Well, as far as the sci-fi thing goes, Spielberg and Lucas have always been into the sci-fi aspects of movie making for years. All 3 of the other Indiana Jones movies had Religious/Supernatural Power involved (The Ark, The Black Magic of Kali and the Power of the Holy Grail to make one Immortal) . Me, I hated the religious/ supernatural Power aspects to the other movies. There have long been speculation that Alien races made contact early civilizations (Chariots of the Gods, etc). I'd rather think Aliens did it that than some God put a curse on a Grail or an Ark of the Covenant. But that is getting into the religion debate and we don't wanna go there.

Let's just leave it at "Like all Indy movies, they needed a "power not of this World" angle and instead of religion, they used Aliens.

I like the way they intergrated Area 51, Roswell and the McCarthy era into the plot. That was cool.