It is that they would only be interested in scientific observation. Based on everything we know interstellar travel will be difficult and have a high energy cost. If aliens were to come themselves I believe it would be far more likely to be to stay than to play hide and seek or just study us. The exception would be with remote robotic probes. While I feel the debate here about robots versus human exploration when humans are clearly far more capable is silly, within a few hundred years that shouldn't even be true for us. I think we are advanced enough now with mechanical, DNA, and forensic technolgy to detect any form of alien amongst us. Or technology has evolved too fast for adaptions to it to be sent from light years away or ships sent years ago to be adapted specifically for it.

While I disagree with Raoul that we are unlikely to be of interest to aliens, I think it is true that it is highly unlikely our presence would have impacted alien colonization of Earth. Just as we study animals to understand our history and are looking for answers on how life originated, aliens might wish to study the evolution of technology. The problem is that humans (H. erectus on) have only existed for 0.03 % of earth's history. Cut that by a factor of 30 (0.001%) for truly modern man and another 0 past that for civilization. While many civilizations might have a hands off policy, the history of evolution suggests that species that expand tend to outlast or overtake those that stick to a limited niche. I think technological civilzations must expand into space or they will fall to a preindustrial level due to a lack of resources. They may even iradicate themselves using hightech in fights over ever diminishing resources. It may well be that evolving intelligence is not the limiting factor, but rather how long it can sustain a technological civilization.

The key point for me is that I believe that if aliens had reached Earth in the past they would have stayed unless they found it too inhospitable. I also doubt that technological civilizations will last long without spreading beyond one planet due to the many long term threats civilization faces.

Fermis paradox is a moot point if you believe technological civilizations will stay bound to their home planet without collapsing.

Regards,
Mark