(A lot of this is my piecing together things from Entangled Life by Merlin Sheldrake alongside other articles. I hope I got it right — ish? It's really fascinating...)

Landed plants. Fungi diverged from other life around 1500 ma, and were some of the first complex life forms on land, mining rocks for mineral nourishment, slowly turning them into what would become soil. Then by the Ordovician era, about 450 ma, they began forming symbiotic relationship with liverworts, the earliest plants. Fungi had helped to create land-dwelling plants

Evolution of trees. Plants gained a new adaptation: they began to create wood out of cellulose and lignin. This led to a huge reduction of CO2 in the atmosphere since all of that carbon was now needed to create gigantic trees. This led to a giant temperature decrease and an instability in the environment. This all happened around the Devonian period, about 350 ma.

Stockpiling wood. The proliferation of large trees led to a giant stockpile of dead trees which simply grew over many millions of years. Without any mechanism for breaking down this vast amount of wood, it simply accumulated and compressed, and became responsible for a large amount of the coal humanity benefits from today.

Evolution of wood decomposers. By the Carboniferous period, about 290 ma, some fungi evolved to process lignin and then feast on the delicious cellulose hidden within these dead trees. Much of the tree mass accumulated during the Devonian period had become inaccessible, buried under sediment due to erosion. However, newly dead trees were now being munched on by these hungry fungi (white rot). Decomposition was in full swing.

Interestingly, fungal decomposition of plants is by far a larger source of carbon dioxide emissions into the atmosphere than anything human caused. By an order of magnitude?

Sources:

• https://en.wikipedia.org/wiki/Timeline_of_plant_evolution
• https://en.wikipedia.org/wiki/Silurian-Devonian_Terrestrial_Revolution
• Study on Fungi Evolution Answers Questions About Ancient Coal Formation and May Help Advance Future Biofuels Production
• White Rot Fungi Slowed Coal Formation
• Mycorrhizal fungi helped plants make the transition from water to land
• Fungi Are Responsible For Life On Land As We Know It