The two-domain system is a biological classification by which all organisms in the tree of life are classified into two domains, Bacteria and Archaea. [1] [2] [3] It emerged from development of knowledge of archaea diversity and challenges the widely accepted three-domain system that classifies life into Bacteria, Archaea, and Eukarya. [4] It was preceded by the eocyte hypothesis of James A. Lake in the 1980s, [5] which was largely superseded by the three-domain system, due to evidence at the time. [6] Better understanding of archaea, especially of their roles in the origin of eukaryotes through symbiogenesis with bacteria, led to the revival of the eocyte hypothesis in the 2000s. [7] [8] The two-domain system became more widely accepted after the discovery of a large group (superphylum) of Archaea called Asgard in 2017, [9] which evidence suggests to be the evolutionary root of eukaryotes, thereby making eukaryotes members of the domain Archaea. [10]
While the features of Asgard archaea do not completely rule out the three-domain system, [11] [12] the notion that eukaryotes originated within Archaea has been strengthened by genetic and proteomic studies. [13] Under the three-domain system, Eukarya is mainly distinguished by the presence of "eukaryotic signature proteins" that are not found in Archaea and Bacteria. However, Asgard archaea contain genes that code for multiple such proteins. [3]
Classification of life into two main divisions is not a new concept, with the first such proposal by French biologist Édouard Chatton in 1938. Chatton distinguished organisms into:
These were later named empires, and Chatton's classification as the two-empire system. [15] Chatton used the name Eucaryotes only for protozoans, excluded other eukaryotes, and published in limited circulation so that his work was not recognised. His classification was rediscovered by Canadian bacteriologist Roger Yates Stanier of the University of California in Berkeley in 1961 while at the Pasteur Institute in Paris. [14] The next year, Stanier and his colleague Cornelis Bernardus van Niel published in Archiv für Mikrobiologie (now Archives of Microbiology ) Chatton's classification with Eucaryotes eloborated to include higher algae, protozoans, fungi, plants and animals. [16] It became a popular system of classification, as John O. Corliss wrote in 1986: "[The] Chatton-Stanier concept of a kingdom (better, superkingdom) Prokaryota for bacteria (in the broadest sense) and a second superkingdom Eukaryota for all other organisms has been widely accepted with enthusiasm." [17]
In 1977, Carl Woese and George E. Fox classified prokaryotes into two groups (kingdoms), Archaebacteria (for methanogens, the first known archaea) and Eubacteria, based on their 16S ribosomal RNA (16S rRNA) genes. [18] In 1984, James A. Lake, Michael W. Clark, Eric Henderson, and Melanie Oakes of the University of California, Los Angeles described what was known as "a group of sulfur-dependent bacteria" as a new group of organisms called eocytes (for "dawn cells") and created a new kingdom Eocyta. With it they proposed the existence of four kingdoms, based on the structure and composition of the ribosomal subunits, namely Archaebacteria, Eubacteria, Eukaryote and Eocyta [19] Lake further analysed the rRNA sequences of the four groups and suggested that eukaryotes originated from eocytes, and not archaebacteria, as was generally assumed. [20] This was the basis of the eocyte hypothesis. [6] In 1988, he proposed the division of all life forms into two taxonomic groups: [5]
In 1990, Woese, Otto Kandler and Mark Wheelis showed that archaea are distinct group of organisms and that eocytes (renamed Crenarchaeota as a phylum of Archaea [22] but corrected as Thermoproteota in 2021 [23] ) are Archaea. They introduced the major division of life into the three-domain system comprising domain Eucarya, domain Bacteria and domain Archaea. [24] With a number of revisions of details and discoveries of several new archaea lineages, Woese's classification gradually gained acceptance as "arguably the best-developed and most widely-accepted scientific hypotheses [with the five-kingdom classification] regarding the evolutionary history of life." [25]
