We now know a lot about organic molecules, amino acids and simple proteins to have a fairly good idea on how life began.
The question for most scientists is not so much how did life form, we will probably never know %100 for sure until we recreate it and even then there is no way to know if that was exactly what happened.
While the below is not proof that there was once life on mars. It has all the scientists winking at each other, we are close to proof.
http://www.iflscience.com/space/were-these-mars-rocks-made-microbes
So the question has to be asked, if we "do" find that Mars once held life, if our neighboring planet held life, then to assume we are the only life in the universe would be an almost indefensible position. Suggest everyone watch the new doco series called Cosmos
http://en.wikipedia.org/wiki/Cosmos:_A_Spacetime_Odyssey to understand how "billions" of planets is a massive understatement.
Now, I don't believe there are aliens visiting us as there is no evidence when there should be a lot. But I have very little doubt that we are not alone in the universe, just need the proof.
