Fungi are closer to animals than to plants

For most of the history of science, fungi were placed alongside plants. They do not move about like animals, they grow out of a substrate and they have cell walls, so the resemblance seemed natural. Only comparisons of cell structure, of modes of nutrition, and then of gene sequences showed that the resemblance is superficial. Fungi and animals belong to a great branch of eukaryotes called the Opisthokonta, and share a common ancestor more recent than the common ancestor of either with plants.

This does not mean a fungus is "almost an animal". Hundreds of millions of years have passed since the lineages separated, and the two kingdoms developed entirely different organisms. Relatedness in biology describes the order of branching on the tree of life, not similarity of everyday appearance. The closest single-celled relatives of animals are the choanoflagellates, while at the edge of the fungal lineage stand, among others, small aquatic organisms. The shared group is wider than the two familiar kingdoms.

Several characters show the difference from plants clearly. Fungi have no chlorophyll and do not produce sugars by photosynthesis. They secrete enzymes into their surroundings, break material down outside the cell and then absorb the dissolved compounds. Their cell walls contain chitin, whereas plant walls rest above all on cellulose. As an energy store, fungi and animals accumulate glycogen rather than the starch typical of plants.

This knowledge has practical consequences. Drugs aimed at fungal cells have to exploit differences large enough not to damage human cells, which is often harder than fighting bacteria. In ecology, meanwhile, fungi cannot be treated as passive "vegetation": they are consumers and decomposers, without which the cycling of carbon and mineral nutrients would stop working. Phylogenetic trees are still being refined as new genomes arrive, but the position of animals and fungi on a shared branch does not rest on a single character or a single study.

What the evidence actually shows

This relationship does not mean fungi “descended from animals.” Analyses of thousands of genes place both kingdoms within Opisthokonta: fungi and animals share a more recent common ancestor with each other than either shares with plants. The two branches have evolved independently since that ancient split. Similarities such as glycogen storage, or chitin in fungal cell walls and arthropod exoskeletons, are interesting but do not by themselves resolve ancestry; the strongest evidence comes from multi-gene phylogenies with broad taxon sampling. Fungi remain a distinct kingdom. They are generally osmotrophs, secreting enzymes outside their bodies and absorbing the resulting small molecules, whereas animals usually ingest food. “Closer” is therefore a precise statement about the branching pattern of common ancestry, not a claim that fungi are animals or that one modern kingdom evolved from the other.