[fnl]

I'd say the life form that dominates this planet are single cellular organisms, i.e., mostly bacteria. They have run this place for billions of years, deciding who gets to stay in symbiosis with them or not and most likely will continue to do so for a few billion more, long after mammals and similar life forms will have disappeared again. They inhabit the entire biosphere, at least 1km below and up to 50km above earth, filling every cubic meter with millions and often billions of them. They can "communicate" (evolve their latest genetic modifications) across the whole planet in a matter of weeks for particularly mission-critical mutations. Every other life-form just passes by.