The three-domain concept did not, however, resolve the issues with the relationship between Archaea and eukaryotes. [12] [26] As Ford Doolittle, then at the Dalhousie University, put it in 2020: "[The] three-domain tree wrongly represents evolutionary relationships, presenting a misleading view about how eukaryotes evolved from prokaryotes. The three-domain tree does recognize a specific archaeal–eukaryotic affinity, but it would have the latter arising independently, not from within, the former." [4]
The two-domain system relies mainly on two key concepts that define eukaryotes as members of the domain Archaea and not as a separate domain: eukaryotes originated within Archaea, and Asgards represent the origin of eukaryotes. [27] [28]
The three-domain system presumes that eukaryotes are more closely related to archaea than to Bacteria and are sister group to Archaea, thus, it treats them as separate domain. [29] As more new archaea were discovered in the early 2000s, this distinction became doubtful as eukaryotes became deeply nested within Archaea. The origin of eukaryotes from Archaea, meaning the two are of the same larger group, came to be supported by studies based on ribosome protein sequencing and phylogenetic analyses in 2004. [30] [31] Phylogenomic analysis of about 6000 gene sets from 185 bacterial, archaeal and eukaryotic genomes in 2007 also suggested origin of eukaryotes from Euryarchaeota (specifically the Thermoplasmatales). [32]
In 2008, researchers from Natural History Museum, London and Newcastle University reported a comprehensive analysis of 53 genes from archaea, bacteria and eukaryotes that included essential components of the nucleic acid replication, transcription, and translation machineries. The conclusion was that eukaryotes evolved from archaea, specifically Crenarchaeota (eocytes) and the results "favor a topology that supports the eocyte hypothesis rather than archaebacterial monophyly and the 3-domains tree of life." [26] A study around the same time also found several genes common to eukaryotes and Crenarchaeota. [33] These accumulating evidences support the two-domain system. [22]
In 2019, research led by Gergely J. Szöllősi assistant professor at ELTE has also concluded that two domains are the correct system. The studies conducted used simulations of more than 3,000 gene families. The study concluded that eukaryotes probably evolved from a bacterium entering an Asgard host (probably from the phylum Heimdallarchaeota). [34] [35] [36]
One of the distinctions of the domain Eukarya in the three-domain system is that eukaryotes have unique proteins such as actin (cytoskeletal microfilament involved in cell motility), tubulin (component of the large cytoskeleton, microtubule) and the ubiquitin system (protein degradation and recycling) that are not found in prokaryotes. However, these so-called "eukaryotic signature proteins" [3] are encoded in genomes of TACK (comprising the phyla Thaumarchaeota, Aigarchaeota, Crenarchaeota and Korarchaeota) archaea, but not encoded in other archaea genomes. [22] The first eukaryotic proteins identified in Crenarchaeota were actin and actin-related proteins (Arp) 2 and 3, perhaps explaining the origin of eukaryotes by symbiogenic phagocytosis, in which an ancient archaeal host had an actin based mechanism by which to envelop other cells, like protomitochondrial bacteria. [37]
Tubulin-like proteins named artubulins are found in the genomes of several ammonium-oxidising Thaumarchaeota. [38] Endosomal sorting complexes, required for transport (ESCRT III), involved in eukaryotic cell division, are found in all TACK groups. [39] The ESCRT-III-like proteins constitute the primary cell division system in these archaea. [40] [41] Genes encoding the ubiquitin system are known from multiple genomes of Aigarchaeota. [42] Ubiquitin-related protein called Urm1 is also present in Crenarchaeota. [43] DNA replication system (GINS proteins) in Crenarchaeota and Halobacteria are similar to the CMG (CDC45, MCM, GINS) complex of eukaryotes. [44] The presence of these eukaryotic proteins in Archaea indicates their direct relationship and that eukaryotes emerged from Archaea. [22] [45]
