Fungi are more closely related to us than to plants
Though mushrooms look like plants, evolutionary biology shows they are far closer to animals. Fungi do not perform photosynthesis; instead, they absorb nutrients from their surroundings, much like animals digest food. Cell walls in fungi are made of chitin, the same tough material found in insect exoskeletons, rather than plant cellulose.
The Longstanding Illusion of the Plant Kingdom
For centuries, naturalists categorized fungi alongside mosses, ferns, and flowering plants. The resemblance was intuitive: mushrooms sprout upright from soil, remain anchored in place, and do not possess eyes, limbs, or nervous systems. Early botanical taxonomies treated them as 'cryptogams' or lower plants lacking flowers and seeds. Because early classification relied heavily on visible morphology and immobility, the superficial similarities between a toadstool rooted in the forest floor and a neighboring fern masked a completely different evolutionary path.
The fundamental flaw in this traditional view was exposed when scientists shifted their focus from gross anatomy to cellular architecture, biochemistry, and genetic analysis. Under microscopic and molecular inspection, the characteristics that define true plants—such as photosynthesis powered by chlorophyll and rigid cell walls built predominantly from cellulose—are entirely absent in fungi. Instead, the deeper mechanisms operating inside fungal cells reveal an organism that branched away from plants hundreds of millions of years ago, following an evolutionary trajectory that ran closely parallel to the ancestors of animals.
The Opisthokont Branch of the Tree of Life
In modern biological taxonomy, fungi and animals are united within a major evolutionary supergroup known as the opisthokonts. This grouping, confirmed by molecular phylogenetics and comparative genomics, separates both animals and fungi from the lineages that gave rise to green plants, red algae, and various other eukaryotic groups. The genetic sequences encoding essential cellular machinery, including ribosomal RNA and structural proteins, show far greater similarity between a human and a baker's yeast than between that yeast and an oak tree.
The ancestral link is also preserved in microscopic morphology. The defining characteristic of opisthokonts is the presence of a single, posterior flagellum on motile cells—a structure that propels the cell from behind like a ship's propeller, rather than pulling it from the front like the flagella found in many plant-like protists. While most complex fungi have lost flagella over evolutionary time, primitive lineages such as chytrids still produce swimming spores with this distinctive posterior tail, mirroring the swimming mechanism of animal sperm cells.
Chitin and Glycogen: Shared Biochemical Signatures
The biochemical building blocks of fungi provide striking evidence of their kinship with animals. The cell walls of fungi are primarily reinforced by chitin, a tough nitrogenous polysaccharide. In the animal kingdom, chitin is the primary structural component of the hard exoskeletons of insects, spiders, and crustaceans. True plants do not synthesize chitin; they construct their cell walls using cellulose and pectin. The presence of chitin across both fungal walls and arthropod shells reflects shared metabolic pathways for synthesizing complex amino sugars.
Another critical shared trait is the method of energy storage. When plants accumulate excess carbohydrates through photosynthesis, they store that energy as starch. Fungi and animals, by contrast, store surplus energy as glycogen, a highly branched polymer of glucose. In human bodies, glycogen is held in the liver and muscles for rapid mobilization, and fungal cells rely on the exact same chemical reserve to fuel their growth and spore production. Furthermore, fungal cell membranes contain ergosterol, a steroid molecule functionally comparable to the cholesterol found in animal cell membranes, distinct from the phytosterols typical of plants.
Absorptive Heterotrophy Versus Photosynthesis
Plants are autotrophs, manufacturing their own food directly from sunlight, carbon dioxide, and water using chloroplasts. Fungi, like animals, are obligate heterotrophs, meaning they must acquire pre-formed organic carbon from their environment to survive. However, while most animals ingest food and digest it internally within specialized cavities or digestive tracts, fungi feed through a process known as absorptive heterotrophy or osmotrophy.
A fungal mycelium—a vast underground network of microscopic, thread-like filaments called hyphae—grows directly into and through food sources. These hyphae secrete powerful digestive enzymes into the surrounding substrate, breaking down complex polymers such as lignin, cellulose, and proteins into simple sugars, amino acids, and minerals outside the organism. The hyphae then absorb these dissolved nutrients across their cell membranes. In effect, fungi digest their environment externally before taking it inside, performing the exact chemical breakdown that animal stomachs accomplish internally.
Why the Evolutionary Connection Matters in Medicine
The close evolutionary proximity between fungi and animals creates a major challenge in modern human medicine. When physicians treat bacterial infections, antibiotics can easily exploit the radical differences between bacterial cells (which are prokaryotes) and human cells (which are eukaryotes). Antibiotics target bacterial cell walls, distinct bacterial ribosomes, or unique metabolic pathways without causing catastrophic toxicity to the patient.
Because fungi and humans are both eukaryotic opisthokonts, they share fundamental cellular machinery, including similar ribosomes, DNA replication processes, protein synthesis pathways, and organelle structures. Developing antifungal drugs that kill pathogenic fungi without harming human tissues is exceptionally difficult. Most effective antifungal medications must target the few remaining biochemical differences—such as the synthesis of ergosterol in fungal membranes or chitin in their cell walls—because disrupting broader fungal metabolic processes frequently produces severe toxic side effects in human patients.
Ecological Partnerships and Evolutionary Distinctions
Despite their closer genetic relationship to animals, fungi maintain an ancient, indispensable ecological relationship with plants. Fungi act as primary decomposers in terrestrial ecosystems, breaking down dead plant matter that few other organisms can digest and recycling vital nutrients back into the soil. Many plants depend on mycorrhizal fungi that wrap around or penetrate their root systems, swapping soil-derived phosphorus and nitrogen for plant-manufactured sugars.
Distinguishing fungi from plants has also cleared up long-standing confusion surrounding other organisms. Groups such as oomycetes (water molds) and myxomycetes (slime molds) were once grouped with fungi due to similar growth forms and spore production. Genetic sequencing has demonstrated that water molds are more closely related to brown algae and diatoms, while slime molds represent entirely different amoebozoan lineages. True fungi stand alone as a unified kingdom within the opisthokonts, occupying a unique evolutionary space that bridges microscopic unicellular ancestors with the complex multi-tissued branches of animal life.
Key takeaways
•Fungi and animals belong to the evolutionary clade Opisthokonta, sharing a common ancestor that diverged from the plant lineage.
•Unlike plants, fungi do not photosynthesize; they are heterotrophs that store energy as glycogen and build structural walls with chitin, the same compound found in arthropod exoskeletons.
•Because fungal cells share extensive cellular and genetic machinery with animal cells, developing antifungal drugs that do not harm human hosts is far more difficult than developing antibacterial treatments.
•Primitive fungi retain cellular features like a single posterior flagellum on swimming spores, directly mirroring the motility mechanism of animal sperm cells.