A mushroom looks more like a plant than a person, but fungi are closer relatives of animals than of plants.1Science. World’s oldest fungi, found in fossils, may rewrite Earth’s history That family resemblance is mostly hidden inside cells, in shared building materials and chemistry, and it has real consequences for medicine.
Where Fungi and Animals Sit on the Tree of Life
Scientists place fungi and animals in a single large group of complex-celled life called the opisthokonts.2PubMed. Phylogenomics Reveals Convergent Evolution of Lifestyles in Close Relatives of Animals and Fungi The group divides into two main lines: animals with their single-celled relatives, and fungi with theirs.3PubMed. Phylogenomics Reveals Convergent Evolution of Lifestyles in Close Relatives of Animals and Fungi Despite sharing that branch, the two ended up with radically different body forms.4PubMed Central. Divergent genomic trajectories predate the origin of animals and fungi
The name refers to a single tail at the back of swimming cells, like a sperm cell. Animal sperm swim with one rear tail, and some fungal cells do too. This shared swimming design was one early clue linking the two groups. Plants’ swimming cells, where they exist at all, are built differently.
Plants split from the shared line earlier, so they are more distant cousins to both fungi and animals. When classification books once grouped mushrooms with plants, they were going by looks rather than by ancestry. But relatedness is measured by shared ancestry recorded in DNA. DNA comparisons settled the fungus question decisively in favor of animals.
To build such a tree, researchers compare the same genes across many species and group together those whose sequences differ least, because more similar sequences usually mean a more recent shared ancestor. Genome comparisons back this up with many genes at once, so the animal-fungus link does not rest on one gene.
Being a closer relative means a more recent shared ancestor, not that fungi are partly animal. Fungi are not evolving into animals, or the reverse. Both are equally modern lineages that kept changing after the split.
The Single-Celled Ancestors Behind the Split
Each side has a living single-celled cousin. Animals’ closest living relatives are choanoflagellates, tiny water-dwelling cells that live alone or in small colonies, so the first animals probably came from creatures much like them.5The Biology Primer. The Origin and Evolution of Fungi Fungi’s closest relatives are nucleariids, a group of small amoebae.6Wikipedia. Nucleariid
A choanoflagellate is a single cell with one whipping tail ringed by a collar of tiny projections. The tail stirs the water so bacteria get caught on the collar and become food. Very similar collared cells line sponges, among the simplest animals. Some species clump into colonies, a glimpse of how many-celled life may have begun.
The genetic paths diverged too. The line leading to fungi tended to lose genes tied to building many-celled bodies, while the line leading to animals gathered more of them.7El·lipse. New light into the evolutionary separation of animals and fungi – El·lipse
Fungi branched off from animals more than 1 billion years ago.8Science. World’s oldest fungi, found in fossils, may rewrite Earth’s history Because they are sister groups, the split had to come before the first animals, which appear in fossils from roughly 570 million years ago.9Wikipedia. Evolution of fungi Possible fungal fossils from the Congo date to between 715 and 810 million years ago, when land was likely bare except for bacterial films.10Science. World’s oldest fungi, found in fossils, may rewrite Earth’s history But the oldest undisputed fungal fossils are about 400 million years old, so much of the early story is a gap.11Science. World’s oldest fungi, found in fossils, may rewrite Earth’s history
The gap exists because early fungi were soft, microscopic threads that rarely fossilize. Dating relies partly on comparing DNA to estimate split times. Disputed fossils need chemical tests to confirm they are fungal and not other microbes.
Nobody has a fossil of the shared ancestor, a single-celled organism, so any picture of it is a reconstruction from its living descendants. Today’s choanoflagellates and nucleariids are cousins, not the ancestor itself, since they have also kept changing for a billion years. Features found in both modern lines are the best guide to what the ancestor likely had, while features found in only one side may have been added later.
Shared Building Materials and Chemistry
Chitin is a tough sugar-based material that forms fungal cell walls and the hard shells of insects and crabs.12Wikipedia. Chitin It is built from chains of a sugar unit bundled into fibers of about 20 chains each.13PubMed. The cellular basis of chitin synthesis in fungi and insects: common principles and differences Fungi and insects make chitin with largely the same cellular machinery.14PubMed. The cellular basis of chitin synthesis in fungi and insects: common principles and differences
Chitin is why a crab shell is hard and a mushroom cap holds its shape when you slice it. In insects and crustaceans it is often combined with other materials, so shells range from flexible to rigid, while in fungi it sits in the wall around each cell.
Plants build their cell walls mainly from cellulose instead. Both are long chains of sugar units, but chitin’s units carry an extra nitrogen-containing group that cellulose lacks. That group lets neighboring chains bond more strongly, which helps make chitin tough and shell-like.
Animals and fungi both store energy as glycogen, while plants store it as starch.15PubMed. Structural evolution and functional adaptation of energy-storage polysaccharides in animals, plants and fungi Your liver and muscles keep glycogen as quick-release fuel, and baker’s yeast stockpiles it when food is plentiful. Starch in bread and rice is the plant equivalent your body breaks down.
