Like people have said alien life is more likely of the bunch, but on the second hand the probability of so many things happening right from planet capable of supporting life to evolution starting off and not failing - it makes the probability very small. Also have to take the time scale into consideration here. Maybe there were aliens but their local star went supernova and everyone went extinct.How could you not believe in aliens though? Considering the youth of our planet, it's so likely that alien life evolved somewhere else billions of years ago.