Where fungi sit
For most of scientific history, fungi were classified as plants. They were sessile, they didn't move, they grew from the ground — plant enough. Linnaeus put them in the plant kingdom in 1753 and there they stayed, more or less, for the better part of two centuries.
The problem is that fungi are not plants in any meaningful sense. They have no chlorophyll, no photosynthesis, no cellulose cell walls. They feed by secreting enzymes and absorbing the breakdown products. They store energy as glycogen, not starch. Their cell walls are made of chitin — the same material in insect exoskeletons.
Molecular phylogenetics — the ability to compare DNA sequences directly — finally settled the matter in the 1990s. Fungi and animals diverged from a common ancestor approximately 1.5 billion years ago. Fungi and plants diverged from a common ancestor approximately 1.1 billion years ago. Fungi are, in a very real sense, more closely related to you than to a fern.
They now occupy their own kingdom: Fungi. Separate from Plantae, separate from Animalia, separate from Protista and Bacteria and Archaea. Six kingdoms total, and fungi are one of them.
The tree of life
All organisms with cells containing a nucleus. Includes animals, plants, fungi, and protists. Distinguished from Bacteria and Archaea by cellular complexity.
Heterotrophic eukaryotes with chitinous cell walls. Feed by extracellular digestion and absorption. Reproduce by spores. Estimated 2–6 million species. Formally described: ~150,000.
The five major phyla. Ascomycota and Basidiomycota together make up the Dikarya — the "higher fungi" — and contain the vast majority of described species including almost all cultivated mushrooms.
Fungi vs Plants vs Animals
| Feature | Fungi | Plants | Animals |
|---|---|---|---|
| Cell wall | Chitin | Cellulose | None |
| Energy storage | Glycogen | Starch | Glycogen |
| Nutrition | Absorptive (external digestion) | Photosynthesis | Ingestive (internal digestion) |
| Growth pattern | Hyphal (tip growth) | Meristematic | Embryonic development |
| Motility | None (spores disperse) | None (pollen/seeds disperse) | Most are motile |
| Carbon source | Organic compounds | CO₂ (photosynthesis) | Organic compounds |
| Closest relative | Animals | Green algae | Fungi |
