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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Eocyte hypothesis | 1/2 | https://en.wikipedia.org/wiki/Eocyte_hypothesis | reference | science, encyclopedia | 2026-05-05T09:07:10.485941+00:00 | kb-cron |
The eocyte hypothesis in evolutionary biology proposes that the eukaryotes originated from a group of prokaryotes called eocytes (later classified as Thermoproteota, a group of archaea). 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. Lake's hypothesis was based on an analysis of the structural components of ribosomes. It was largely ignored, being overshadowed by the three-domain system which relied on more precise genetic analysis. In 1990, Carl Woese and his colleagues proposed that cellular life consists of three domains—Eucarya, Bacteria, and Archaea – based on the ribosomal RNA sequences. The three-domain concept was widely accepted in genetics, and became the presumptive classification system for high-level taxonomy, and was promulgated in many textbooks. Resurgence of archaea research after the 2000s, using advanced genetic techniques, and later discoveries of new groups of archaea revived the eocyte hypothesis; consequently, the two-domain system has found wider acceptance.
== Description == In 1984, James A. Lake, Michael W. Clark, Eric Henderson, and Melanie Oakes of the University of California, Los Angeles described a new group of prokaryotic organisms designated as "a group of sulfur-dependent bacteria." Based on the structure and composition of their ribosomal subunits, they found that these organisms were different from other prokaryotes, bacteria and archaea, known at the time. They named them eocytes (for "dawn cells") and proposed a new biological kingdom Eocyta. According to this discovery, the tree of life is represented by four kingdoms, Archaebacteria, Eubacteria, Eukaryote and Eocyta. Following analyses of the rRNA sequences of the four groups, Lake concluded in 1988 that eukaryotes were closely related to eocytes so that the two groups constitute the same (monophyletic) group, meaning that eukaryotes originated from eocytes and not archaebacteria, as was generally assumed. This was the establishment of the eocyte hypothesis. In 1988, Lake proposed a systematic classification of all life forms into two taxonomic groups, which he later mentioned as superkingdoms:
Karyotes (that include eukaryotes and proto-eukaryotic organisms such as eocytes) Parkaryotes (that consists of eubacteria and two groups of archaea known at the time, halobacteria and methanogens)
== Development and competition == Lake's classification was not widely recognised, but the eocyte hypothesis gained considerable attention after its introduction due to the interest in determining the origin of the eukaryotic cell. However, the concept faced a problem because it was not known that eocytes, the main organism group on which the hypothesis was based, were archaea. For example, studies in the late 1980s and early 1990s still treated eocytes as separate group from archaea. As Lake also argued, the rival hypothesis was called archaebacterial tree (as introduced by Carl Woese of the University of Illinois in 1987) or archaebacterial theory, which (supposedly) stated that eukaryotes originated from archaea, and not eocytes. Due to such confusion, some studies appeared to invalidate the hypothesis. For example, Japanese scientists reported in 1990 their study on the elongation factors Tu(EF-Tu) and G(EF-G) from various organisms that showed that eukaryotes are most closely related to archaea (methanogen and halobacteria), and not eocytes. Other studies also supported the eukaroyte-archaea relationship and rejected the eocyte hypotheses. Ribosomal RNA sequencing in 1989 also opposed the eocyte tree as the origin of eukaryotes.
=== Three-domain system === The most important blow to the eocyte hypothesis and Lake's classification was the development of ribosomal RNA sequencing that became a reliable determinant in biological classification. Introduced in 1977 by Carl Woese and George E. Fox in classification, the technique indicated that archaea (with only methanogens known at the time) and bacteria were distinct groups of organisms. Two kingdoms, Archaebacteria (archaea) and Eubacteria (for bacteria) were established. Based on further studies, Woese, Otto Kandler and Mark Wheelis introduced the concept of "domain" in 1990 as the highest level of biological classification, and proposed the three-domain system consisting of Eucarya, Bacteria and Archaea. With it they classified eocytes as archaea under the phylum Crenarchaeota (which was renamed Thermoproteota in 2021). The classification gradually gained acceptance and was recognised as "arguably the best-developed and most widely-accepted scientific hypotheses [with the five-kingdom classification] regarding the evolutionary history of life." It became a scientific concept and general taxonomy in textbooks. Although Lake continued to advocate his eocyte taxonomy and hypothesis instead of conceding that eocytes were archaea, the hypothesis was largely neglected and support of it waned in favour of the three-domain system.
== Archaeal studies == In addition to a Thermoproteota origin of eukaryotes, some studies have suggested that eukaryotes may also have originated in the Nitrososphaerota (formerly Thaumarchaeota). A superphylum TACK has been proposed that includes the Nitrososphaerota, Thermoproteota, and other groups of archaea, so that this superphylum may be related to the origin of eukaryotes. It is seen that eukaryotes share a large number of proteins with members of the TACK superphylum and that these complex archaea may have had rudimentary phagocytosis abilities to engulf bacteria. As a result of metagenomic analysis of material found nearby hydrothermal vents, another superphylum — Asgard — has been named and proposed to be more closely related to the original eukaryote and a sister group to TACK more recently. Asgard consists of phyla Lokiarchaeota (found first), Heimdallarchaeota (possibly related closest to eukaryotes) and others.