Two trains of thought on this right now:
1.) The "Earth" we saw wasn't our Earth, and they will discover a planet, and call it Earth, and that will be our Earth
2.) The "Earth' we saw was our planet, and we are thousands of years in the future
Except you are forgetting that when they first showed "Earth", we saw the landmasses, and they were exactly as they are on our Earth. You could easily make out North America, for example. So it has to be 2.
If you subscribe to the theory that there are infinite numbers of parallel universes, in which every possible iteration of history that could happen DOES happen in one of them, then an Earth with a North American shaped continent could develop in a multitude of different realities. BSG
is in one of them, where humans evolved on Kobol and colonized a planet with a continent that shape but not in our own reality, where humans evolved on the same planet with a continent that shape.