Somebody asked me why Abiogenesis happened only once.
My response was that it may very well have happened multiple times but that the Earth's atmosphere transitioned from 'reduced' to 'non-reduced' when fotosynthesis evolved and that this resulted in the extinction of all the other strains of life (the Great Oxygenation Event).
However, this can not account for it wholy because what about life forms that do chemosynthesis? How much have these life forms been examined? Are they known to be part of that same life strain (and simply well shielded from the non-reduced atmosphere and therefor unaffected? Or is the research pointing in another direction? A second abiogenssis? A remainder from the first? A return to the 'basics' comparable to the return of the whale to water?