It's a statistic impossibility that life hasn't developed elsewhere. People keep saying that conditions have to be just right for life, but they're wrong. They keep considering that life can only be just like us, when that is incredibly unlikely. Any sort of life can appear in enviornments, and they adapt to it. Carbon-based life-forms would need conditions similar to us, yes, but what about Silicon-based? Argon-based? etc.
Whether extra-terrestial life has come to this planet? It remains to be seen, but people keep using excuses based on our level of knowledge saying why it's not possible. I don't like that people can be so narrow minded to believe our scientific knowledge is absolute.
So yeah, I believe there is alien life out there