hit the 12 Colonies. (Yes, I know it wasn’t a widely held opinion, but this pretty much rules it out) (Compare the ruins of Starbuck’s return to Caprica to this)
That leaves two options; an inter-human war or another Cylon attack pre-dating the skinjobs. I do not think the destruction of Earth is simply the inevitable having happened. (Someone also posited that this may have been the Cylon homeworld and they’ve gone and nuked themselves dead. Or a world in which humans and cylons lived in peace.)
(One hopes the inevitable tandry jokes about “jumping the Ape, as it were” are not factual. If Moore has given us a four year long rerun of Planet of the Apes, there will be blood.)
Now, on to speculation.
My theory: Human survivors exist. They orchestrated the activation of the Four, Starbuck’s return and the signal drawing the Four’s attention to Starbuck’s faux Viper. The Final Fifth would be someone not in the Fleet at the time, by what we now know. (She was very specific. “Four. Four in your fleet.” And evaded Roslin’s eyebrowed follow-up.) So, the candidates are Adama, Roslin, Baltar and Helo from the prominent cast.
The Three was also specific that the fifth was not in the fleet, which could indicate a civilian if you want to get literal about it. The person would also have had to be in civvies before the Three was disabled, which excludes Lee. Besides, Lee makes no sense from a storytelling perspective; it’d be redundant to go after Adama again and Kara has already been hurt by Anders. Technically, it implicates Dr. Cottle, Tom Zarek, Romo Lampkin and some bit-part characters.
The possibility is also, of course, open that the Final Cylon remains on Earth and is a hereto undiscovered player, responsible for orchestrating the arrival of two amicable and joined factions of Humans and Cylons. This would, for me, be disappointing.
My personal feeling is that it’s Doc Cottle. They’ve played their hand with Adama, they’ve played their hand with Roslin (D’anna’s mindfuck), Baltar would be an god-awful choice, Helo is not interesting enough as a character (some say interchangeable with Anders; it’s the steely-eyed broad shouldered thing), Lampkin has a played-out backstory and Tom Zarek haven’t been involved except in the Quorum subplot.
Doc Cottle, on the other hand, has had moments with religious overtones with a dying Cylon, have patched up his fair share of Humans, gets a lot more screentime than a bit character like that is owed, has no backstory we know of, is close to the decision-makers (Adama and Roslin), has been sage and wise in almost all his decisions and fits the age frame for the Cylon injects.
So, I’m putting $10 on Doc Cottle being the cyborg with his finger on the music button. I also posit that he’s aware of it, and have been working the backstage all along.