I want to add a bit more about fungus, which is the family that includes mushrooms and yeasts. The numbers are so vast and sometimes so microscopic that they aren't widely studied. Even though about 150,000 species of fungi have been discovered, it is estimated there could be between 3 and 12 million species. 95% of fungal diversity has never been documented. To say the least, it is a grossly understudied branch of the tree of life. Animals are more closely related to fungus than plants are. The fungi and animal branches diverged a good deal later than that of plants.
Apparently when dinosaurs disappeared was a time when fungi really took off in diversity. That is because as is surmised, with the sun blocked out for an extended period, things that didn't need the sun survived and with plants and animals dying all over the globe, things that eat decaying matter flourished. That would be things like fungi, slugs and snails. Animals ingest their food, fungi absorbs it. Some fungi can kill, some makes you high, some are nutritious and some live on and in our bodies. That's what dandruff is.