I am assuming by UFO, you mean existence of intelligent life form and them visiting (or visited) our planets.
I believe that we cannot be alone in this universe, that is for sure. Did some of the intelligent life forms visit us, now that is a million dollar question.
If you look at the old artficats, ancient scriptures, cave drawings, Inca's air strip and those images that are only visible from above the ground, it is quite possible that we were visited by Aliens in the past.
Now if you are talking about UFO sightings then that is exactly what they are, UFO (Unidentfied Flying Objects). Just because they are flying and common man could not identify them does not make them an alien spacecraft all the times.
There is one place in Soth America(?) where mysterious carvings can be seen.They are so large that those carvings are believed to be meant for UFO landings.I thought it was Inca,as u mentioned.But not able to find out.