Fungi are more closely related to animals than to plants. This conclusion, now firmly established in biology, was one of the bigger surprises of late-twentieth-century molecular science. For centuries, fungi were classified alongside plants because they grow in soil, stay rooted in place, and seem to sprout like vegetation. But when researchers compared the proteins and genes of fungi, animals, and plants, the data consistently showed that fungi and animals share a more recent common ancestor, placing them on the same branch of the tree of life while plants sit on their own separate limb.
How We Know Fungi and Animals Are Siblings
The strongest evidence comes from molecular phylogenetics, which uses protein and DNA sequences to reconstruct evolutionary relationships. A landmark study published in the Proceedings of the National Academy of Sciences compared four separate proteins across animals, fungi, plants, and a broad range of single-celled organisms. All four proteins placed animals and fungi together as a single group, excluding plants and protists entirely. The researchers concluded that animals and fungi are sister groups, while plants form an independent evolutionary lineage.1PubMed. Animals and fungi are each other’s closest relatives: congruent evidence from multiple proteins
That finding has been replicated and reinforced by decades of subsequent work using larger gene sets, whole-genome comparisons, and increasingly sophisticated computational methods. The conclusion has not wavered. The supergroup that contains both animals and fungi is called Opisthokonta, a name that hints at what unites them biologically.
The Opisthokont Ancestor
The name Opisthokonta comes from Greek words meaning “rear pole,” and it refers to the position of a whip-like flagellum on the cell. The common ancestor of animals and fungi was a single-celled organism that had a flagellum at the back of its body, pushing the cell forward through water. You can still see this arrangement today in animal sperm cells, where the tail propels the cell from behind.2Current Biology. Divergent trajectories predate the origins of animals and fungi
Most fungi have since lost their flagellum altogether. But a few ancient fungal lineages called chytrids still produce swimming cells with rear-mounted flagella, giving us a direct window into what the shared ancestor looked like.3Current Biology. Chytrids: an evolutionary primer Plants, by contrast, never had this rear-flagellum arrangement. When plant cells have flagella (as in some sperm cells of mosses and ferns), the structure is organized differently. This shared ancestral feature is one of many molecular and structural markers that group fungi with animals and away from plants.
When Did the Split Happen
The three-way parting of plants, animals, and fungi took place deep in the Proterozoic, long before anything we would recognize as a plant or animal existed. A study estimating divergence times using molecular clocks placed the split at roughly 1.6 billion years ago, with a margin of uncertainty of about 88 million years on either side.4PubMed Central. Divergence time estimates for the early history of animal phyla and the origin of plants, animals and fungi Plants branched off first from the lineage that would go on to produce both animals and fungi. After plants departed, the animal and fungal lines continued as one for some further stretch of evolutionary time before eventually diverging from each other. That is what makes animals and fungi closer relatives: they shared an exclusive common ancestor that plants never belonged to.
To put this in perspective, the split between fungi and animals happened hundreds of millions of years before the oldest known animal fossils. For over a billion years after these lineages diverged, they were all still single-celled or colonial organisms living in oceans. The complex multicellularity we associate with mushrooms, oak trees, and tigers evolved independently in each kingdom.
Biochemical Fingerprints Shared by Fungi and Animals
The kinship between fungi and animals is not just an abstraction read off genetic sequences. It shows up in how their cells actually work. Several biochemical traits link fungi and animals while distinguishing them from plants.
- Energy storage: Animals and fungi both store energy as glycogen, a highly branched sugar molecule that allows rapid energy release. Plants store energy as starch (amylopectin and amylose), which has a different branching pattern and crystallinity. Fungal glycogen sits in an intermediate zone between animal glycogen and plant starch in terms of chain length, but its high proportion of short chains makes it well suited for quick mobilization of fuel.5Elsevier (Carbohydrate Polymers). Structural evolution and functional adaptation of energy-storage polysaccharides in animals, plants and fungi
- Sterol synthesis: Animals and fungi both build their membrane sterols starting from the same precursor molecule, lanosterol. In animals the end product is cholesterol; in most fungi it is ergosterol. But the early enzymatic steps are shared. Plants take a different route entirely, starting from a different cyclic intermediate called cycloartenol.6PLoS One. Phylogenetic Distribution of Fungal Sterols This divergence in sterol chemistry is part of why antifungal drugs can be tricky to design: the biochemical machinery they target in fungi can resemble the machinery in our own cells.
