I've always thought it was mankind's ego that made them believe that we were the lone inhabitants in a universe filled will billions, no trillions of stars. Since there are portions of the universe that are far older than our solar system, it is very possible that a civilization or civilizations could have evolved that are far advanced to ours, and they would look at us in the same way that we look at a chimp, or possibly even something less evolved.
If such a civilization were to be discovered, or even the remnants of one were to be discovered, I'm convinced that many would still be unwilling to accept it, for doing so would mean that mankind isn't the greatest or the top of the food chain. It would be similar to the reaction of the Catholic Church when Galileo attempt to convince it that the sun, not the earth, was the center of the solar system. I'm sure there would be some who might attempt to offer alternative explanations or merely dismiss the evidence as something else, but for those who believe the evidence, it would cause them to rethink many of their beliefs, including religious ones, and that would totally change humanity forever.