When scientists classify living things, they use a structured system to organize the vast diversity of life on Earth. This system helps to understand the relationships between different organisms, from the smallest bacteria to the largest whales. At the highest and broadest level of this biological classification system is a category known as a “domain,” which groups organisms based on fundamental differences in their cellular structure.
The Three Domains of Life
The current classification system recognizes three overarching domains of life: Archaea, Bacteria, and Eukarya. This three-domain system was proposed by microbiologist Carl Woese in the late 1970s, based on ribosomal RNA gene sequences, which revealed evolutionary distinctions. The most fundamental difference among these domains lies in their cellular organization.
Archaea and Bacteria are both composed of prokaryotic cells, which are simpler in structure. These cells lack a true nucleus, meaning their genetic material is not enclosed within a membrane. They also do not contain other membrane-bound organelles found in eukaryotic cells. Examples include methanogens, a type of Archaea that produces methane, and Escherichia coli, a common bacterium found in the intestines.
In contrast, organisms within the Eukarya domain consist of eukaryotic cells. These cells are characterized by a membrane-bound nucleus that houses genetic material. They also contain other membrane-bound organelles, such as mitochondria and endoplasmic reticulum, which perform specialized functions. Humans, trees, and mushrooms all belong to the Eukarya domain.
The Eukarya Domain and Its Kingdoms
Within the Eukarya domain, life is further organized into several major groups called kingdoms, representing the next level of biological classification. These kingdoms group eukaryotic organisms based on specific shared characteristics, such as their mode of nutrition and cellular organization.
Four major kingdoms are recognized within the Eukarya domain: Protista, Fungi, Plantae, and Animalia. Protista is a diverse kingdom encompassing single-celled or simple multicellular eukaryotes, like amoebas and algae. Fungi, such as mushrooms and yeasts, are typically multicellular organisms that obtain nutrients by absorbing organic material from their surroundings.
Plantae includes multicellular organisms that produce their own food through photosynthesis, like trees and ferns. The Animalia kingdom comprises multicellular organisms that move and obtain nutrients by consuming other organisms.
Defining Characteristics of the Animal Kingdom
Organisms classified within the Kingdom Animalia share several defining characteristics that distinguish them from other forms of life. A primary trait is multicellularity, meaning animals are composed of many cells organized into tissues, organs, and organ systems, even in the simplest forms like sponges. This complex organization allows for specialized functions within the organism.
Animals are also heterotrophic, meaning they obtain nutrients by ingesting other organisms or organic matter. Unlike plants that produce their own food, or fungi that absorb nutrients, animals consume their food, digest it internally, and assimilate usable components. This mode of nutrition drives many of their behavioral and anatomical adaptations.
Most animals exhibit motility at some life stage, meaning they can move independently. This movement can range from the slow crawling of a snail to the rapid flight of a bird, facilitating activities like foraging for food, escaping predators, or seeking mates. Even sessile animals, like corals, typically have a motile larval stage.
Animals lack cell walls, unlike the rigid cellulose cell walls found in plants and chitin cell walls in fungi. This lack of a cell wall contributes to the flexibility and diverse shapes observed in animal cells and tissues. Animal cells are instead supported by an extracellular matrix, composed of collagen.
Another unifying feature is their characteristic embryonic development, including the formation of a blastula. The blastula is a hollow ball of cells that forms early in embryonic development, unique among multicellular organisms. This developmental pathway is observed across a wide range of animals, from sea urchins to humans.
Distinguishing Animals from Other Eukaryotes
Comparing animals to other major eukaryotic kingdoms, such as plants and fungi, clarifies their definition. Modes of nutrition represent a divergence. Animals are ingestive heterotrophs, meaning they consume food internally, requiring specialized digestive systems.
In contrast, plants are autotrophs, producing their own food through photosynthesis using sunlight, water, and carbon dioxide. Fungi are absorptive heterotrophs, releasing digestive enzymes externally onto their food source and then absorbing the broken-down nutrients. This difference in energy acquisition shapes their biology.
Cellular structure also provides distinct points of comparison. Animal cells lack a rigid cell wall, giving them flexibility and allowing for complex cell-to-cell junctions that form diverse tissues. Plant cells, however, possess an outer cell wall made of cellulose, which provides structural support and protection. Fungi also have cell walls, but these are composed of chitin, a different polysaccharide.
Mobility is another distinguishing factor. While many animals are motile throughout their lives, exhibiting complex movements for various purposes, plants are sessile, meaning they remain fixed in one place. Fungi are also sessile organisms, growing in place and spreading through hyphae rather than moving as a whole. These differences in movement capabilities reflect their distinct ecological roles and survival strategies.