Cell membranes are similar but not identical. Animal membranes use cholesterol, while fungal membranes use a close chemical cousin called ergosterol. Ergosterol has three double bonds versus cholesterol’s one, which makes it stiffer and more reactive with oxygen.16Biochemistry (Moscow), Supplement Series A: Membrane and Cell Biology. Evolutionary Choice between Cholesterol and Ergosterol
How Fungi and Animals Differ
Fungi cannot swallow food like animals or make food from sunlight like plants.17University of California Museum of Paleontology. Fungi: Life History and Ecology Instead, they release digestive chemicals and absorb nutrients from their surroundings.18University of California Museum of Paleontology. Fungi: Life History and Ecology
On a fallen log, a fungus releases enzymes into the wood, which break it down into small molecules the fungus then takes up through its surface. Threadlike growth gives it a very large surface area relative to its volume, which suits absorbing food from outside. That is why fungi are so good at decomposing leaves and dead wood, and why bread or fruit left out gets moldy.
The mushroom itself is only the fruiting part, like an apple on a tree. The main body is a hidden web of threads in soil or wood, spreading and absorbing food over large areas. Most fungal bodies are networks of thin threads rather than organs, so a fungus has no heart, brain, or stomach. Animals typically develop a body that moves and senses its surroundings, while fungi grow toward food and spread through it.
Mushrooms are complex many-celled structures that evolved independently of animal bodies.19PubMed Central. Transcriptomic atlas of mushroom development reveals conserved genes behind complex multicellularity in fungi Even so, genes controlling growth and signaling expanded in mushroom-forming fungi much as they did in plants and animals.20PubMed Central. Transcriptomic atlas of mushroom development reveals conserved genes behind complex multicellularity in fungi This is convergent evolution: unrelated lineages reach similar solutions to the same problem, as bird and bat wings evolved separately yet both fly.
Some fungi blur the line. Chytrids, an early branching group, show traits found in both animals and fungi,21Current Biology / Elsevier. Chytrid fungi such as swimming spores with a single tail.
Why the Kinship Makes Fungal Infections Hard to Treat
Because fungal and human cells are built so similarly, many chemicals that harm fungi also harm people. That is why there are far fewer antifungal drugs than antibiotics for bacteria. Bacteria are far more distant from us, with cell walls and ribosomes quite different from ours, so many antibiotics can strike those differences without hurting us. Fungal cells are complex cells like ours, with a nucleus and similar internal machinery, so fewer features are unique to them. Antibiotics do nothing against a fungal infection, and doctors need a different kind of drug.
Ergosterol is one key difference, and several common antifungals work by attacking it or blocking its production. Some stop the fungus from making ergosterol, so its membranes end up faulty and leaky. Others bind to ergosterol directly and punch holes in the membrane. Human cells use cholesterol instead, so these drugs mostly spare our cells, though the similarity means they do not spare them entirely. Human cells have no cell wall at all, which makes the fungal wall another target, and another class of drugs goes after it.
Athlete’s foot, ringworm, nail fungus, and vaginal yeast infections (caused by Candida) are everyday examples. Many creams for these list drugs that target ergosterol.
Side effects arise partly because the drugs touch processes we share with fungi. Some oral antifungals affect the liver and need monitoring, so doctors may choose creams first to limit whole-body exposure. Some antifungals interact with other medicines, so mention everything you take.
Disease-causing fungi can quickly pile up genetic changes.22PubMed. Complexity and dynamics of host-fungal interactions That means they can dodge the immune system, become more harmful, and resist antifungal drugs.23PubMed. Complexity and dynamics of host-fungal interactions So finishing a prescribed antifungal course matters even after symptoms ease.
Fungi Living Inside and Alongside Animals
Fungi in plant-eating mammals’ guts diversified alongside their hosts as those mammals evolved, including cattle’s four-chambered stomachs.24PubMed Central. Patterns and determinants of the global herbivorous mycobiome In humans and other great apes, some gut fungi follow the family tree of their hosts.25PubMed Central. Gut fungal profiles reveal phylosymbiosis and codiversification across humans and nonhuman primates Gut fungi are also key players in the networks of microbes living inside animals.26bioRxiv. Fungal microbiomes are determined by host phylogeny and exhibit widespread associations with the bacterial microbiome
Your own body hosts a small fungal community on the gut and skin, alongside bacteria. Most are harmless and kept in balance by other microbes. Antibiotics can upset that balance and let yeast overgrow.
Fungus farming evolved separately in ants, termites, and several times in ambrosia beetles, and these farmers work in strikingly similar ways despite evolving apart.27Annual Review of Ecology, Evolution, and Systematics. The Evolution of Agriculture in Insects Leafcutter ants cut leaves not to eat but to feed a fungus garden underground. Worker ants carry fresh pieces down, where other workers chew them and add them to the fungus bed as growing material. The fungus converts the leaves into food rich enough for the colony, and the ants eat parts of it, tending it like a crop. They also weed out invading molds, remove waste, and spot infections. The fungus depends on the ants for food, protection, and spread.
Aphids carry genes for making colorful pigments that came from fungi, so red aphids get their color from a fungus-derived gene that green aphids lack.28PubMed. Lateral transfer of genes from fungi underlies carotenoid production in aphids Gall midges gained similar genes, which may have helped them feed on plants.29PubMed. Carotenoids in unexpected places: gall midges, lateral gene transfer, and carotenoid biosynthesis in animals
Genes normally pass from parents to offspring, so a jump between species requires an unusual event. A fungus and an insect in close contact, such as inside a plant or an insect’s body, give such an event a chance. For the gene to persist, it must end up in cells that make eggs or sperm, so descendants inherit it. If it then helps the animal survive, it can spread through the population.