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
BTW, humans did not come from Earth. Everyone came from Kobol, the 13th tribe was a tribe of Cylons the Kobol people made, they settled the "Earth" we saw, the other 12 races were Humas and settled the 12 colonies.