The underlying belief for this idea is evolution. Evolution as origin account and as the developmental aspect preceding reincarnation into new body goes back to Babylon and ancient Egypt. After the dispersion evolution showed up in what is today Asia where it was highly developed in the Upanishads. Eventually it made its way around the world as an important element within all ancient Mysteries. At the end of the Medieval era it showed up in the heart of Christendom.
Belief in existence of ETs depends upon the idea that evolution has been happening for billions of years, therefore it's logical to imagine that highly evolved lifeforms must exist in deep space (the 2nd heaven)
Some believers imagine that ETs brought life to earth and after seeding the waters with life tended to its evolution. The movie, "The Day the Earth Stood Still" and its remake are examples of many movies and sci-fi books based in the idea of highly evolved aliens.
Notice ETs in modern times morphing from little green men to interdimensional beings.
Now where do we read about them I wonder? /s