There have been no proof of other life forms in the universe -- the closest that humans know regarding extraterrestrial life is that there are a few planets that may be livable, but no further studies have seen anything that resembled life in the cosmos.
Still, despite the lack of proof of extraterrestrial life, some people still believe that aliens do live in deep space somewhere. As Ancient-Code asked - how could it be possible that one moment, people are living in caves, and the next, they were building complex cities?
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