Mushrooms are much, much more closely related to animals than they are to plants.

A billion and a half years ago , the only organisms on Earth were single-celled bacteria and archaea living in the ocean. Bacteria and archaea are very similar. It's rather technical, but the slight differences between bacteria and archaea have to deal with genes for membrane and cell wall construction. Both types of organisms were very simple, and, like all life, share a common ancestor, thought to trace its ancestral lineage back to a single RNA molecule around 4 billion years ago. The ancestor to all life on earth is called LUCA (the Last Universal Common Ancestor) in biology. Despite their similarity and simplicity, both groups of organisms had remarkable variety and diversity, but bacteria had some more "advanced" techniques.

For example, one bacterium (closely related to modern-day rickettsia) had developed a very complex energy-generation procedure with various enzymes. One step in the process used energy from the environment (glucose and other simple carbohydrates were probably the primary food source) to concentrate protons (hydrogen ions) into a membrane, which creates an electrical gradient. Electrical gradients flow from positive to nagative, so by concentrating protons, it creates an outward force from the electrical potential . An enzyme called ATP synthase is believed to rotate, like a motor, at the force of these protons flowing from one side of the membrane to the other. The enzyme accepts ADP (a precursor molecule to ATP ). It uses the energy of rotation to catalyse the chemical reaction which turns ADP into ATP . ATP is the chemical used for energy storage by cells. This sophisticated bacterium is called the protomitochondrion .

In 2010, scientists drilled a cylindrical core out of a deep-sea hydrothermal vent (hydrothermal vents are fractures in the ocean floor where the action of tectonic plates causes the expulsion of boiling water into the ocean). In the core sample, scientists found modern-day archaea which , remarkably, can be shown through multiple converging lines of inquiry to be in a single line of descent with plants, animals, and fungi. The group was named "Lokiarchaeota," after the Norse shape-shifting deity, to refer to the lineage's ability to "shape-shift" over evolutionary time into so many diverse groups.

A billion and a half years ago, an archaeote, from the Lokiarchaeota lineage, ate the aforementioned bacterium (the protomitochondrion), and rather than destroying the bacterium, simply fed off of its amazing capacity to generate ATP. The protomitochondrion became a symbiotic organism within the archaea, and, over time, was incorporated as an organelle - the mitochondrion, the "powerhouse of the cell." This new method of internal energy generation is what allowed complex life - animals, plants, and fungi, to evolve. Organisms descended from the lokiarchaeote which ate a protomitochondrion are called " Eukaryotes ."

The same thing happened many millions of years later when one of the decedents of a lokiarchaeote ate a different bacterium (called a cyanobacterium ), capable of using energy from sunlight to catalyse chemical reactions which turns carbon, hydrogen, and oxygen atoms from CO2 and H2O molecules into energy storage molecules. Thus, the ancestor to all plants gained the ability to photosynthesise. But at that time, a different descended lineage of lokiarchaeota, the one which would give rise to animals and fungi, had already split off.

A group of flagellum-wagging, single-celled Eukaryotes called Opisthokonts split into two groups of organisms, holomycota and holozoa . Holomycota eventually picked up the habit of forming colonies (or, potentially, but controversially, inherited it from Opisthokonta), and from those colony-forming cells descended all fungi . For animals, it's a bit more complicated, and we have a lot of "cousin" single-celled organisms one might be forgiven for mistaking for animals. Basically, Holozoa split into two groups, ichthyosporea and filozoa . Filozoa split into two groups, filasteria and choanozoa . Choanozoa split into two groups, choanoflagella , and animals .

Animals and fungi are the descendants of the opisthokonts. The opisthokonts and the plants are the descendants of the Eukaryotes. Eukaryotes and Lokiarchaeota are descended from older archaea, which is related to bacteria (and, thus, all life on Earth) because both descended from LUCA (the Last Universal Common Ancestor).