Biological classification, or taxonomy, organizes the immense diversity of life on Earth into meaningful groups, helping researchers understand evolutionary relationships. Modern classification systems use the domain as the highest rank. This broad category reflects fundamental differences in cellular structure and biochemistry, separating all known organisms into three distinct lineages.
The Molecular Basis for Three Domains
Life was historically divided into two main categories: prokaryotes and eukaryotes. This system was dramatically revised in the 1970s by microbiologist Carl Woese and his colleagues, who shifted classification away from physical appearance. They analyzed the molecular composition of the cell’s machinery by sequencing the gene for ribosomal RNA (rRNA), a molecule present in all cellular life that changes very slowly over evolutionary time. Comparing the unique molecular signatures of rRNA revealed that prokaryotes were not a single, unified group, demonstrating that life evolved along three major, separate lines from a common ancestor. This genetic evidence established a tree of life containing three separate domains: Bacteria, Archaea, and Eukarya.
Defining Features of Domain Bacteria
Domain Bacteria encompasses a vast collection of single-celled organisms, often called true bacteria. These cells are prokaryotic, lacking a membrane-bound nucleus and complex internal compartments. A defining characteristic of nearly all bacteria is the presence of peptidoglycan in their cell walls, a polymer that provides structural support and is absent in the other two domains.
Bacteria exhibit a wide range of metabolic capabilities, allowing them to thrive in virtually every environment. Some are photoautotrophs, like cyanobacteria, while others are chemosynthetic or heterotrophs that consume organic matter, such as Escherichia coli. Their cell membranes are constructed of unbranched fatty acid chains connected to glycerol by ester linkages.
Defining Features of Domain Archaea
Organisms in Domain Archaea are single-celled and prokaryotic, but their molecular makeup sets them apart as a distinct lineage. The most significant difference is the composition of their cell membranes, which are made of branched hydrocarbon chains (isoprene units). These chains are connected to glycerol by ether bonds, a structure chemically more stable than the ester linkages found in bacteria and eukarya.
This unique membrane chemistry allows many archaea to survive in harsh conditions, earning them the name extremophiles. Examples include hyperthermophiles, halophiles, and methanogens that produce methane gas as a metabolic byproduct. Archaea also lack peptidoglycan in their cell walls, and their genetic machinery shares a closer evolutionary relationship with Eukarya than with Bacteria.
Defining Features of Domain Eukarya
Domain Eukarya includes all organisms whose cells are defined by the presence of a membrane-bound nucleus. The nucleus houses the cell’s genetic material, and the cell interior is highly organized with complex, specialized organelles, such as mitochondria and the endoplasmic reticulum. This compartmentalization allows for greater cellular complexity and larger cell sizes than those found in prokaryotes. Eukaryotic organisms can be unicellular, like amoebas, or highly complex and multicellular, such as plants and animals.
The domain Eukarya contains four traditional kingdoms that represent the diversity of its members:
- Kingdom Protista is the most varied group, containing mostly single-celled organisms like algae and protozoa.
- Kingdom Fungi includes yeasts, molds, and mushrooms, which are generally multicellular and absorb nutrients from their environment.
- Kingdom Plantae consists of multicellular organisms that primarily perform photosynthesis.
- Kingdom Animalia comprises multicellular organisms that ingest food and are motile at some stage of their lives.