In my opinion, if we discover some life on Mars, but biochemical analysis says that it is compatible with Earth life (uses DNA, same chirality on the molecules, clear analogs with Earth life in the design, probably could even find an ancestor), we have to stick with the theory that Mars did not originate life. If Mars did independently originate life, it should be clearly obvious.<p>"Mars is able to natively host small amounts of Earth-derived life" is still interesting and still says some things about the prospect of life in the universe, but I don't think that would really add much to what we already know, the way a truly independent branch of life would.<p>Good cases have been made that RNA and DNA may be the only suitable bases for life, which would imply that finding those wouldn't be absolute proof, but there's enough other stuff in Earth's base DNA catalog that should be different that sequencing should immediately reveal a different origin. (Not to mention the different starting conditions should result in other choices for basic functioning.)<p>(It is not necessary that RNA/DNA be the only choices for life; it simply means that if the arguments are even partially true it tweaks the probability that truly foreign life might still be RNA/DNA based up. But above that level I'd expect significant differences; "ontogeny recapitulates phylogeny" may be false at the macro level, but billion-year-old decisions of life are still written into our DNA at deep levels. Truly independent life should have made at least some different decisions.)