Another possibility is that Yellowstone or some other supervolcano erupted. The radionuclei could have come from within the Earth and shot into the atmosphere during the eruption.

Both types of events could easily lead to the “nuclear winter” depicted. Both events could lead to the moderate radionuclei count detected by the landing party.

Given the state of disrepair, the city was probably abandoned within 200 years of the fleet entering orbit. There was no showing of the ISS, so it probably had fallen out of orbit.

I know that a natural event is not as “sexy” as a war, but could again explain the natural balance and rebirth of humanity/cylons.

I respectully suggest that the 5th Cylon is Lee Adama and this is why…

Lee was about to be wasted by a Raider. He was out of ammo and was totally outflown. The Raider was at point blank range. It would have been a clean kill. Then the Raider scanned him and broke off the attack. Lee was shaken to the core. When Lee returned to the Galactica, he turned in his wings and went into politics.

The only reason for the Raider to break of the attack was that it detected that Lee was a Cylon.

If you assume that the Fleet refers to the Galactica and the military, you could say that Lee was not in the fleet. Lee was part of the civilian government.

++++

What if someone from the Galactica stumbled upon the original humans the 12 are based upon. The Human 12 could be in a high state of decay or so absorbed by the baseship that they could never be set free without killing them. That may even be how the Final Cylon is revealed – through an accidental discovery like this.

A discovery like this would be more than enough for the Galactica and Baseship to get into a point blank slugfest – destroying both.

Kara and Helo (and/or a few other couples) could be on the surface of Earth at the time. Kara’s special destiny is to be the next Eve in the rebuilding of humanity.