For those of us who watched the series finale we know that:
Both human and organic Cylon helped form what we know as the human race on earth. They also freed the Cylon Centurions and gave them control over the basestars. The Centurions leave and head out into space. Earth is colonized and humanity grows and prospers for 150,000 years.
Now here's the link (my theory and my own opinion):
The original BSG takes place between the 1960s and 1980s (earth standard time)
The humans have been in a 1000 year war with the Cylons, created by a reptilian race.
The original trek to earth took place around the time of the first moon landing and finished in 1980.
Now if we subtract 1000 years (for the war) and approximately 40 years (from the exodus in the original to 2009 reimagined series finale) we still have 148,960 years separating.
Many years after the fall, survivors made their way back to the 12 colonies. In that time the radiation on the planets eventually dissipates and become habitable again. The survivors carry with them stories of the ones who went to Earth, the stories dissipate and change (there are 140,000 years for this to happen).
Eventually the Cylons return thus starting up the 1000 year war (all this has happened and all this shall happen again).
That's the short short version of how I see it.