Fungi and animals appear vastly different, yet evolutionary biology reveals a surprising connection between the two kingdoms. For a long time, fungi were mistakenly grouped with plants due to their stationary nature and presence of cell walls. Modern genetic analysis, however, has fundamentally reshaped this view of the tree of life. This research confirms that fungi are more closely related to animals than they are to plants. This unexpected kinship points to a shared ancestral organism that lived hundreds of millions of years ago.
Placing Fungi and Animals on the Tree of Life
The phylogenetic placement of Fungi and Animals establishes them as sister kingdoms within a large supergroup of eukaryotes known as the Opisthokonta. This classification, confirmed through extensive molecular study, signifies that they share a more recent common ancestor with each other than either does with plants or other eukaryotic lineages. The Opisthokonta is divided into two major branches: the Holomycota, which contains Fungi and their closest unicellular relatives like the nucleariids, and the Holozoa, which includes Animals and their relatives such as the choanoflagellates.
This arrangement excludes the plant kingdom, which belongs to a different major branch of eukaryotes. Opisthokonta is recognized as a monophyletic group—meaning it contains a common ancestor and all of its descendants. This means that the last organism that was an ancestor to all fungi and all animals was not an ancestor to plants. The evolutionary split between the Opisthokonta and the lineage leading to plants occurred much earlier.
Molecular Signatures of Kinship
The close evolutionary relationship between Fungi and Animals is supported by several distinct molecular and genetic characteristics that are absent in other kingdoms. One compelling piece of evidence is the presence of a unique 12-amino acid insertion within the gene for translation elongation factor 1 alpha (EF-1alpha). This specific genetic signature is found in virtually all animals and fungi, but not in plants or other major groups of eukaryotes, suggesting a shared inheritance from a single ancestor.
Both kingdoms also share specific structural features in their motile cells. Flagellated cells, such as the sperm of most animals and the spores of primitive chytrid fungi, propel themselves with a single flagellum located at the posterior end. This posterior position of the flagellum is a defining characteristic of the entire Opisthokonta supergroup, contrasting with other eukaryotes that often have anterior or multiple flagella.
Furthermore, both Fungi and Animals utilize the structural polymer chitin. In fungi, chitin forms the rigid component of the cell walls, providing structural support. While animals do not have cell walls, chitin is a component of the exoskeletons of arthropods, such as insects and crustaceans, demonstrating a shared biochemical pathway.
Shared metabolic pathways also highlight the kinship, including the specific triple-fusion of three enzymes involved in pyrimidine synthesis. This unusual combination of carbamoyl phosphate synthetase, dihydroorotase, and aspartate carbamoyltransferase is found in opisthokonts and distinguishes them from the plant lineage. These shared molecular and genetic traits provide strong, independent lines of evidence for the sister relationship between the two kingdoms.
The Last Common Ancestor
The last common ancestor of Fungi and Animals was likely a single-celled organism that existed in aquatic environments approximately 800 million to 1 billion years ago. This ancestor was a protist, meaning it was a simple eukaryotic organism, and it possessed the defining Opisthokonta characteristic: a single, posterior flagellum. This organism was capable of moving through water, a trait retained in the mobile gametes of animals and the spores of basal fungi.
The ancestor’s closest living relatives provide a model for its likely characteristics. Choanoflagellates are the closest extant relatives to animals, and nucleariids are the closest to fungi. Choanoflagellates are simple, colonial protists that structurally resemble the feeding cells of sponges. The ancestral organism was heterotrophic, meaning it obtained nutrients by consuming organic matter, a characteristic passed down to both its animal and fungal descendants.
The ancient organism likely utilized phagocytosis, the process of engulfing food particles, a method of nutrient acquisition still used by many single-celled protists. This ability to engulf food was later retained and refined in the animal lineage as ingestive feeding. The evolutionary trajectory from this single-celled ancestor involved a divergence: one branch moved toward complex multicellularity and ingestion, and the other toward a stationary, absorptive lifestyle.
Defining the Great Divergence
The transition from the common ancestor to the modern kingdoms involved a significant divergence in structure and nutritional strategy. The animal lineage (Holozoa) developed ingestive heterotrophy, consuming food and digesting it internally. This strategy required the evolution of complex organ systems, mobility, and the loss of rigid cell walls to allow for flexibility and complex tissue development.
Conversely, the fungal lineage (Holomycota) evolved absorptive heterotrophy. They excrete powerful digestive enzymes into the environment, which break down complex organic matter externally before the resulting smaller molecules are absorbed across the cell membrane. This mode of nutrition led to the development of a mycelial body plan, characterized by a network of stationary, filamentous cells called hyphae.
Structural differences also became pronounced. Fungi retained chitin, forming rigid, protective cell walls that anchor the organisms in place. Animals, in contrast, developed specialized tissues and complex intercellular junctions, enabling movement and behavioral complexity. The chemical composition of cell membranes also diverged, with animals using cholesterol as their primary membrane sterol, while fungi utilize ergosterol. These fundamental biological shifts cemented the separation of the two kingdoms from their shared, aquatic ancestor.