- Lysine production: Fungi synthesize the amino acid lysine through a unique biochemical route called the alpha-aminoadipate pathway. This pathway is found only in fungi and is not shared with plants or animals. Because animals cannot make lysine at all (we must get it from food), this pathway has attracted attention as a potential target for antifungal drugs: block it, and you starve the fungus of a building block without affecting human biochemistry.7PubMed. The alpha-aminoadipate pathway for lysine biosynthesis in fungi
The lysine pathway also illustrates why “more related to animals” does not mean “basically the same as animals.” Fungi have evolved their own distinctive metabolic toolkit over a billion-plus years of independent evolution. The kinship is about shared ancestry, not shared lifestyles.
Why Textbooks Used to Get This Wrong
Before molecular tools were available, biologists classified organisms primarily by how they looked and where they lived. By those criteria, fungi seem plant-like. They grow in soil. They have rigid cell walls. They do not run away when you poke them. Many fungi produce branching, root-like structures. For centuries, fungi were housed in the plant kingdom, and botany departments traditionally studied them alongside mosses and ferns.
But these surface similarities are misleading. Fungal cell walls are made of chitin, the same tough polysaccharide that forms insect exoskeletons and crustacean shells. Plant cell walls are made of cellulose, a completely different molecule. Fungi do not photosynthesize. They obtain food by secreting enzymes that digest organic matter externally and then absorbing the nutrients, a feeding strategy more comparable to an animal’s digestive system turned inside out than to a plant soaking up sunlight. The sessile, rooted-in-place appearance of fungi is a case of convergent evolution: fungi and plants arrived at similar growth habits for different reasons, not because they inherited them from a common ancestor.
Even how fungi divide their cells differs from plants in instructive ways. During cell division, many fungi use a structure called the spindle pole body to organize their chromosomes. This structure has no physical resemblance to the centrosome that animal cells use for the same purpose, yet the two share a small set of conserved molecular components that reflect their common ancestry. Plants use yet another arrangement. The spindle pole body is a reminder that deep homology can persist even when surface structures have diverged beyond recognition.8Current Biology. Spindle pole body
Fungi and Plants as Ancient Partners
If fungi and plants are not close relatives, how did they end up so intimately intertwined in nature? The answer is symbiosis rather than shared ancestry. The relationship between fungi and plant roots, known as mycorrhiza, is one of the oldest and most consequential partnerships on land. Genetic evidence suggests that the genes required for this symbiosis were already present in the common ancestor of all land plants, meaning the partnership likely existed from the very moment plants began colonizing land roughly 450 to 500 million years ago.9PubMed. Presence of three mycorrhizal genes in the common ancestor of land plants suggests a key role of mycorrhizas in the colonization of land by plants
In this arrangement, fungi extend vast networks of thread-like filaments through the soil, vastly increasing the effective surface area through which a plant can absorb water and mineral nutrients like phosphorus. In return, the plant supplies the fungus with sugars produced through photosynthesis. Some researchers argue that plants could not have survived on dry land at all without fungal partners to help them access soil nutrients during those earliest, harshest stages of terrestrial life.10PubMed Central. The dawn of symbiosis between plants and fungi
The identity of the original fungal partners is itself an active area of research. For decades, a group called Glomeromycota was assumed to be the ancient mycorrhizal fungi. But fossil and molecular evidence now shows that fungi from a different group, Mucoromycotina, are symbiotic with some of the most ancient living land plant lineages like liverworts, raising the possibility that Mucoromycotina were the original partners and Glomeromycota came later.10PubMed Central. The dawn of symbiosis between plants and fungi Either way, the deep entanglement of fungi and plants in ecosystems is the product of a long ecological marriage, not a sign of recent shared ancestry.
