I know that the Neanderthals didn't create a civilization, yet I also know that they were intelligent and that some of them had sex with humans. So, for a brief period of history, humans were in contact with and interacting with another intelligent life form. Why do we forget that?
I have to point out also that Neanderthal were human. In addition, the conclusion that the two ever mated is highly controversial and disputed. I know anthropologists that are proponents of the theory and those that think it's scientific nonsense.