Assuming the theories that Mars was once a much nicer place ages ago
are still correct.

Just to keep the flame burning: There are simple creatures on Earth
that live in extreme conditions and something could survive buried way
underground, but that will take a serious colony setup to search for
those. The fossils, however, might be easier.

We focused on Mars for so long as a place for life because in the pre-
spacecraft days it looked most promising. But now I think we need to
take our search further out to certain moons around the Jovian planets.

At least when we establish colonies on Mars, we probably won't have to
worry about contamination in either direction.