Fungi exist as single celled yeasts or multicellular structures made of filamentous tubes or 'hyphae', both with cell walls composed of glucose polymers (most commonly chitin). Although as a groupo fungi are distinct from plants and animals, molecular evidence strongly supports quite a close relatiopnsihp to the animals (planst are more distant). This divergence occured at least 900 Ma ago, while plant-associated forms (Glomales) have been found in the Ordovician (460 Ma). Fungi gain their nutrition from surrounding plant or animal tissue, principally through exporting hydrolytic enzymes and absorbing the resulting simple molecules. Fungal reproduction occurs by dispersal and germination of mitotic (clonal) or meiotic (sexual) spores, mating partners finding each other through oligopeptide or complex organic pheromones. Interestingly, several fungi have independently evolved explosive mechanisms for launching spores (at an acceleration that would flatten a human!), whereas others rely on wind or water for dispersal.

The classification of fungi has undergone some radical shifts as new molecular data have appeared in recent years. Four major divisions of fungi are now recognised, including ascomycetes, basidiomycetes, zygomycetes and the less diverse chytridomycetes. Within the basidiomycetes we find the largest living organism on Earth, namely Armillaria (honey-fungus), where a single clonal individual may occupy almost 1000 hectares and be over 8,500 years old. The chytrids are the most primitive fungi, retaining a flagellum and also possessing rhodopsin as a sensory molecule. An important subsidiary group is the 'Glomeromycota', which is very closely related to the zygomycetes and includes the mycorrhizal fungus Glomus and its allies. Of significance regarding convergence, certain very fungus-like groups apparently have protistan origins, including water moulds (e.g. late potato blight, Phytophthora infestans), cellular slime moulds (e.g. the intriguing 'social' microbe Dictyostelium), plasmodial slime moulds and parasitic Microsporidia.

Virtually all types of fungi form associations, both symbiotic and pathogenic with plants and animals, among which we find myriad examples of convergence. Mutualistic associations between fungi and plants have been important since plants moved to land, especially as arbuscular mycorrhizal (AM) fungi in plant roots or endophytes in the main plant body. Disastrous fungal plant pathogens are rife, and include the famous Dutch Elm disease, potato blight and wheat rust. Around 25% of fungal species form lichens, which are symbiotic associations with cyanobacteria or green algae, and here too convergences are frequent. An increasing number of animal-fungus associations are known, including fungi in the guts and ectodermal 'pockets' of various unrelated insects and crustacea, cultivation of fungi by ants, termites and ambrosia beetles for nourishment, and by scale bugs for protection. Humans suffer from numerous fungal pathogens, for example athlete's foot, ringworm, dandruff and in those with weakened immune systems, infections by Aspergillus fumigatus and Candida albicans can be very dangerous. Fungi are also of immense benefit to humans, as domesticated yeasts are used in making bread, beer and wine, Penicillium in certain cheeses and the life-saving antibiotic penicillin, and several fungi are genetic 'model' organisms (e.g. budding yeast Saccharomyces cerevisiae, fission yeast Shizosaccharomyces pombe, and filamentous Neurospora crassa).