The 'common ancestor' dates back to about the same time as the late heavy bombardment. It's been theorised that earth may have been suitable for life earlier th

The 'common ancestor' dates back to about the same time as the late heavy bombardment. It's been theorised that earth may have been suitable for life earlier than originally hypothesised, but the bombardment wiped the record. If this is true, then there may have been many independently evolved microorganisms pre-bombardment, but only a few colonies survived because they happened to live deep underwater near volcanic vents. Perhaps also, in this era of reduced genetic diversity, one colony was especially capable of adapting to the new environment and quickly outcompeted all the other surviving colonies. Or maybe aliens did it. Who knows.