Question

What type of scientific proof is there that Earth is the only location in the universe that life has ever existed on?

Although this is a good question, it is posed in such away that is not congruent with the way scientific research works. In order to prove that Earth is the only location for life in the universe, we would have to check all of the other locations first! At our current state of technological development, we only have the means to directly search for life on the nearest bodies in our solar system, such as Mars, Europa, and Titan (and even this is done with spacecraft, not with direct human presence). We do have the means to look for signs of life astronomically, but this is a different scientific approach. (Refer to the question “What are some ways in which scientists gather evidence of life on other planets?” at http://nai.arc.nasa.gov/astrobio/astrobio_detail.cfm?ID=249). At this point, scientists can neither confirm nor deny the existence of life elsewhere—but they have several good reasons to believe that it does exist in places besides Earth. These reasons fuel the current motivation for so much of the exciting new research going on in astrobiology. So although your question is a good one, perhaps an even better one is “how can scientists verify that life exists (or existed) elsewhere?” David Lamb, Science Projects Specialist, NAI
November 8, 2002