There are cities in the Indus valley in india where the bodies are found radioactive...
Humans in their current incarnation from Einstein to hawking have been around 240,000 years or so according to anthropologist.
plenty of time for a few super advance civs to rise and fall, all with out alien involvement.
I was trying to think of the show, but the History channel ran it, on what would happen after humans where gone, after say 10,000 years you could almost never tell there was a civilization of man.
All that was left would be the stone and granite structures... Sounds kinda familiar huh?
edit on 16-5-2012 by benrl because: (no reason given)