Wouldn't there be radio wave evidence of such?
In addition, going on how sloppy humans are, I refuse to believe that this planet has been visited by any ETs, whether it be 1947 in Roswell, NM or ancient times. There would have to be some solid and obvious evidence left behind, such as a cigarette butt (poor example, sorry, but it is America's favorite litter, followed by the aluminum soda can). If there was evidence, or a visit, such as Roswell, NM in 1947, I also refuse to believe that the US government is efficient enough to 100% cover it up with NO piece of evidence left untouched and unavailable for the public. The only thing government does efficiently is inefficiency.
There are megalithic structures all over the earth as old as 10,000 years, but they are not in good repair. Anything less than megalithic structures would not last even that long. There wouldn't be much left of anything older than that. If something happened to be living here 100,000 years ago and didn't happen to build megalithic structures, it would all be gone back to nature. If we found some fossil bones, we might not recognize them as alien, especially if they were related to us in the first place.