The closest relatives of fungi are thought to be the animals. On top of that, not plants. Not bacteria. Animals.
That surprises most people. Fungi look like plants — they're stationary, they grow in soil, they have cell walls. But the cell walls are made of chitin, not cellulose. They store energy as glycogen, not starch. And when you trace the molecular evidence back far enough, the tree of life puts fungi and animals on the same branch, separate from plants, separate from protists, separate from everything else Simple, but easy to overlook..
It's one of those facts that rewrites how you see the world once you actually absorb it. The mushroom on your pizza is more closely related to you than it is to the oregano sprinkled on top But it adds up..
What Is the Fungal-Animal Connection
The relationship isn't obvious from the outside. That's why animals are multicellular, ingestive heterotrophs that swallow food whole and digest it internally. Fungi are multicellular (mostly), absorptive heterotrophs that secrete enzymes onto food and slurp up the results. The lifestyles couldn't look more different Not complicated — just consistent..
But at the cellular and molecular level, the similarities stack up fast.
Both groups:
- Store reserve energy as glycogen
- Use chitin as a structural polymer (in fungal cell walls and animal exoskeletons)
- Lack chloroplasts and photosynthetic ability
- Share similar mitochondrial DNA sequences
- Have comparable ribosomal RNA structures
- Use similar protein synthesis machinery
- Share key metabolic pathways absent in plants
The smoking gun came from molecular phylogenetics in the 1980s and 90s. On top of that, when researchers compared ribosomal RNA sequences across the tree of life, fungi and animals consistently clustered together — not with plants, not with slime molds, not with anything else. They form a clade called Opisthokonta Which is the point..
The Opisthokont Clade
Opisthokonta gets its name from a shared ancestral trait: a single posterior flagellum on motile cells. "Opistho" means behind, "kontos" means pole. The flagellum pushes from the rear rather than pulling from the front like in most other eukaryotes.
This trait shows up in:
- Animal sperm cells
- Fungal spores (in early-diverging groups like chytrids)
- Choanoflagellates — the closest living relatives of animals
- Several other unicellular protist lineages
The opisthokonts split from other eukaryotes over a billion years ago. Then, somewhere along that lineage, the ancestors of fungi and animals diverged from each other. Current estimates put the split around 900 million to 1.2 billion years ago — deep in the Precambrian, long before either group left a clear fossil record And that's really what it comes down to..
Why It Matters
This isn't just taxonomic trivia. The fungal-animal connection reshapes how we understand evolution, cell biology, and even medicine.
For starters, it explains why fungal infections are so notoriously hard to treat. Fungi are eukaryotes, like us. Their cellular machinery — ribosomes, cytoskeletons, metabolic pathways — looks uncomfortably similar to ours. Antibiotics target bacterial differences (prokaryotic ribosomes, peptidoglycan cell walls). In real terms, antifungals have to find the few molecular gaps between two closely related eukaryotic kingdoms. That's why antifungal drugs tend to have more side effects and narrower therapeutic windows than antibiotics.
The relationship also illuminates the origins of multicellularity. Both fungi and animals evolved complex multicellular bodies independently from unicellular opisthokont ancestors. Here's the thing — comparing how they did it — what genes they co-opted, what developmental toolkits they built — reveals general principles about how multicellularity evolves. Spoiler: it involves a lot of gene duplication, new regulatory networks, and repurposing ancient adhesion molecules Less friction, more output..
And there's the practical side. Fungi produce compounds that affect animal nervous systems because we share neurotransmitter pathways. Psilocybin, ergot alkaloids, aflatoxins — these molecules interact with receptors that exist in animals because those receptors evolved in our common opisthokont ancestor. Fungi didn't "evolve to drug us." They evolved to defend themselves or compete with microbes, and their chemical weapons happen to cross-react with animal physiology because the targets are conserved Took long enough..
How the Relationship Was Discovered
The fungal-animal link wasn't obvious to early taxonomists. The reasoning was simple: they don't move, they grow in the ground, they have cell walls. Linnaeus put fungi in the plant kingdom, and they stayed there for over two centuries. Case closed Turns out it matters..
Early Hints
A few dissenting voices appeared in the late 1800s. German botanist Anton de Bary noted that fungi lack chlorophyll and absorb nutrients — animal-like traits. But without molecular data, these were just morphological arguments, and morphology lies. Convergent evolution is rampant. Slime molds look like fungi at certain life stages but are amoeboid protists. Water molds (oomycetes) look like fungi but are actually stramenopiles, closer to brown algae and diatoms.
The real breakthrough required molecular clocks.
The Molecular Revolution
In the 1980s, researchers like Carl Woese, Mitchell Sogin, and Thomas Cavalier-Smith began comparing small subunit ribosomal RNA (18S rRNA) across eukaryotes. This molecule changes slowly, making it ideal for deep phylogeny. The results were unambiguous: fungi and animals grouped together with strong statistical support And that's really what it comes down to. Less friction, more output..