Surprisingly Animal-Like Behaviors
Beyond shared chemistry, fungi occasionally exhibit behaviors that seem more at home in the animal kingdom than the plant kingdom. One striking example is electrical signaling. Researchers who inserted electrodes into the mycelium of oyster mushrooms recorded voltage spikes that resemble the action potentials neurons use to transmit information. Each spike showed the classical phases: a rapid rise in voltage, a return to baseline, and a refractory period during which the cell could not fire again.11Scientific Reports. On spiking behaviour of oyster fungi Pleurotus djamor No one is claiming mushrooms think the way animals do, but the presence of this signaling machinery suggests that electrical communication across cells is an ancient and broadly useful innovation, possibly inherited from the shared opisthokont ancestor.
Fungi also produce melanin, the same class of pigment responsible for skin and hair color in humans and other animals. In fungi, melanin serves as a shield against ultraviolet light, radiation, and chemical stress. Some fungi even synthesize melanin from L-DOPA, the same precursor molecule that mammalian skin cells use. Recent research has found that fungal melanin may be assembled in internal compartments that resemble the melanosomes found in mammalian cells, hinting at another piece of shared cellular machinery conserved since before the animal-fungal split.12PubMed Central. Synthesis and assembly of fungal melanin
Where Fungi Truly Stand Alone
Acknowledging that fungi are animals’ closest kingdom-level relatives can overcorrect the old myth that they are just weird plants. Fungi are neither. Over more than a billion years of independent evolution, they have developed a biological toolkit that belongs to them alone.
Their body plan is unique. Most fungi grow as a mycelium, a sprawling network of microscopic filaments called hyphae that can colonize enormous volumes of substrate. A single individual can span hectares underground. This growth form has no real counterpart in either the animal or plant kingdoms, though it superficially resembles root systems. Their reproductive strategies are also distinctive: many fungi can reproduce both sexually and asexually, and some species have thousands of mating types rather than the simple two-sex system animals use.
At the molecular level, fungi possess their own version of the RNA-silencing machinery that animals and plants both use to regulate genes. Fungi have Dicer and Argonaute proteins, the key players in this system, but the way they use them has quirks not seen in either kingdom. In the bread mold Neurospora crassa, for instance, some small RNA molecules depend on both Dicer and an Argonaute protein called QDE-2 for their production, while others bypass Dicer entirely. And unlike in plants, the Argonaute protein in Neurospora is needed to produce precursor molecules, not just to process them into their final form.13International Journal of Molecular Biology: Open Access. Biogenesis and function of microRNAS from plants, animals and fungi applicable in health and food biotechnology These differences in gene regulation reflect a long period of evolutionary tinkering within the fungal lineage.
What This Means for Medicine and Everyday Life
The close evolutionary relationship between fungi and animals has real practical consequences, especially in medicine. Because fungal cells share so much biochemistry with our own, designing drugs that kill fungi without harming human cells is genuinely difficult. Antibiotics work well against bacteria because bacterial cells are biochemically distant from ours, offering plenty of targets that human cells lack. Fungi offer far fewer such targets. The shared lanosterol-based sterol pathway, for instance, means that drugs disrupting fungal membrane synthesis can sometimes interfere with human cholesterol metabolism as a side effect.
The uniquely fungal lysine biosynthesis pathway is one of the rare points of clear biochemical divergence between fungi and animals, which is precisely why it has attracted interest as a drug target.7PubMed. The alpha-aminoadipate pathway for lysine biosynthesis in fungi If researchers can find compounds that block this pathway effectively, they could potentially produce antifungals with fewer side effects than current options. The difficulty of treating fungal infections in immunocompromised patients makes this more than an academic exercise.
In agriculture, understanding the fungal-animal kinship matters less, but the fungal-plant symbiosis matters enormously. Roughly 80 to 90 percent of land plant species form mycorrhizal partnerships. Disrupting those partnerships through overuse of fungicides, soil compaction, or intensive tillage can quietly undermine crop health even when the plants appear fine above ground. Recognizing that fungi are not plants but rather their indispensable ecological partners shifts how we think about soil management and sustainable farming. The fungi beneath a farmer’s field are not just decomposers clearing away dead matter; they are a living network actively feeding the plants above, a relationship that stretches back to the earliest days of terrestrial life.