Unless of course Mars never had life to begin with, if it did at
least start ages ago, then there may be lots of fossil remains
all over the planet, which could be found once a larger and
more systematic search begins - probably when humans start
setting up there in earnest.

And if any of those suspicious looking formations the rovers
have imaged over the last four years turn out to be actual
fossils of once living creatures, the proof may come even
sooner than thought.

While I think we should keep exploring Mars, I alao think we
should start improving our odds of finding alien life in the Sol
system by looking at more hospitable places, such as a
certain ice-encrusted moon of Jupiter with a global ocean
of liquid water (and a briny one at that) extending 60 miles
beneath.

And maybe Enceladus and Titan.