For much of history, fungi were grouped with plants because they appeared rooted in the ground and did not move like animals. This classification was based on superficial observation rather than biological relatedness. Modern biology, informed by microscopic and molecular evidence, now recognizes Fungi as a distinct kingdom of life. Fungi share a more recent common ancestor and many fundamental characteristics with the Animal Kingdom than they do with plants. This closer evolutionary link is evident in key differences, from how they obtain nutrients to the molecules they use for structure and energy storage.
The Key Difference: How Fungi Eat
The most fundamental characteristic linking fungi and animals is their mode of nutrition, known as heterotrophy. This means they must consume pre-existing organic carbon sources. Plants, by contrast, are autotrophs, producing their own food through photosynthesis using sunlight and inorganic compounds. This shared necessity to acquire nutrients from the environment marks a major split from the Plant Kingdom for both fungi and animals.
However, the specific methods of heterotrophy differ significantly. Animals are ingestive heterotrophs, taking food into their bodies, typically through a mouth, before breaking it down internally. Fungi, on the other hand, are absorptive heterotrophs. They remain fixed in their location and secrete powerful digestive enzymes, called exoenzymes, directly into their external environment.
These enzymes break down complex organic materials, such as decaying wood or dead organisms, into smaller, absorbable molecules. The fungus then absorbs these simple nutrients through its cell membranes, essentially performing digestion outside of its body. This external digestion and subsequent absorption is the defining nutritional trait of the Fungi kingdom, differentiating it from the internal digestion common to most animals.
Hidden Similarities: Structure and Energy Storage
The relationship between fungi and animals is reinforced at a cellular level through shared structural components and metabolic strategies. One example is the composition of their protective cell walls. Plant cells rely on cellulose, a polysaccharide that provides rigidity and support, as the primary component of their cell walls.
Fungal cell walls, conversely, are constructed mainly from chitin, a tough nitrogen-containing polysaccharide. Chitin is the same polymer that forms the hard, supportive exoskeletons of arthropods, including insects, crabs, and lobsters. The use of this durable molecule for structural support is a shared feature found in both Fungi and Animalia, but not in plants.
Another parallel is how both organisms manage their excess energy. Plants convert surplus glucose into starch, a relatively less-branched polysaccharide ideal for long-term storage in structures like roots and seeds. Both fungi and animals, however, store their excess glucose in the form of glycogen.
Glycogen is a highly-branched polysaccharide often referred to as “animal starch.” The extensive branching allows for rapid breakdown when energy is needed quickly, benefiting the metabolically active lifestyles of animals and the rapid growth phases of fungi. This shared metabolic compound highlights a common biochemical heritage.
Shared Ancestry: The Opisthokont Connection
The most conclusive evidence for the close biological relationship between fungi and animals comes from molecular data. Genetic sequencing has allowed scientists to trace the evolutionary history of life by comparing DNA, providing a clear map of ancestry. This research places Fungi and Animalia together within a supergroup of eukaryotes called the Opisthokonta.
The term Opisthokonta means “posterior pole,” referring to a characteristic feature of the group’s motile cells. The most primitive fungi, such as chytrids, and the sperm cells of nearly all animals, possess a single flagellum positioned at the rear that pushes the cell forward. This unique flagellar structure is a signature trait inherited from their last common ancestor, a trait not found in plants.
Phylogenetic analyses consistently show that the most recent common ancestor shared by fungi and animals is more recent than the common ancestor they share with plants. This molecular evidence effectively refutes the old botanical classification, confirming that animals and fungi form a true evolutionary unit.