I think that it is hubris to think that humans are the onlg intelligent life in the universe.

Whether or not we have been visited by alien beings is another question altogether.

There are those who believe that life on earth was seeded by aliens, they tend to point to the ancient structures around the world and how the older portions are built with superior quality than the more recent additions.

Thay would likely be explained by being taught a masted process and then the gradual forgetting of the techniques.

I agree, at most we can speculate, but i prefer to think that the universe is abundant with life.