The natural world is often seen as neatly divided into animals, plants, and fungi. While these groups appear distinct, their evolutionary relationships reveal a surprising connection that challenges traditional assumptions. This prompts the question: is an animal or a plant more closely related to a fungus?
Understanding Life’s Major Groups
Scientists categorize life into broad groups, known as kingdoms, based on shared characteristics. Among eukaryotes, organisms with cells containing a nucleus and other membrane-bound organelles, three kingdoms are well-known: Animalia, Plantae, and Fungi. Animals are typically multicellular, obtaining nutrients by ingesting other organisms, and most can move. Plants are primarily multicellular, generating food through photosynthesis, and usually remain rooted. Fungi, which can be unicellular or multicellular, acquire nutrients by absorbing dissolved organic molecules from their environment, often by secreting digestive enzymes externally.
Shared Characteristics Linking Fungi and Animals
Fungi and animals share several biological similarities. Both are heterotrophs, meaning they cannot produce their own food and must obtain nutrients from external sources. Animals typically ingest their food, while fungi secrete enzymes to digest organic matter externally before absorbing the resulting smaller molecules. This shared reliance on external food sources distinguishes them from photosynthetic plants.
Fungi possess cell walls primarily composed of chitin, a complex carbohydrate. Chitin is also a major component of the exoskeletons of many animals, such as insects and crustaceans, providing structural support. Animal cells, in contrast, do not have cell walls, allowing for greater flexibility and specialized cell shapes.
Furthermore, both fungi and animals store their energy reserves in the form of glycogen, a branched polysaccharide. Glycogen serves as a readily accessible energy storage molecule in both groups. These shared traits — nutritional mode, cell wall chemistry, and energy storage compound — indicate a closer evolutionary partnership between fungi and animals.
Why Plants Stand Apart
Plants have characteristics that set them apart from both animals and fungi, illustrating their distinct evolutionary path. Plants are autotrophic, meaning they produce their own food through photosynthesis. This process utilizes sunlight, water, and carbon dioxide to synthesize organic compounds, facilitated by chloroplasts containing pigments like chlorophyll.
Plant cells also have rigid cell walls made primarily of cellulose, which provides structural support and protection. Unlike animals and fungi, plants store their energy as starch, another complex carbohydrate. These attributes underscore the biological divergence of plants from the animal and fungal lineages.
Genetic Evidence for Relatedness
Modern understanding of evolutionary relationships relies on molecular evidence, particularly comparisons of DNA and protein sequences. Genetic analysis supports the close kinship between animals and fungi. Scientists compare genetic codes and protein structures to trace ancestry. Molecular phylogenies consistently demonstrate that fungi and animals share a more recent common ancestor than either does with plants.
Specific genetic markers reinforce this relationship. For instance, a unique 12-amino acid insertion in the translation elongation factor 1 alpha protein is found in both animal and fungal genes, but not in plants or other eukaryotes. Similar shared insertions or deletions in other proteins, such as enolase, also point to a common evolutionary history. This cumulative genetic evidence, derived from comparing numerous genes and proteins, establishes fungi and animals as sister groups that diverged from a common ancestor after their lineage separated from that of plants.