I think that panspermia is the most likely hypothesis to be eventually proven correct.
According to this hypothesis, very simple "living" structures can be found throughout space, scattered around planets, meteorites, and perhaps even in space dust. This dust can be thought of as fungal spores. When they land in a planet with suitable conditions, these systems can evolve into more complex life forms.
The alternatives to panspermia generally imply that Earth is a super special planet in which certain special conditions conspired at a point in time to allow an extremely unlikely event to happen. Panspermia suggests that, instead of a being a statistical anomaly, life is probably rather common throughout the universe.