(The reimagined series; Caprica and Blood & Chrome)
Earth is a rocky planet formerly inhabited by organic and mechanical lifeforms.
Earth was settled on some 3600 years prior to the Fall of the Twelve Colonies by the Thirteenth Tribe, a cybernetic species which left Kobol for a colony of its own to flourish in away from humanity. In the 1600 years following the exodus, the Thirteenth Tribe built large cities across the planet and began constructing increasingly sophisticated AI technology. These machines later achieved sentience and started a slave revolt, which ended with Earth's nuclear holocaust and the apparent mutual destruction of both peoples, with the exception of five survivors of the Thirteenth Tribe.
In the 2000 years following this conflict, a radioactive trace was still detectable in the ecosystem. When the planet was discovered by a refugee fleet consisting of humans, Humanoid Cylons and four of the Thirteenth Tribe survivors, these soil and water contaminants were deemed unsafe for colonisation. After a few days, the fleet abandoned Earth, leaving behind only a Number Three Cylon who requested to stay behind.