Seeing as how this has gone into discussion on aliens I'll add my two cents.
The two big things I point to as evidence that there may be aliens, I've probably raised these before, are things such as the Egyption pyramids and Area 51. To go into detail the pyramids were built really not long after man first evolved on Earth, whichever way you want to go by (Adam & Eve, evolve from apes, ect). Now the story goes that Pharoh had his slaves build the pyramids and statues, however even with technology today we cannot do what the Egyptions did and there is a strong belief that aliens were involved. I'm not saying they were, but the possibility is there. The other strong indicator is Area 51 and all that was involved with it, Roswell, ect. For decades the government had denied the existence of Area 51 despite eyewitness testimonies and many believed this was where aliens were being kept, most likely the ones that had meant to have crashed in Roswell New Mexico. Only recently had it been revealed that Area 51 did exist, I'm not sure if this is the official reason but one of the rumors was that they tested experimental aircraft there. I'm willing to go with this theory (and the crash in Roswell was pilot error as opposed to aliens), especially when considering one important fact. If aliens are so advanced that they can travel lightyears through space they would hardly be captured by such a primitive world.