I've pondered this topic since the game was originally released. There are a few things to me in Civilization V that hint at the game being set in the far future, and humanity is rebuilding after some sort of catastrophe. Ancient Ruins are the strongest piece of evidence I think; one can explore them for cultural artifacts from past empires, scientific marvels already discovered or your units can find advanced weaponry inside. Therefore, the technology has been researched before by civilisations past and unknown. Not only that, but the narrator often refers to your empire "rising again" and "once again building a civilisation." Sounds clear to me that this isn't your civilisation's first time in the planet! Just an interesting little thought experiment. The game seems to be referring to the potential cyclical nature of the universe, with history playing out slightly differently each time. What say you? Have you any other evidence?