The discovery of Asgard, described as "eukaryote-like archaea", [46] in 2012 [47] [48] and the following phylogenetic analyses have strengthened the two-domain view of life. [49] Asgard Archaea called Lokiarchaeota contain even more eukaryotic protein-genes than the TACK group. Initial genetic analysis and later reanalysis showed that out of over 31 selected eukaryotic genes in the archaea, 75% of them directly support eukaryote-archaea grouping, meaning a single domain of Archaea including eukaryotes; [50] [51] although the findings did not completely rule out the three-domain system. [52]
As more Asgard groups were subsequently discovered including Thorarchaeota, Odinarchaeota and Heimdallarchaeota, their relationships with eukaryotes became better established. Phylogenetic analyses using ribosomal RNA genes indicated that eukaryotes stemmed from Asgards, and that Heimdallarchaeota are the closest relatives of eukaryotes. [9] [53] Eukaryotic origin from Heimdallarchaeota is also supported by phylogenomic study in 2020. [13] A new group of Asgard found in 2021 (provisionally named Wukongarchaeota) also indicated a deep root for eukaryotic origin. [54] A report in 2022 of another Asgard, named Njordarchaeota, indicates that Heimdallarchaeota-Wukongarchaeota branch is possibly the origin group for eukaryotes. [55]
The Asgards contain at least 80 genes for eukaryotic signature proteins. [56] In addition to actin, tubulin, ubiquitin and ESCRT proteins found in TACK archaea, Asgards contain functional genes for several other eukaryotic proteins such as profilins, [57] ubiquitin system (E1-like, E2-like and small-RING finger (srfp) proteins), [58] membrane-trafficking systems (such as Sec23/24 and TRAPP domains), a variety of small GTPases [49] (including Gtr/Rag family GTPase orthologues [59] ), and gelsolins. [60] Although this information do not completely resolve the three-domain and two-domain controversies, [46] they are generally considered to favour the two-domain system. [3] [13] [61]
The two-domain system defines classification of all known cellular life forms into two domains: Bacteria and Archaea. It overrides the domain Eukaryota recognised in the three-domain classification as one of the main domains. In contrast to the eocyte hypothesis, which proposed two major groups of life (similar to domains) and posited that Archaea could be divided to both bacterial and eukaryotic groups, it merged Archaea and eukaryotes into a single domain, Bacteria entirely in a separate domain. [4]
It consists of all bacteria, which are prokaryotes (lacking nucleus), thus, Domain Bacteria is made up solely of prokaryotic organisms. [62] [63] Some examples are:
It comprises both prokaryotic and eukaryotic organisms. [68]
Archaea
Archaea are prokaryotic organisms, some examples are:
Eukarya
Eukaryotes have a nucleus in their cells, and include:
In biological taxonomy, a domain, also dominion, superkingdom, realm, or empire, is the highest taxonomic rank of all organisms taken together. It was introduced in the three-domain system of taxonomy devised by Carl Woese, Otto Kandler and Mark Wheelis in 1990.
The three-domain system is a taxonomic classification system that groups all cellular life into three domains, namely Archaea, Bacteria and Eukarya, introduced by Carl Woese, Otto Kandler and Mark Wheelis in 1990. The key difference from earlier classifications such as the two-empire system and the five-kingdom classification is the splitting of Archaea from Bacteria as completely different organisms. It has been challenged by the two-domain system that divides organisms into Bacteria and Archaea only, as eukaryotes are considered a clade of Archaea.
The Thermoproteota are prokaryotes that have been classified as a phylum of the domain Archaea. Initially, the Thermoproteota were thought to be sulfur-dependent extremophiles but recent studies have identified characteristic Thermoproteota environmental rRNA indicating the organisms may be the most abundant archaea in the marine environment. Originally, they were separated from the other archaea based on rRNA sequences; other physiological features, such as lack of histones, have supported this division, although some crenarchaea were found to have histones. Until 2005 all cultured Thermoproteota had been thermophilic or hyperthermophilic organisms, some of which have the ability to grow at up to 113 °C. These organisms stain Gram negative and are morphologically diverse, having rod, cocci, filamentous and oddly-shaped cells. Recent evidence shows that some members of the Thermoproteota are methanogens.
