Two-domain system
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.
[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.
[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.
[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.
"[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.
[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 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.
[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.
[22] In 2019, research led by Gergely J. Szöllősi assistant professor at ELTE has also concluded that two domains are the correct system.
[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.
[38] Endosomal sorting complexes, required for transport (ESCRT III), involved in eukaryotic cell division, are found in all TACK groups.
[52] As more Asgard groups were subsequently discovered including Thorarchaeota, Odinarchaeota and Heimdallarchaeota, their relationships with eukaryotes became better established.
