Many people consider plants, with their visible structures and green leaves, to be more akin to humans than fungi. However, this common perception differs from scientific understanding. Humans and other animals share a more recent common ancestor with fungi than with plants, placing them closer on the tree of life. This connection is revealed through shared biological characteristics and genetic evidence.
Unpacking the Evolutionary Tree
The vast diversity of life on Earth is organized into a hierarchical system of classification, beginning with three fundamental domains: Bacteria, Archaea, and Eukarya. Humans, fungi, and plants all belong to the domain Eukarya, meaning their cells possess a nucleus and other membrane-bound organelles. Within Eukarya, life further branches into various kingdoms, including Animalia (animals), Fungi, and Plantae (plants).
The “Tree of Life” illustrates evolutionary relationships among all living organisms, with branching points representing common ancestors. Animals and fungi are grouped together in a supergroup called Opisthokonta. This classification signifies that animals and fungi share a more recent common ancestor with each other than either group does with plants. The name Opisthokonta refers to a distinctive feature: a single posterior flagellum found in the motile cells of many organisms within this group, such as animal sperm and certain fungal spores.
Shared Biological Heritage
The close evolutionary ties between animals and fungi are supported by several biological and genetic similarities. Both animals and fungi are heterotrophs, meaning they obtain nutrients by consuming organic matter, a stark contrast to plants that produce their own food through photosynthesis. Animals ingest their food, while fungi absorb nutrients from their surroundings after secreting digestive enzymes. This mode of nutrition sets them apart from the autotrophic nature of plants.
Molecular comparisons provide further evidence of this shared ancestry. Genetic analyses have revealed specific genes and molecular pathways that are more similar between animals and fungi than between either and plants. For example, studies comparing proteins like actin and tubulin, and enzymes like enolase, show that animals and fungi form a monophyletic group, meaning they share a common ancestor not shared with plants. These genetic resemblances extend to cellular machinery, including certain enzyme pathways and the structure and function of mitochondria.
Another shared characteristic involves chitin. While fungi use chitin as a primary component of their rigid cell walls, animals do not possess cell walls. Chitin is found in various animal structures, such as the exoskeletons of insects and crustaceans. The biochemical pathways for chitin synthesis are also present in both animals and fungi. Additionally, both animals and fungi lack chloroplasts, the organelles responsible for photosynthesis in plants. This absence is a direct consequence of their heterotrophic lifestyle, highlighting their divergence from the plant lineage.
Divergent Paths and Unique Adaptations
Despite their shared evolutionary heritage, humans and fungi exhibit significant differences, due to millions of years of independent evolution and adaptation to distinct ecological niches. A primary structural difference lies in their cellular architecture. Fungi possess rigid cell walls primarily composed of chitin, which provides structural support and protection. Animal cells, in contrast, lack cell walls, allowing for greater flexibility, diverse cell shapes, and the development of complex tissues and organs.
Their modes of life also vary significantly. Animals are mobile organisms that ingest food, leading to the evolution of complex nervous systems and muscular structures. Fungi, on the other hand, are sessile, acquiring nutrients by secreting enzymes externally and absorbing the digested material. This absorptive heterotrophy has led to the development of filamentous growth forms, known as hyphae, which maximize surface area for nutrient uptake.
The development of multicellularity has also taken different paths. Animals evolved complex multicellularity with specialized tissues, organs, and organ systems, enabling intricate body plans and physiological functions. Fungi, while also multicellular in many forms, develop filamentous structures or exist as single-celled yeasts. Their multicellularity often involves a network of hyphae rather than the highly differentiated tissues seen in animals. These divergent adaptations reflect the distinct ecological roles animals and fungi play, from consumers and predators to decomposers and symbionts.