There may well be advanced civilizations on other planets.
It's also possible they have visited earth and even had an impact on us.
On the other hand, the popular evidence usually presented to support this possibility is largely based on junk science.
Believe it or not, human beings - even those from ancient civilizations - are capable of advanced science.
Rather than speculate that we are the recipients of the largess of some unknown ancient aliens, we'd be much better served to understand, appreciate and in some cases even emulate the advanced science and engineering our own ancestors.
Best to watch this and even download it now. I've heard the History Channel likes to challenge people who use this video in a fair use context
All the claims are sourced at the website: http://ancientaliensdebunked.com
Produced by Chris White with commentary from Dr. Michael Hesier.
The producer has distributed it for free on the internet and it is a completely non-profit project.