HT, I think your synopsis is a bit off the mark. The disaster on Kobol was the start of the cycle, and was not a disease but the very first clash of humanity with its creation (the Pithean scrolls talked about one Lord of Kobol wanting to be elevated above the others - this is likely the humans use of their Cylons as the slaves they were created to be). The war on Kobol led to a split, where the humans went one way and their Cylons the other (the Colonies and Earth, respectively), where events repeated themselves fairly simultaneously. There is never any suggestion that it all started on our Earth.
Well, I'm just attempting to put the events of the series in a (plausible) context with our known Earth history
without someone implanting all the fossilized remains of humanity having evolved here, along with seeding the planet with humans.
Or, heck, from biological standpoint life on Earth is one continuum with very much in common, and from a molecular level it's almost impossible that such similar life would evolve elsewhere. Humans as a species can't really have been dumped here by the unknown entity (which doesn't like being called God), unless you want to assume two of quite annoying options to be true:
1. The whole evolution leading to human species was guided by the unknown entity in BSG, along with the machinations that led to the ragtag fleet discovering our Earth. This would mean that the unknown entity is older than life on Earth.
2. The whole history of evolution as we see it from fossilized remains is a fabrication implanted for us to find by
the Flying Spaghetti Monster the unknown entity (so yeah, YEC... in a way)
.
While in the series' context valid options, I'd rather try to somehow make sense out of the finale in the context of physical reality, leaving as little as possible to be explained by the noodly appendage.
So basically if we want to assume that the Colonials really found our Earth and assuming that the history of life and evolution as recorded from fossils is valid, then the conclusion is that the Earth that they found actually is the planet where humanity evolved. Extrapolating from this, and neutralizing the timetravel mess, my current hypothesis is as follows...
*ahem*
Terran life and consequently humans evolved in Earth. Somewhere around 10000 years before the events of the series, Kobolds encountered recently emerged humanoid species on a backwater planet (the one you're standing on). They took a small population with them for whatever purposes aliens abduct people for. Consequently, it's likely that they reverse-engineered human biology into synthetic species (aka skinjobs, or specifically the "final five" variety). About the society and culture on Kobol I have no idea; they could have incorporated humans into the society as peers, slaves or worker caste or anything in between. Conversely, same applies to skinjobs. We have no idea of the actual origins of the skinjobs, just an assumption that since they call themselves cylons (a name attached to the
mechanical variety by their creators, Colonial humans).
4000 (?) years ago, something happened on Kobol that made continued life there unfeasible. Whatever happened, remains a mystery. A fullscale war with nuclear holocaust is unlikely, considering the state of Kobol's ecosystem just after four millennia, although possible depending on the scale of destruction. Whatever the reason, Kobolds (or Lords of Kobol or whatever you want to call them) ended up mostly extinct or otherwise absent, while the human population set for the Colonies and the synthetic copies of the humans set for 13th colony, to be named "Earth" in the Pythian scriptures. Considering a map was left to Kobol to point out the Pseudo-Earth's location for searchers, it is likely that the 13th tribe did not intend to be entirely cut off from the Colonies, but it is possible that some sort of schism developed between the skinjob and human populations that led to the decision to separate the "species" for the time being. Possible cause for this might be the disease encountered in the series (the one that targeted skinjobs and biological cylons in general), which would suggest to some kind of quarantene procedure.
So, 13th tribe went to "Earth", developed a culture, and destroyed themselves in about 2000 years (if I remember the numbers right), and 2000 years before the series' events they nuked themselves to death and the Final Five set for the Colonies in their... colony ship.
Meanwhile, the Colonials managed not to destroy themselves, before they developed the mechanical Cylons, entered into war with them and generally screwed up stuff, until the Final Five encountered the mechanical cylons, adopted the name, allied themselves with the chromejobs, signed and armistice and proceeded to create more synthetic humanoid cylons based on themselves and the mechanic cylons' research on the Hybrid, and gave the synthorganic cylons capacity to download, or resurrect.
However, Cavil betrayed the Five, killed the Daniel, killed the Five (nuked their memory) and sent them to the Colonies for some reason. At this point I would say the Unknown Entity (probably a surviving Kobold) interrupted the events by placing a failsafe mechanism of sorts on the sleeper Five (aka. All Along the Watchtower) that would trigger their cylon identity and later on probably open some of their old memories.
Events leading to Kara's resurrection and implanted knowledge of the original Earth's location was likely the second important act by the Unknown Entity. This eventually led to Kara being able to lead the ragtag fleet to the original Earth. After which she was beamed up. Or a leopard ate her.
Meanwhile on Earth... nothing really happened all this time. Humanity on Earth continued their hunter-gatherer routine. 5000-10000 years can be a pretty short time in a prehistoric pre-cultural setting in terms of change. The Colonial Human branch was little more than a footnote on the pages of history of humanity.
For the sake of abandoning time travel I'm going to ignore the obvious discrepancies in astronomy... for now.
