Early life and early earth
I
Fungus was born in the reaches of space, a little lump of carbon imbued with a will. It fell to earth some 4 billion years ago, where it landed in a stromatolite. Fungus decided to become one with the microbial masses of earth, and cloaked themself in a skin of algae and bacteria. They lived alone for quite some time before they met another sentient organism, or perhaps a god of sorts, who they became friends with. Their friendship was short-lived as a meteor struck their friend and scattered their parts across earth. To save their friend, they created a dimension within themself for their friend to survive in.
The hunt
II
Fungus now had to feed. It takes a lot of energy to run a pocket dimension, you see. In the beginning, all that was there was microbial mats. So, they harvested those for many years. As life became more complex, Fungus watched. And fed. Eventually, they grew tired of hunting for food by themself. They had an idea. They created a couple of miniature, microbial entities like themself, and offered them to certain creatures. The creature would bring food to Fungus, and the microbial entity would provide the creature invulnerability if it could live within the body of the host. And so, this was Fungus's business model for millions of years.
Business is booming
III
Fungus took a bit of an emotional hit in the K-Pg extinction event, as both they and the inhabitants of earth were very successful up until then. They were taking the form of a sauropod back then, and continued using that shape for millions of years after. They experimented with being a mammoth and a ground sloth but when humans came around, they took the form of one. They also may or may not have caused a spontaneous event where humans suddenly got non-mammalian phenotypic traits all at once. Don't ask them how it happened, it just did.
