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.
Based on differences in the nucleotide sequences of ribosomal RNA, all cellular life can be classified into three distinct domains — Bacteria, Archaea, and Eukarya.
These three domains originated from a common ancestor approximately 3.5 billion years ago, with significant horizontal gene transfer.
The eukaryotic cells appeared later, approximately 2.5 billion years ago.
The domain Bacteria encompasses prokaryotes that have peptidoglycan in their cell walls. It includes all pathogenic and non-pathogenic prokaryotes, as well as phototrophic bacteria.
The domain Archaea includes prokaryotes that lack peptidoglycan in their cell walls. These organisms are often found in extreme environments and include methanogens, extreme halophiles, and hyperthermophiles.
Methanogens are anaerobes that produce methane from carbon dioxide and hydrogen.
Extreme halophiles require high salt concentrations for survival.
Hyperthermophiles thrive in extremely hot environments.
The domain Eukarya contains nucleated organisms, including protozoans, algae, fungi, plants, and animals.
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