Tigh the Cylon
People have been asking about how Colonel Tigh can be a Cylon so I thought I would provide some background on that. To me it was not a surprise, but to many viewers it was a giant shock.
For some time, the show has been telling us that the "Final Five" Cylons are quite special and different from the 7 humanoids we first met. Indeed, the first thing we find out is that according to Six, none of the 7 have seen the Final Five. We learn that they do not talk about them, and to even be curious is forbidden and seemingly there is programming against it. We watch Three (D'Anna) follow a quest to see them, first killing herself hoping to see them in "the place between life and death" and eventually risking (and losing) all to be in the Temple of Five when it activates. There she sees them, recognizes them and apologizes profusely to one. That the 7 had never seen the 5 was the first big clue that these were a very different class of Cylon. Not clear to everybody was the fact that they must not have participated in the war -- otherwise they would have been seen, of course. They are tied to the Cylon God, and are tied to the 4,000 year old Temple of Five, built by Earthlings. Indeed, while one can imagine they stole the temple, the more direct explanation is that they built it, and are themselves at least 4,000 years old.
Why is that more direct? Well, the temple was built to activate when a predicted nova would explode in 4,000 years. It seems the five who appeared when it activated should be connected to the builders. And those builders, it turns out were also five in number (there are few coincidences in TV drama) and they followed a special God who felt he was the one true god.
Now I was actually expecting this, because I never felt the Cylons could have "evolved" the way we first see on the show. Their biotech is incredibly advanced, and yet they don't seem to have much of a grasp on it. They're unable to breed and obsessed with breeding. They manage it once. It would not make sense to have developed such technology from scratch in just a few years, to be able to grow a body in a tank -- that's impressive -- but not be able to do it the old fashioned way. This told me they got the biotech from elsewhere. And what better place than an earlier generation of Cylons? After all, like in Peter Pan, "All this has happened before." How can that be if there were not Cylons before? This society has had advanced technology like interstellar travel for 4,000 years. Indeed, their ancestors on Kobol and Earth had even more advanced technology, with holo-projection chambers and the works. Of course AIs (Cylons) have been made before.
So you put it all together -- ancient Cylons and a secret F5 that are ancient. These Five have been around a long, long time. Infiltrating both humanity, and the Cylons, for a long time. They (or their god) helped the other Cylons "evolve" and programmed them to know there were 5 other models, with numbers assigned at random, but that they should not be curious.
So they've been part of the tribes since before they left Kobol. And their biotech is much better than what they gave the first 7. They can breed reasonably easily (Tyrol gets it first try.) They can resist the virus that killed the others. They can age.
Moore confirmed it in his interviews, "A very different kind of Cylon." He also confirmed, in a roundabout way, that Tigh did fight in CW1, he didn't just have false memories.
So of course Colonel Tigh can be one. Adama could be one. Anybody can, except the 7 Cylons we met first, and, apparently, Starbuck, who has her own destiny.
Now we know a lot less about their agenda, and I provide some analysis of that in other messages. However, they didn't directly get involved in the genocide, so it is not the simple hostile agenda we've assumed for the 7. They are manipulating both the 7 and the colonials to their own purpose, or that of their god. It goes back a long way -- they fought in the war, they may have even been involved in the development of the metal robot Cylons. Their reasons for all this, we'll learn in the future.