What do we do if we find life on Mars? It is possible that martian life is on the same tree of life as Earth life because of the exchange of meteorites between the two planets. Alternatively, it may be that life on Mars represents a second genesis--an independent origin of life. Contamination by even one Earth bacterium may be a serious issue of environmental ethics. Furthermore, if we find evidence of a second genesis, then this may open discussions of warming Mars to help that alien life to flourish. Scientists and policy-makers who consider this choice will have to deal with any contamination left on Mars by previous explorers, so that it does not flourish instead.-- from Biologically Reversible Exploration by Christopher P. McKay.
(See also Should Mars be treated like a wildlife preserve?. Some Mars posts here)