The hydrogen hypothesis is a model proposed by William F. Martin and Miklós Müller in 1998 that describes a possible way in which the mitochondrion arose as an endosymbiont within a prokaryotic host in the archaea, giving rise to a symbiotic association of two cells from which the first eukaryotic cell could have arisen (symbiogenesis).
Viral eukaryogenesis is the hypothesis that the cell nucleus of eukaryotic life forms evolved from a large DNA virus in a form of endosymbiosis within a methanogenic archaeon or a bacterium. The virus later evolved into the eukaryotic nucleus by acquiring genes from the host genome and eventually usurping its role. The hypothesis was first proposed by Philip Bell in 2001 and was further popularized with the discovery of large, complex DNA viruses that are capable of protein biosynthesis.
The prokaryotic cytoskeleton is the collective name for all structural filaments in prokaryotes. It was once thought that prokaryotic cells did not possess cytoskeletons, but advances in visualization technology and structure determination led to the discovery of filaments in these cells in the early 1990s. Not only have analogues for all major cytoskeletal proteins in eukaryotes been found in prokaryotes, cytoskeletal proteins with no known eukaryotic homologues have also been discovered. Cytoskeletal elements play essential roles in cell division, protection, shape determination, and polarity determination in various prokaryotes.
A prokaryote is a single-cell organism whose cell lacks a nucleus and other membrane-bound organelles. The word prokaryote comes from the Ancient Greek πρό (pró), meaning 'before', and κάρυον (káruon), meaning 'nut' or 'kernel'. In the two-empire system arising from the work of Édouard Chatton, prokaryotes were classified within the empire Prokaryota. However in the three-domain system, based upon molecular analysis, prokaryotes are divided into two domains: Bacteria and Archaea. Organisms with nuclei are placed in a third domain: Eukaryota.
Archaea is a domain of organisms. Traditionally, Archaea only included its prokaryotic members, but this sense has been found to be paraphyletic, as eukaryotes are now known to have evolved from archaea. Even though the domain Archaea includes eukaryotes, the term "archaea" in English still generally refers specifically to prokaryotic members of Archaea. Archaea were initially classified as bacteria, receiving the name archaebacteria, but this term has fallen out of use.
The eukaryotes constitute the domain of Eukaryota or Eukarya, organisms whose cells have a membrane-bound nucleus. All animals, plants, fungi, seaweeds, and many unicellular organisms are eukaryotes. They constitute a major group of life forms alongside the two groups of prokaryotes: the Bacteria and the Archaea. Eukaryotes represent a small minority of the number of organisms, but given their generally much larger size, their collective global biomass is much larger than that of prokaryotes.
The eocyte hypothesis in evolutionary biology proposes that the eukaryotes originated from a group of prokaryotes called eocytes. After his team at the University of California, Los Angeles discovered eocytes in 1984, James A. Lake formulated the hypothesis as "eocyte tree" that proposed eukaryotes as part of archaea. Lake hypothesised the tree of life as having only two primary branches: prokaryotes, which include Bacteria and Archaea, and karyotes, that comprise Eukaryotes and eocytes. Parts of this early hypothesis were revived in a newer two-domain system of biological classification which named the primary domains as Archaea and Bacteria.
Lokiarchaeota is a proposed phylum of the Archaea. The phylum includes all members of the group previously named Deep Sea Archaeal Group, also known as Marine Benthic Group B. Lokiarchaeota is part of the superphylum Asgard containing the phyla: Lokiarchaeota, Thorarchaeota, Odinarchaeota, Heimdallarchaeota, and Helarchaeota. A phylogenetic analysis disclosed a monophyletic grouping of the Lokiarchaeota with the eukaryotes. The analysis revealed several genes with cell membrane-related functions. The presence of such genes support the hypothesis of an archaeal host for the emergence of the eukaryotes; the eocyte-like scenarios.
Proteoarchaeota is a proposed archaeal kingdom thought to be closely related and possibly ancestral to the Eukaryotes.
