We had fungus growing here on Earth before we had plants and trees. In fact, plants and trees depend on mycelium for nutrient uptake and decomposition of biological matter.
Ask any organic gardener if he feels mycelium is important for his plants’ well being.
Fungus is a very interesting organism and it has its own intelligence we are only now beginning to research on a more serious level. Fungi is not dependent on sunlight. So it would be safe to say that the “interstellar space invaders” already arrived a long time ago on Earth. Fungus came first.
Fungus create some of the most potent medicinal healing compounds, and it can also generate some of the most deadly.
The most highly regarded medicine in Chinese medicine is the Reishi Mushroom. In that culture there are even paintings of the “gods” coming down on a cloud with a Red Reishi in his hands for the people: