So is the Earth in BSG, our future or past? I'm not sure we got a good glimpse of the continents, so i guess it could be many many years in one direction of the other.
Put it this way:
It's a tv show. It's not our universe. We didn't come from the stars, we evolved here.
The way it's implied in the show is it isn't our Earth, or if it is, it's our far future (after we leave here and find Kobol, and start the ball rolling).