What if the technological advancements in the history of mankind were in fact given to us by beings from another planet and our ancestors simply called them angels and Gods because they had no other way to define what they were. Ancient texts state that their ancestors were visited by and given knowledge from other worldly beings that came from the stars. Look no further than the similarity in temple construction around the ancient world. For civilizations that had no contact with each there construction designs are oddly similar. The only logical explanation is that some alien civilization came down and taught the people of Earth how to build these monolithic structures. I don't think that it could be anymore clear. Ancient human beings were visited, in the past, by aliens.
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Intelligent life has never visited this planet. period. I'll make one small example. Let's say in La La Land there is intelligent life and they have the ability to travel the stars in great distances. Let's also say they found our measly little planet among the trillions of planets which are in our own galaxy. They see our planet can support life and they see there is intelligent life populating it. They would never come here to set foot on the surface. You know why? Because we would kill it the first time it would land out of fear just like they would if the tables were turned. It doesn't matter though. The more realistic thing is if they existed, they would be too far for us to know they were even there and we would be too far away for them to know we were here.