Is there a point in Earth’s history where if you were to visit you’d find it was just a fungus planet, with no other form of life? Could there have been fungal type creatures that lived, died, and left no trace?
Blue-green algae have colonized fresh water and all moist spaces on the continents billions of years before the appearance of fungi and of the terrestrial green plants. They were accompanied by the ancestors of several groups of heterotrophic bacteria, e.g. Firmicutes and Actinobacteria (the Gram-positive bacteria), which had developed abilities like making spores that could survive dryness and be spread by wind, like also fungi would do later.
In fact, it is likely that blue-green algae have appeared for the first time on continents, either in fresh water or moist rocks, and only much later they have spread into marine environments, which had been previously dominated by anoxygenic phototrophic bacteria, which were oxidizing sulfur, iron or manganese, not water. The transition to oxidizing water is likely to have been necessary for the spreading of the blue-green algae on continents, where the fresh water had only a very small content of substances that could be oxidized unlike seawater, which at that time was rich in hydrogen sulfide and in Fe(II) and Mn(II) ions. When the blue-green algae have expanded back into the oceans, that must have been after the oxygenation of the atmosphere has modified the composition of the oceans, by precipitating most of the iron and manganese and oxidizing sulfide to sulfate, which would have deprived the anoxygenic phototrophic bacteria of their food.
Then, in the oceans, at some point in time a blue-green alga has become symbiotic with the ancestor of red algae and green algae, which have then dominated for many hundred million years the oceans, before the much later appearance of other phototrophic groups, like diatoms and brown algae. Multicellular red algae and green algae already existed in the oceans around one billion years ago, when no other multicellular eukaryotes existed.
When the fungi have appeared through a transition to a terrestrial lifestyle, that could happen only if on land they could find great amounts of dead living matter, which had been produced by blue-green algae. It is not known for sure whether fungi have appeared a long time before the first terrestrial green plants or about the same time, but it seems that already for the most ancient terrestrial green plants, symbioses with fungi that enhanced the absorptive capabilities of their roots have been important. For aquatic plants, roots had only a fixation function, not the function of absorbing nutrients, so perhaps symbiosis with some already existing terrestrial fungi might have been necessary, not optional, for the first terrestrial green plants, until better absorbing roots have evolved.
So for a long time in the history of the planet, the drier parts of the land would be barren, but wherever there was moisture you would find mats or crusts of blue-green algae, with associated heterotrophic bacteria and viruses.
Then, probably not earlier than the Cambrian, there would be also fungi, and even later the first moss-like terrestrial green plants would appear.
