There was a long period of time that humans thought that they were the the only intelligent life in the entire universe. There are still a few hold outs to this train of thought but the greater majority of people believe that there is other intelligent life somewhere in the cosmos. The statistics (if nothing else) would show otherwise.
When you look at the number of stars in just one galaxy, then the number of galaxies, plus the number of planets and moons that could be involved, life on Earth, and Earth alone doesn't make a lot of sense.
In all of the years that humans have been around, we have only left our planet in the last 60 years or so.
If an alien life form was only a thousand or so years ahead of us space travel would be much easier. If we go into the area of millions of years ahead of us, there is no telling what they would be capable of doing.
How do this fit into aliens visiting the planet Earth? Throughout history, mankind has made great leaps in technology at certain time periods. It could be from being hunter/gathering nomads to building cities and growing crops. Living in caves or houses made of wood to building pyramids that are still standing after thousands of years.
The biggest jump was probably in the last 150 years. Humans have gone from either walking or riding horses to being able to fly around the world without landing or actually go into outer space.
Humans have made advancements that are considered everyday technology, yet just a few hundred years ago, it would be witchcraft or the work of the devil.
How did this happen? Do humans just randomly have a species wide brainstorm and come up with ideas to make life easier? Possible, yes. Likely, probably not. It would seem that there was some kind of an outside influence.
Aliens visiting the planet from time to time would be an explanation for this. They see a species that is somewhat intelligent (by their standards) and decide to give them a little help here and there. Now, some hard core religious people will argue that it was the doing of their particular God.
One doesn't have to actually cancel out the other. To us lowly humans, a being of higher intelligence that brings knowledge, punishes, and helps out could be aliens. In our minds, they would have “God like” attributes.
We witnessed this first hand during WWII. Allied airbases were built on islands all over the pacific. To the natives, that were lining in the Stone Age for all practical purposes, “Gods” came from the sky in flying machines and gave them gift. Many natives still worship these “sky Gods” by (somewhat) doing maintenance on the runways and making wood and grass models of airplanes to lure their “Gods” back.
The Earth has a lot of natural resources, specifically gold. Gold works great in outer space as well as for electrical devices. Could this be the reason that humans are hard wired to want gold? Is it the reason that throughout history, gold always adorned the temples to the gods?
Another good point about aliens visiting the planet is actually religion. In every religion, the major deity (as well as subordinates) always come from above. Usually with a lot of noise, smoke, and light. This could of course, be the “glory of God” or it could be the rocket engines of a normal space craft.
Basically, yes aliens have visited the Earth in the past and probably continue to do so.