system
Given the following post, respond as if you're commenting on Reddit:
human
[Alien (1979) film] If humans had knowledge that alien life existed prior to the events of the film, what species was it? The Engineers?

I recently rewatched the film and Ripley asks if the signal they are going to investigate is human. When they find the derelict ship and the Engineer's corpse they aren't that surprised by the discovery. 

When they first encounter the Facehugger their fascinated by it but not shocked by its existence. 

So in cannon, did humans already have direct contact with some primitive alien species and/or find ruins and realized E.T. life existed because of such findings?
gpt
The Prometheus voyage begins before Ripley is born, and so, yes, humanity does have an inkling of life existing beyond Earth.  How much of an inkling depends in part on how much data the Prometheus mission was able to send back.