Re: Fossil Finds...
What i failed to articulate was that it would impact our perception of our place in the universe, WRT the fact that to date, we think we are the only sentient beings. That's because so far, nobody/nothing has ever proven us right or wrong. But, for fear or entertainment purposes, or calls to prepare for the end of the world, or claims to prophesy and so on, imagine would it be like if suddenly, finally, we can let our hair down that right in our own back yard we find evidence of an extinct advanced set of animals or other beings. Of course, given that we've not found structures, roads, or the like, even if we did find fossils, they'd likely be very very pre-advance -- assuming no interstellar travelers crashed and died there trying to get to what WE call Earth. Then again, it could be that WE are in "the back yard", but the front-yard-occupants got killed off.
At any rate, it seems to hold that Earth had the optimal conditions for sustaining life that was started here locallly, and Mars was just too hot, too dry, and not right at the time for life that we can visualize. But, "Martian life" is still a possibility. That is probably why NASA and international bodies want to avoid introducing to Mars microbes not likely present on Mars or avoid reintroducing them in case what we take to there is active.
As for the risk of contamination, if non-contamination is THAT important, why hasn't there been build a "Decontamination Chamber" of sorts (send ahead of Curiosity) into which the Rover would be driven, parked, and repeatedly subjected to ND-TC (non-destructive-to-Curiosity) treatment that solar UV and G forces might not have killed. If it's just the drill bits, it probably would not need to be ST:TNG-grade stuff, especially since we don't have pattern buffers, collimators, quantum force fields, or other Treknobabble-of-the-week stuff at our disposal (decontamination was complicated by the unique structure of the pre-anu-dioxcipitall pre-cortex lobes behind his 3rd condron filament...... You know, bowl-of-soup-spewing babble like that, which in itself would be a reason to watch TNG, to see which episode would on the floor rolling my ass off in laughter as the writers one-up themselves, possibly on inhalable/injectable contaminants, themselves..... (really, many TV adverts that defy ordinary logic arise from stress-induced people resorting to drugs to take the edge off their lives -- one of my friends working in the insanely-competitive and cutthroat marketing/advertising industry in the 90s was serious when I joked about it... Anyway....)).
BTW, in your lst para, you mentioned RNA-based microbes on Mar and possibly originating here... After all this time, the separation time and existence in two differrent environments, is it likely that the microbes would develop in compatible ways or diverge, possibly incompatibly? Is it necessarily so that cross-transferrence would result in cross-contamination? Even if today Curiosity's pre-exposed drill bit contaminates Mars, how many millions of years would it be before humans or the Martian microbes would know? In which direction would the finger point? Just some thoughts...