Modern biology confirms a deep, shared evolutionary history between humans and mushrooms. While they appear vastly different, genetic and cellular evidence reveals that the animal and fungal kingdoms are more closely related to each other than either is to plants. This kinship means the last common ancestor we share with a mushroom lived far more recently than the ancestor we share with any tree or flower.
Placing Fungi and Animals on the Tree of Life
For centuries, fungi were mistakenly classified alongside plants because they are non-mobile and grow rooted in the soil. Modern science relies on cellular structure, biochemistry, and genetic analysis to map evolutionary relationships, categorizing organisms into three Domains: Bacteria, Archaea, and Eukarya.
The Eukarya Domain includes the four complex kingdoms: Animalia, Fungi, Plantae, and Protista. Fungi were officially separated from plants because they lack chloroplasts and do not perform photosynthesis. Plants are autotrophs, making their own food, while animals and fungi are heterotrophs, meaning they must consume or absorb organic carbon from their environment.
The Shared Evolutionary Group
The closer link between animals and fungi led to their grouping into the supergroup Opisthokonta. This name derives from a shared ancestral trait: a single, posterior flagellum found in motile cells. While most modern fungi and animals lack this structure, the sperm cells of most animals and the spores of certain primitive fungi (chytrids) still use a single flagellum for propulsion from the rear.
This characteristic distinguishes Opisthokonts from plants and other Eukaryotes, whose motile cells use flagella in an anterior position. Phylogenetic studies consistently show that animals and fungi form a monophyletic group, sharing a common ancestor not shared by plants. This ancestor likely lived about 1.1 billion years ago, diverging into the Holozoa lineage (animals) and the Holomycota lineage (fungi). The Fungi kingdom is considered the sister group to the Holozoa, confirming fungi are our closest non-animal relatives.
Molecular and Genetic Evidence of Kinship
The relationship is supported by specific molecular and genetic details shared between the two kingdoms.
Shared Biochemical Pathways
Animals and fungi both utilize the polysaccharide glycogen for energy storage, whereas plants store energy as starch. This shared biochemical pathway points to a common metabolic heritage. Furthermore, both groups share the use of chitin, a tough, nitrogen-containing polysaccharide. Fungi use chitin as a main component of their rigid cell walls for structural support. While animal cells lack cell walls, chitin is a component in the exoskeletons of arthropods, such as insects and crustaceans.
Unique Genetic Markers
Genetic comparisons provide compelling proof, revealing specific protein structures unique to the Opisthokonta group. For example, a particular 12-amino acid insertion is found in the gene for elongation factor 1-alpha (EF-1α), a protein involved in synthesis, in both animals and fungi, but not in plants. Both kingdoms also possess genes for tyrosine kinase receptors, proteins that play a role in cell-to-cell communication and growth control, a feature largely absent in other kingdoms.
Defining the Biological Divergence
Despite the deep evolutionary connection, the two kingdoms followed dramatically different paths, leading to the profound biological differences observed today. The primary divergence lies in their mode of acquiring nutrients.
Nutritional Strategy
Animals are ingestive heterotrophs; they consume food whole and digest it internally within a specialized digestive system. Fungi, conversely, are absorptive heterotrophs. They excrete powerful enzymes into their environment to break down organic matter externally, then absorb the resulting smaller molecules through their cell walls. This difference in feeding strategy led to significant changes in body plan: animals developed mobility to seek food, while fungi evolved a sessile, thread-like mycelium for broad absorption.
Cellular Architecture
The cellular structure also marks a clear division. Fungi retain rigid cell walls, primarily made of chitin, to maintain their fixed shape. Animal cells lack a cell wall, which allows for the flexibility and movement necessary for complex tissues, organs, and overall mobility.