I think the universe is way too big for Earth to be the only planet with life, but the universe is so big that other life might never find us, and vice versa.
I don't think aliens left their mark on ancient civilisations. We recognise the pictures you refer to as spaceships or whatever because we see a lot of spaceships in fiction and real life. The original artists were probably trying for something very different.
I don't think any of the more recent evidence of UFOs is terribly conclusive proof of aliens, but a lot of them are certainly unidentified flying objects as far as the people with the cameras are concerned.
- SmartLX
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what "ufo" means
Remember, "UFO" means "unidentified flying object", not "spaceship from Mars".