This is something that has been tossed around for some time, but something occurred to me: If ancient humans did have advanced technology, such as space travel, we would know about it.
The reason I say we would know about it is because they would most certainly have left objects in orbit around earth. Yes, the low-orbit objects would have eventually fallen back due to the drag of the upper atmosphere. Examples of this would be the GPS satellites, the space station, and the space shuttle. However, the high-orbit objects would stay in orbit indefinably. An example of this would be objects like the communication satellites in geo-synchronous orbit.
A few years ago I stumbled onto an interesting Astronomy related hobby. Basically people use their telescopes to look at satellites in orbit. One could have easily spotted the satellites in geo-synchronous orbit using a telescope available during Isaac Newton?s time. The reason it would be so easy is because they stay in the exact same location in the sky all the time. When you look at them through a telescope they are the only objects that appear to be sitting still while the stars move past them. This most certainly would have been a curiosity for the first telescope builders several hundred years ago and they most certainly would have written about them. Even today they would be something we could see using a back-yard telescope.