One this topic there is so much rock transfer between Mars and Earth (about 100kg a year) that both planets are effectively one ecosystem.
1) Life that has a shared lineage with Earth life.
2) Life that is unrelated to Earth life.
The thing about #1 is that we're almost certain it's completely plausible. We have extremophiles on Earth that live in conditions just as harsh as some of the places on Mars. The tricky part would be getting them there. Sadly, biological transfer between planets doesn't really help us fill in any of the variables in the Drake equation...
But #2 - that would be incredible. If we were to find life on Mars and we could prove that it had no relation to life on Earth, that forcibly changes everyone's cosmic perspective. Proving that life evolved independently twice in the same solar system would mean the universe must be teeming with it.
One of the interesting things about bacteria is that there are two major lineages here on earth (the archea and the true bacteria). Some people have speculated that one arose on Earth and the other on Mars, but I doubt we will ever find good evidence for this speculation.
Here's a fun article about reviving supposedly 250 million year old spores trapped in salt crystals: http://www.its.caltech.edu/~bi1/Bi1__Micro-_to_Macro-Biology...