You know that science fiction trope of the deadly planet full of life where every last life form is trying to kill every other life form, and they've evolved a frightful array of offenses and defenses to do it?
That planet is Earth. The real Earth. We perceive our ecosystem as friendly only because we've co-evolved with it. It is in objective terms a vicious, vicious place, with every cubic centimeter of any useful resource contended for by thousands or millions of organisms, fighting each other in every conceivable chemical, biological, and physical manner, and in particularly sophisticated corners of the Earth ecosystem, even social manners. It may look peaceful from the apex, but it's not when you get close.
The likelihood of some sort of Martian bacteria getting here and causing amazing problems can never quite be eliminated, but it's very, very low. Plus, if Martian life ever existed, it actually won't be entirely disconnected from Earth life due to what you could call "transpermia": http://users.tpg.com.au/users/horsts/swaprock.html
In fact if life is discovered on Mars, the next question is whether it is just Earth life that hitched a ride and managed to adapt to conditions that aren't necessarily all that much harsher than conditions we've found life on here on Earth. It's interesting either way, but the implications are hugely different between some hitchhiking Earth life vs. life that clearly came from a different abiogenesis event.
That planet is Earth. The real Earth. We perceive our ecosystem as friendly only because we've co-evolved with it. It is in objective terms a vicious, vicious place, with every cubic centimeter of any useful resource contended for by thousands or millions of organisms, fighting each other in every conceivable chemical, biological, and physical manner, and in particularly sophisticated corners of the Earth ecosystem, even social manners. It may look peaceful from the apex, but it's not when you get close.
The likelihood of some sort of Martian bacteria getting here and causing amazing problems can never quite be eliminated, but it's very, very low. Plus, if Martian life ever existed, it actually won't be entirely disconnected from Earth life due to what you could call "transpermia": http://users.tpg.com.au/users/horsts/swaprock.html
In fact if life is discovered on Mars, the next question is whether it is just Earth life that hitched a ride and managed to adapt to conditions that aren't necessarily all that much harsher than conditions we've found life on here on Earth. It's interesting either way, but the implications are hugely different between some hitchhiking Earth life vs. life that clearly came from a different abiogenesis event.