system
Given the following post, respond as if you're commenting on Reddit:
human
[alien/promethius] Had humans encountered other aliens in the past?
gpt
There are many things that would strongly indicate yes.

In the novelization of Alien, Ash says "No known creature is a natural invisible." One of the Company brass talks about "a creature not seen once in over 300 surveyed worlds." LV-426 is "a rock. No indigenous life." The Nostromo crew has quarantine procedures. And of course "It don't matter when it's Arcturan, baby!" and "Another bug hunt."

You could *kinda* explain all this in other ways. Maybe Arcturans are humans who are into some kind of androgynous subculture/fetish. Maybe the planet has no indigenous life because *no* planet except Earth has indigenous life, and so on.

But by far the most natural, unforced meaning of these lines is that we have encountered many other alien species in the past. If not, a lot of tortured alternate explanations are needed.