Life on Earth exhibits an astonishing array of forms. To make sense of this diversity, scientists employ biological classification systems. The broadest level of this classification is the domain, which categorizes all known life based on fundamental cellular and genetic characteristics. There are three such domains.
Domain Bacteria
Organisms in the Domain Bacteria are prokaryotes, meaning their cells lack a true nucleus and other membrane-bound internal compartments; their genetic material typically resides freely within the cytoplasm. Most bacterial cells have a cell wall primarily composed of peptidoglycan, a unique polymer of sugars and amino acids, which provides structural support. Bacteria exhibit metabolic versatility, thriving in nearly every environment on Earth. Some are photosynthetic, while others obtain energy by breaking down compounds. Examples include Escherichia coli and Cyanobacteria; many play beneficial roles, while others can cause diseases.
Domain Archaea
Organisms in the Domain Archaea are prokaryotes, like Bacteria, lacking a membrane-bound nucleus and organelles. Archaea are distinct from Bacteria in several fundamental ways. Their cell membranes are composed of unique lipids with ether linkages, and archaeal cell walls do not contain peptidoglycan. Many Archaea survive in extreme environments, earning them the moniker “extremophiles”; examples include methanogens and halophiles. Evolutionary studies suggest that Archaea are more closely related to Eukarya than to Bacteria.
Domain Eukarya
The Domain Eukarya encompasses all organisms whose cells possess a true nucleus enclosed within a nuclear membrane, along with various other membrane-bound organelles. These internal compartments, such as mitochondria and chloroplasts, allow for specialized functions and increased cellular complexity. This domain includes familiar life forms organized into major groups:
- Animals are multicellular and heterotrophic, consuming other organisms for energy.
- Plants are multicellular and autotrophic, producing their own food through photosynthesis, and possess rigid cell walls made of cellulose.
- Fungi, such as yeasts and mushrooms, are largely multicellular and heterotrophic, absorbing dissolved organic molecules.
- Protists are a diverse collection of mostly single-celled organisms, including algae, amoebas, and paramecia, that do not fit neatly into the other eukaryotic kingdoms.
How Scientists Classified Life into Three Domains
Historically, biological classification systems primarily divided life into two main groups: prokaryotes and eukaryotes; however, the widely accepted five-kingdom system, which included Monera, Protista, Fungi, Plantae, and Animalia, grouped all bacteria into a single kingdom, overlooking significant underlying differences. A revolutionary shift occurred with the work of Carl Woese and his colleagues in the late 1970s, who analyzed ribosomal RNA (rRNA) genetic sequences to establish evolutionary relationships. Ribosomal RNA, found in all cells and essential for protein synthesis, changes slowly, making it an excellent “molecular clock” for tracing ancestry. Woese’s rRNA analysis revealed that organisms previously classified as prokaryotes were two distinct groups: Bacteria and Archaea, which were no more closely related to each other than to Eukarya. This led to the proposal of the three-domain system in 1990, reflecting three distinct evolutionary lineages that originated from a common ancestor.