Life on Earth exhibits vast diversity, from microscopic organisms to complex ecosystems. To understand this variety, biologists classify organisms based on shared characteristics and evolutionary relationships. The “domains of life” represent the most fundamental divisions among all living things.
The Basis of Biological Classification
Historically, life was classified into broad categories like plants and animals, and later into a five-kingdom system. However, with advances in molecular biology, these older systems proved insufficient for capturing true evolutionary relationships. Carl Woese’s work in the late 1970s marked a significant shift. He and his colleagues analyzed ribosomal RNA (rRNA) genetic sequences, a molecule present in all living cells.
By comparing rRNA sequences, Woese discovered that prokaryotes, previously a single group, comprised two distinct lineages. This evidence led him to propose a new classification system in 1990, establishing three domains: Bacteria, Archaea, and Eukarya. This three-domain system accurately reflects life’s deepest evolutionary branches.
Domain Bacteria
Organisms in the Domain Bacteria are single-celled prokaryotes, lacking a membrane-bound nucleus and other internal organelles. Bacteria are found almost everywhere, from soil and water to the human body. Most bacterial cell walls feature peptidoglycan, a polymer composed of sugars and amino acids that provides structural strength.
The metabolic diversity within Bacteria is extensive, encompassing various ways to obtain energy and nutrients. Some bacteria are photosynthetic, like cyanobacteria, while others are chemosynthetic, deriving energy from chemical reactions. Many are heterotrophic, obtaining nutrients by consuming organic matter. Common examples include Escherichia coli (E. coli), beneficial bacteria involved in digestion, and numerous disease-causing species.
Domain Archaea
The Domain Archaea also consists of single-celled prokaryotic organisms. Genetic and biochemical analyses reveal that Archaea are fundamentally different from Bacteria and are more closely related to Eukarya in their evolutionary lineage. A primary difference lies in their cell wall composition; archaeal cell walls lack peptidoglycan, instead composed of pseudopeptidoglycan, glycoproteins, or other protein-based structures.
Their distinctive cell membrane lipids feature ether-linked branched hydrocarbons, contrasting with the ester-linked unbranched fatty acids in Bacteria and Eukarya. Many archaea thrive in extreme environments, often called “extremophiles.” These include hot springs (thermophiles), highly saline lakes (halophiles), and methane-producing environments (methanogens). Archaea are also abundant in more moderate environments like soils, oceans, and the human gut.
Domain Eukarya
The Domain Eukarya encompasses all organisms whose cells possess a membrane-bound nucleus, housing their genetic material, and various other membrane-bound organelles. These internal compartments, such as mitochondria for energy production and chloroplasts for photosynthesis in plants, allow for higher cellular organization and specialization. Eukaryotic organisms range from microscopic single-celled forms to complex multicellular life.
Within the Domain Eukarya, life is further organized into several kingdoms, including Animalia (animals), Plantae (plants), Fungi (fungi), and Protista (a diverse group of mostly single-celled eukaryotes). This domain includes diverse forms like amoebas, mushrooms, trees, and humans.