Eukaryogenesis, the process which created the eukaryotic cell and lineage, is a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. The process is widely agreed to have involved symbiogenesis, in which an archeon and a bacterium came together to create the first eukaryotic common ancestor (FECA). This cell had a new level of complexity and capability, with a nucleus, at least one centriole and cilium, facultatively aerobic mitochondria, sex, a dormant cyst with a cell wall of chitin and/or cellulose and peroxisomes. It evolved into a population of single-celled organisms that included the last eukaryotic common ancestor (LECA), gaining capabilities along the way, though the sequence of the steps involved has been disputed, and may not have started with symbiogenesis. In turn, the LECA gave rise to the eukaryotes' crown group, containing the ancestors of animals, fungi, plants, and a diverse range of single-celled organisms.
TACK is a group of archaea, its name an acronym for Thaumarchaeota, Aigarchaeota, Crenarchaeota, and Korarchaeota, the first groups discovered. They are found in different environments ranging from acidophilic thermophiles to mesophiles and psychrophiles and with different types of metabolism, predominantly anaerobic and chemosynthetic. TACK is a clade that is sister to the Asgard branch that gave rise to the eukaryotes. It has been proposed that the TACK clade be classified as Crenarchaeota and that the traditional "Crenarchaeota" (Thermoproteota) be classified as a class called "Sulfolobia", along with the other phyla with class rank or order. After including the kingdom category into ICNP, the only validly published name of this group is kingdom Thermoproteati.
Ubiquitin-like proteins (UBLs) are a family of small proteins involved in post-translational modification of other proteins in a cell, usually with a regulatory function. The UBL protein family derives its name from the first member of the class to be discovered, ubiquitin (Ub), best known for its role in regulating protein degradation through covalent modification of other proteins. Following the discovery of ubiquitin, many additional evolutionarily related members of the group were described, involving parallel regulatory processes and similar chemistry. UBLs are involved in a widely varying array of cellular functions including autophagy, protein trafficking, inflammation and immune responses, transcription, DNA repair, RNA splicing, and cellular differentiation.
Asgard or Asgardarchaeota is a proposed superphylum belonging to the domain Archaea that contain eukaryotic signature proteins. It appears that the eukaryotes, the domain that contains the animals, plants, and fungi, emerged within the Asgard, in a branch containing the Heimdallarchaeota. This supports the two-domain system of classification over the three-domain system.
Archaeal translation is the process by which messenger RNA is translated into proteins in archaea. Not much is known on this subject, but on the protein level it seems to resemble eukaryotic translation.
Marine prokaryotes are marine bacteria and marine archaea. They are defined by their habitat as prokaryotes that live in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. All cellular life forms can be divided into prokaryotes and eukaryotes. Eukaryotes are organisms whose cells have a nucleus enclosed within membranes, whereas prokaryotes are the organisms that do not have a nucleus enclosed within a membrane. The three-domain system of classifying life adds another division: the prokaryotes are divided into two domains of life, the microscopic bacteria and the microscopic archaea, while everything else, the eukaryotes, become the third domain.
Christa Schleper is a German microbiologist known for her work on the evolution and ecology of Archaea. Schleper is Head of the Department of Functional and Evolutionary Biology at the University of Vienna in Austria.
Heimdallarchaeota is a group of archaea that in turn forms a distinct group within the superphylum Asgard. Named after the mythical Norse god, Heimdall, one of the sons of Odin, it consist of several archaea that are considered as the closest relatives of eukaryotic organism. The first specimens were discovered from the marine sediments at Loki's Castle and Bay of Aarhus, and some other species from Auka hydrothermal vent field in the Pacific Ocean. Proposed as a phylum, it consists of a class Heimdallarchaeia, that contains at least three orders and three genera. Discovered by team of microbiologists at the Uppsala University, Sweden, led by Thijs Johannes Gerardus Ettema, and reported in 2017, Heimdallarchaeota is the group to which eukaryotes, or more specifically, from where the common ancestor of all eukaryotes emerged.