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Earth is the believed destination of the Thirteenth Tribe of Kobol, as attested in the Sacred Scrolls. Prior to its discovery, its existence is considered purely mythical and legendary within Colonial and Cylon societies. Earth is also the name of a planet Colonial survivors eventually settle after a long and arduous journey across space following the Fall of the Twelve Colonies.
Earth: Home of the Thirteenth Tribe
The legend of Earth begins at least four thousand years ago on Kobol, where Scriptures tell of the exodus of a Thirteenth Tribe of humanity during a period of great turmoil across the planet. It is said that "they traveled far and made their home upon a planet called Earth . . . which circled a distant and unknown star." While not many previously believed in Earth, there are artifacts rumored to point out the general direction to the planet. These rumors are later confirmed after the destruction of the Colonies (TRS: Miniseries, "Kobol's Last Gleaming, Part I").
Settlement and Development
For nearly two thousand years, the Thirteenth Tribe flourished on the planet, creating a civilization comparable to the Twelve Colonies before their fall, including development of their own robotics and artificial intelligence. Unbeknownst to Colonials or most Cylons, the Thirteenth Tribe was a flesh and blood race of artificial beings much like the humanoid Cylons, which were created on Kobol thousands of years ago. They originally relied on organic memory transfer technology to download into new bodies after their deaths. However, sometime after leaving Kobol, they gained the ability to reproduce sexually, and the technology fell out of use (TRS: "No Exit").
Approximately two thousand years BCH, the Tribe's robotic creations rebelled against their creators, in an act of mutually assured destruction. The attack caught the population off guard, with many at work or enjoying leisurely activities. Evidence of people going about their lives is still visible on the remains millennia later. However, prior to the apocalypse, five individuals started seeing visions of strange people warning them of the upcoming cataclysm. The five worked hard to recreate the lost organic memory transfer technology, and built a ship in orbit to carry their experiments. After their deaths, the five were resurrected on this ship, and opted to follow the Tribe's original course back to Kobol, leaving Earth behind (TRS: "Revelations", "Sometimes a Great Notion", "No Exit").
At the conclusion of the war, no survivors remained on either side, and Earth was left burning and irradiated. Despite lingering radioactive contamination, plant life gradually returned, and whatever ruins remained succumbed to nature and time. A large city on the planet visited by both Colonials and rebel Cylons was largely submerged in water and partly buried in sand.
Unable to support human life, the planet is abandoned days after its discovery, and is likely never visited again (TRS: "Kobol's Last Gleaming, Part I", "Kobol's Last Gleaming, Part II", "Revelations", "Sometimes a Great Notion").
A New Earth: The Promised Land
By an infinitesimal chance, a habitable planet is found by pilot Kara "Starbuck" Thrace, after she is able to extrapolate FTL jump coordinates from a song she used to play as a child. Lt. Thrace inputs these FTL coordinates into Galactica's FTL computer in a desperate attempt to escape from the Cylon Colony. Galactica, damaged from battle, limps into orbit of the planet.
After Colonial recon of the planet, Earth is discovered to have a diverse range of wildlife, more than the Twelve Colonies put together. It also features a race of primitive hunter-gatherer humanoids, having evolved here separately from those on Kobol. Upon further study, these humanoids are discovered to be compatible with Colonial DNA, which opens up the possibility of Colonials and Cylons interbreeding with them.
Colonial/Cylon Settlement and Future Evolution
With Galactica in pieces, and the Colonial Fleet not fairing any better, it is decided that the planet will be settled, with the remaining Cylons soon joining them. Plans begin for new cities to be built, but those plans are soon abandoned in favor of starting fresh in an attempt to break the historical cycle of violence. Colonial units scout the planet, and eventually determine the best possible chance for survival is to split the survivors into groups, and spread them across different sites around the globe. These locations are documented, and presented to each of the groups for future use. After the survivors set up their camps, the fleet and any remaining advanced technology are scuttled. The settlers christen this new planet "Earth," in honor of the promised land they have been chasing for four years.
The surviving Colonial and Cylon settlers and their descendants begin interbreeding with the indigenous humanoids, creating a new race, with the first human-Cylon hybrid becoming "Mitochondrial Eve," the most recent common matrilineal ancestor of all living humans. Her fossilized remains are uncovered nearly one hundred fifty thousand years later, around the time that another advanced human society is on the brink of major breakthroughs in robotics - much like the Twelve Colonies, the first Earth, and Kobol before them (TRS: "Daybreak, Part II").