Later studies added more genes — elongation factors, actin, tubulin, RNA polymerase subunits. Whole genome comparisons followed. And every dataset told the same story. Fungi and animals are sister groups Simple, but easy to overlook..
The Choanoflagellate Connection
The picture sharpened further when choanoflagellates entered the genomic era. Consider this: these collar-flagellated protists had long been suspected as animal relatives based on their resemblance to sponge choanocytes. Genome sequencing confirmed it: choanoflagellates are the closest living relatives of animals, and together with animals they form the Holozoa — the animal total group.
Fungi sit just outside Holozoa, as the sister group to the entire holozoan clade. So the full topology looks like this:
Opisthokonta
├── Holomycota (fungi + closest unicellular relatives)
└── Holozoa
├── Choanoflagellata
├── Filasterea
├── Ichthyosporea
└── Animalia
The unicellular relatives of fungi (nucleariids, rozellids, microsporidia) and the unicellular relatives of animals (choanoflagellates, filastereans, ichthyosporeans) are critical. They preserve ancestral traits that were lost in the multicellular crown groups. Studying them is like finding the missing pages of an instruction manual.
What Most People Get Wrong
"Fungi Are Plants"
Still the most common misconception. On the flip side, the historical inertia is massive. It persists because fungi are taught in botany departments, sold in produce aisles, and studied by people who call themselves mycologists but work in plant biology buildings. But it's been wrong for 40 years.
"Fungi Are Primitive"
People hear "single-celled ancestors" and assume fungi are simple or ancient in a primitive sense. Wrong. Fungi are highly derived.
and radiated into an estimated 2–5 million species, making them one of the most diverse kingdoms on the planet. Their metabolic versatility lets them act as decomposers, pathogens, symbionts (think lichens and mycorrhizae), and industrial workhorses (producing antibiotics, enzymes, and biofuels). From the microscopic yeast that ferments our beer to the massive mushroom that towers over a forest floor, fungi have colonized every niche imaginable—soil, water, extreme temperatures, even the bodies of animals. In short, fungi are far from “primitive”; they are a sophisticated, highly evolved lineage that has refined a unique way of life over half a billion years.
“Fungi Are Just ‘Dirty’ Bacteria”
Another common slip is to lump fungi in with bacteria because both can cause disease or rot our food. The truth is that fungi belong to a completely different cellular architecture. Their cells have real nuclei and membrane‑bound organelles, whereas bacteria are prokaryotic. Worth adding, fungi grow as hyphae or yeast, using actin‑based cytoskeletons and complex cell‑wall polysaccharides (chitin and glucans) that have no counterpart in bacterial cell walls. Even their modes of reproduction—spores that can be dispersed by wind, water, or animals—are far more elaborate than the binary fission of bacteria.
“Fungi Are Not Important to Human Health”
Because we often think of fungi only as the cause of athlete’s foot or the mold that spoils our bread, many overlook their profound medical relevance. Conversely, fungi also produce life‑saving antibiotics (penicillin from Penicillium), immunosuppressants (cyclosporine from Tolypocladium inflatum), and cholesterol‑lowering statins. Think about it: antifungal drugs are a cornerstone of modern medicine; without them, organ transplants, chemotherapy, and HIV treatment would be far riskier. The same genomes that make fungi successful pathogens also give us tools to treat disease, underscoring their dual role as both threat and treasure.
The Evolutionary Take‑Home
The molecular clock studies of the 1980s and the subsequent explosion of genomic data have painted a clear picture: fungi are not distant cousins of plants; they are sister to animals, sharing a common opisthokont ancestor. Practically speaking, the unicellular relatives—nucleariids, rozellids, microsporidia, choanoflagellates, filastereans, and ichthyosporeans—serve as living windows into the transitional forms that gave rise to multicellularity in both kingdoms. By comparing the genetic toolkits of these protists with those of modern fungi and animals, scientists have uncovered shared features such as the actin‑myosin contractile system, cAMP signaling, and ribosomal protein structures that predate the split.
Understanding this relationship matters beyond academic curiosity. But , targeting fungal cell‑wall synthesis may affect related pathways in animals) and informs biotechnology (fungal enzymes are already workhorses in industrial processes). g.Which means the shared biology hints at common vulnerabilities that can be exploited for medicine (e. Also worth noting, recognizing fungi as close relatives of animals reshapes how we think about ecosystem services: mycorrhizal networks are not mere plant helpers but extensions of a fungal “internet” that connects and regulates entire landscapes That's the part that actually makes a difference..
Conclusion
Fungi have long been mischaracterized as plants, primitive relics, or mere contaminants. Their evolutionary history is a story of diversification, adaptation, and innovation—not of simplicity. Modern phylogenetics, however, places them firmly within the Opisthokonta, alongside animals and their closest protist relatives. By appreciating fungi for what they truly are—sophisticated, ecologically indispensable, and medically vital organisms—we gain a richer understanding of life’s tree and a deeper appreciation for the complex connections that bind all eukaryotes together.
