That is assuming *we* are even here when the Galactica arrives. It may well be that BSG will return to earth 5,000 or more years in the future.
I am thinking that the colonies are our descendants who fled to Kobol, that Earth was abandoned due to an environmental catastrophe and has repaired itself only over millenia - and is now under the careful eye of its Guardians who await Humanity's return.
The 13th tribe were the original Cylons. The Guardians are those "first Cylons". Artificial humans who now guard Earth awaiting our return.
"All this has happened before; all of this will happen again." Original Cylons will kick the butt of the second Cylons with uber-tech. (And I suppose it is entirely possible that Roslin is, in fact, one of them - as are the rest of the "final five").
That's my shoot from the hip prediction.