Three-Domain System of Life

Ribosomal RNA (rRNA) sequence analysis revealed three distinct groups of cells: eukaryotes, bacteria, and archaea. In 1978, Carl R. Woese proposed the concept of domains, a taxonomic level above kingdoms, to differentiate these groups. He suggested that archaea and bacteria, despite their similar appearance, represent separate domains. Domains differ in rRNA, membrane lipid structure, transfer RNA, and antibiotic sensitivity.

In this classification, animals, plants, and fungi belong to the Domain Eukarya, which also includes all eukaryotic organisms. The Domain Bacteria consists of pathogenic and nonpathogenic prokaryotes, including soil and water species and photoautotrophs. The Domain Archaea encompasses prokaryotes lacking peptidoglycan in their cell walls, often thriving in extreme environments. Archaea include methanogens (methane producers), extreme halophiles (salt-loving organisms), and hyperthermophiles (heat-adapted species).

Fossil evidence suggests prokaryotes emerged 3.5 billion years ago, with eukaryotes evolving around 2.5 billion years ago. The endosymbiotic theory posits that eukaryotes arose when prokaryotes formed symbiotic relationships, with one cell living inside another. Infoldings of the plasma membrane may have led to the formation of a nucleus, as seen in some bacteria, such as Gemmata. Over time, the nucleoplasm evolved, with chromosomes fragmenting and adapting for efficient cell division.

Horizontal gene transfer among the three domains has been observed, with shared genes highlighting their interconnected evolution. For instance, some genes in Thermotoga bacteria likely originated from archaea. The taxonomy system continues to evolve, aiding in understanding organismal relationships and evolutionary history as new species are discovered.