See this article on Fermi's Paradox.

This is the reply I (John Schmidt) posted here on 4/9/05:


1) intelligent life evolved on the order of 1,000,000,000 years ago
2) that intelligent life and its offspring continued to exist for the past 1,000,000,000 years
3) some fragment of those offspring reached Earth long ago
4) we have not found evidence of 1-3 (above)
5) those offspring are still on Earth
then what are the possible explanations of 1-5?

First, there is what I would call the "sociological problem". How would it be possible for any form of intelligen life (biological or artificial) to exist for hundreds of millions of years and still retain an interest in planets like Earth- an interest that would extend over hundreds of millions of years?

It seems likely that for this to happen, there would have to be an effort to design a form of life that was essentially static in terms of its evolution and cultural development. Life as we know it in a natural biosphere is all about change and adaptation. Could a form of life be designed that would not change through time and that would play the role of observing planets like Earth?

Second, even if Earth is being observed, this would not mean that the observers might not want to collect samples of life from Earth. If such samples were collected and "cultured" off of Earth by the alien observers, there might be populations of humans who are currently in contact with the alien observers.

If so, those humans might be able to return to Earth and function as the "eyes and ears" of the alien observers. Such observers might have reason to invent and promote the Fermi Paradox in an effort to make people think that intelligent aliens are unlikely.