Life on Earth exhibits great diversity, from microscopic organisms to vast ecosystems. To understand this variety, scientists classify living things using a hierarchical system. At the broadest level, “domains” represent fundamental distinctions among all known life forms. This system organizes and clarifies evolutionary relationships.
The Domain System
The current understanding of life’s broadest divisions stems from American microbiologist Carl Woese’s work in the late 1970s. Woese and his colleagues introduced the three-domain system, which transformed biological classification. This system was established through genetic analysis, comparing ribosomal RNA (rRNA) sequences.
Before Woese, life was categorized into prokaryotes and eukaryotes. However, rRNA genetic evidence revealed that prokaryotes comprised two distinct lineages. This divergence necessitated a new classification, moving beyond older models that relied on physical characteristics. The three-domain system now groups all cellular life into Archaea, Bacteria, and Eukarya, reflecting these genetic differences.
Domain Bacteria
Organisms in the Domain Bacteria are prokaryotic, lacking a membrane-bound nucleus and other organelles. These single-celled organisms have a rigid cell wall composed of peptidoglycan, which provides structural support and protection. This layer helps bacteria resist osmotic pressure.
Bacteria exhibit diverse metabolic capabilities and inhabit nearly every environment. They play varied ecological roles: as decomposers, pathogens, and symbionts. Examples include Escherichia coli (E. coli), found in intestines, and cyanobacteria, which are photosynthetic. Nitrogen-fixing bacteria, crucial for converting atmospheric nitrogen into usable forms, also belong here.
Domain Archaea
Like Bacteria, organisms in the Domain Archaea are prokaryotic. Despite their superficial resemblance, genetic analysis revealed Archaea are distinct. They share a more recent common ancestor with Eukarya than with Bacteria, indicating a deeper evolutionary split.
Archaea possess unique biochemical characteristics. Their cell membranes have ether-linked lipids with branched chains, differing from those in Bacteria and Eukarya, which contributes to their stability in harsh conditions. Archaeal cell walls lack peptidoglycan, featuring other unique components. Many Archaea thrive in extreme environments, often called extremophiles. Examples include thermophiles in hot springs, halophiles in saline waters, and methanogens producing methane.
Domain Eukarya
The Domain Eukarya encompasses all organisms with cells characterized by a true nucleus and other membrane-bound organelles. These internal compartments, like mitochondria and chloroplasts, allow for specialized functions and greater cellular complexity. Eukaryotic cells are generally larger and more intricate than prokaryotic cells.
This domain includes single-celled organisms to complex multicellular beings. Eukarya’s diversity is categorized into kingdoms: Animalia (animals), Plantae (plants), Fungi (fungi), and Protista. Protista is a diverse group of unicellular and simple multicellular organisms like algae and protozoa. Eukaryotic cells’ defining features underpin the complexity and multicellularity of visible life.