Or there might have been a catastrophe that killed off almost everything. In which case the organisms that survived might not have been very widespread before t

Or there might have been a catastrophe that killed off almost everything. In which case the organisms that survived might not have been very widespread before the catastrophe, or typical of what existed before the catastrophe. If a large asteroid hit the Earth today it might be only a few subterranean extremophile microorganisms that survive. Asteroids were more frequent during the early history of the solar system.