The hierarchy ofbiological classification's eight majortaxonomic ranks.Life is divided into domains, which are subdivided into further groups. Intermediate minor rankings are not shown.
Non-cellular life, most notably theviruses, is not included in this system. Alternatives to the three-domain system include the earliertwo-empire system (with the empires Prokaryota and Eukaryota), and theeocyte hypothesis (with two domains of Bacteria and Archaea, with Eukarya included as a branch of Archaea).
Carl Linnaeus made the classification "domain" popular in the famoustaxonomy system he created in the middle of the eighteenth century. This system was further improved by the studies ofCharles Darwin later on but could not classifybacteria easily, as they have very few observable features to compare to the other domains.[5]
Carl Woese made a revolutionary breakthrough when, in 1977, he compared thenucleotide sequences of the16s ribosomal RNA and discovered that therank "domain" contained three branches, not two as scientists had previously thought. Initially, due to their physical similarities,Archaea andBacteria were classified together and called "archaebacteria". However, scientists now know that these two domains are hardly similar and are internally distinctly different.[6]
A speculatively rooted tree forRNAgenes, showing major branches Bacteria, Archaea, and EukaryotaThe three-domain tree and theeocyte hypothesis (two-domain tree), 2008.[7]Phylogenetic tree showing the relationship between the eukaryotes and other forms of life, 2006.[8] Eukaryotes are colored red, archaea green, and bacteria blue.
Archaea are prokaryotic cells, typically characterized by membrane lipids that are branchedhydrocarbon chains attached to glycerol by ether linkages. The presence of these ether linkages in Archaea adds to their ability to withstand extreme temperatures and highlyacidic conditions, but many archaea live in mild environments.Halophiles (organisms that thrive in highly salty environments) andhyperthermophiles (organisms that thrive in extremely hot environments) are examples of Archaea.[1]
Archaea are relatively small. They range from 0.1 μm to 15 μm diameter and up to 200 μm long, about the size of bacteria and themitochondria found in eukaryotic cells. Members of the genusThermoplasma are the smallest Archaea.[1]
Cyanobacteria andmycoplasmas are two examples of bacteria.Even though bacteria are prokaryotic cells like Archaea, theircell membranes are instead made ofphospholipid bilayers, with none of the ether linkages that Archaea have. Internally, bacteria have different RNA structures in theirribosomes, hence they are grouped into a different category. In the two- and three-domain systems, this puts them into a separate domain.
There is a great deal of diversity in the domainBacteria. That diversity is further confounded by theexchange of genes between different bacterial lineages. The occurrence of duplicate genes between otherwise distantly-related bacteria makes it nearly impossible to distinguish bacterial species, count the bacterial species on the Earth, or organize them into a tree-like structure (unless the structure includes cross-connections between branches, making it a "network" instead of a "tree").[1]
Members of the domain Eukarya – calledeukaryotes – have membrane-bound organelles (including a nucleus containing genetic material) and are represented by fivekingdoms:Plantae,Protozoa,Animalia,Chromista, andFungi.[1]
The three-domain system includes no form of non-cellularlife. Stefan Luketa proposed a five-dominion system in 2012, addingPrionobiota (acellular and without nucleic acid) andVirusobiota (acellular but with nucleic acid) to the traditional three domains.[9]
Theeocyte hypothesis, proposed byLakeet al. (1984),[13] which posits two domains, Bacteria and Archaea, with Eukaryota included as a subordinate clade branching from Archaea.[14][13][15]
