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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Eukaryote | 3/3 | https://en.wikipedia.org/wiki/Eukaryote | reference | science, encyclopedia | 2026-05-05T07:15:19.756158+00:00 | kb-cron |
=== Phylogeny === By the early 21st century, a rough consensus started to emerge from phylogenomic studies. The majority of eukaryotes can be placed in one of two large clades dubbed Amorphea (similar in composition to the unikont hypothesis) and the Diphoda (formerly bikonts), which includes plants and most algal lineages. A third major grouping, the Excavata, has been abandoned as a formal group as it was found to be paraphyletic. The proposed phylogeny below includes two groups of excavates (Discoba and Metamonada), and incorporates the 2021 proposal that picozoans are close relatives of rhodophytes. The Provora are a group of microbial predators discovered in 2022. TSAR is a possible clade that would contain Telonemia and the SAR supergroup.
One view of the great kingdoms and their stem groups. The Metamonada are hard to place, being sister possibly to Discoba or to Malawimonadida or being a paraphyletic group external to all other eukaryotes. Eukaryotes are thought to have emerged within the archaeal phylum Promethearchaeota.
=== Origin of eukaryotes ===
The origin of the eukaryotic cell, or eukaryogenesis, is a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. The last eukaryotic common ancestor (LECA) is the hypothetical origin of all living eukaryotes, and was most likely a biological population, not a single individual. The LECA is believed to have been a protist with a nucleus, at least one centriole and flagellum, facultatively aerobic mitochondria, sex (meiosis and syngamy), a dormant cyst with a cell wall of chitin or cellulose, and peroxisomes. An endosymbiotic union between a motile anaerobic archaean and an aerobic alphaproteobacterium gave rise to the LECA and all eukaryotes with mitochondria. A second, much later endosymbiosis with a cyanobacterium gave rise to the ancestor of plants, with chloroplasts. The presence of eukaryotic biomarkers in archaea points towards an archaeal origin, except for mitochondrial DNA, which is bacterial in origin. The genomes of Promethearchaeota archaea have plenty of eukaryotic signature protein genes, which play a crucial role in the development of the cytoskeleton and complex cellular structures characteristic of eukaryotes. In 2022, cryo-electron tomography demonstrated that Promethearchaeota archaea have a complex actin-based cytoskeleton, providing the first direct visual evidence of the archaeal ancestry of eukaryotes.
=== Fossils === The timing of the origin of eukaryotes is hard to determine. Multiple different fossils that may be early eukaryotes have been suggested, but remain contested. Fossils that are clearly related to modern groups start appearing an estimated 1.2 billion years ago, in the form of red algae, though fossilized Vindhyan filamentous algae have been suggested to be as much as 1.6 to 1.7 billion years old, rather than Cambrian as previously thought. Fossils from the Ruyang Group of China, dating to approximately 1.8-1.6 billion years ago, may be the oldest known eukaryotes. One possible earliest multicellular eukaryote fossil is Qingshania magnifica from North China, which lived 1.635 billion years ago. This would suggest that the crown group eukaryotes originated in the late Paleoproterozoic (Statherian). Other early unicellular eukaryotes, Tappania plana, Shuiyousphaeridium macroreticulatum, Dictyosphaera macroreticulata, Germinosphaera alveolata, and Valeria lophostriata from North China, lived approximately 1.65 billion years ago.
Some acritarchs are known from at least 1.65 billion years ago, and a fossil, Grypania, which may be an alga, is as much as 2.1 billion years old. The "problematic" fossil Diskagma has been found in paleosols 2.2 billion years old. The Neoarchean fossil Thuchomyces shares some similarities with fungi. It especially resembles the problematic fossil Diskagma, with hyphae and multiple differentiated layers. However, it is over 600 million years older than all other possible eukaryotes, and many of its "eukaryote features" are not specific to the clade, meaning it is almost certainly a microbial mat instead. Structures proposed to represent "large colonial organisms" have been found in the black shales of the Palaeoproterozoic such as the Francevillian B Formation, in Gabon, dubbed the "Francevillian biota" which is dated at 2.1 billion years old. However, the status of these structures as fossils is contested, with other authors suggesting that they might represent pseudofossils. The presence of steranes, eukaryotic-specific biomarkers, in Australian shales previously indicated that eukaryotes were present in these rocks dated at 2.7 billion years old, but these Archaean biomarkers have been rebutted as later contaminants. The oldest valid biomarker records are only around 800 million years old. In contrast, a molecular clock analysis suggests the emergence of sterol biosynthesis as early as 2.3 billion years ago. The nature of steranes as eukaryotic biomarkers is further complicated by the production of sterols by some bacteria. Whenever their origins, eukaryotes may not have become ecologically dominant until much later; a massive increase in the zinc composition of marine sediments 800 million years ago has been attributed to the rise of substantial populations of eukaryotes, which preferentially consume and incorporate zinc relative to prokaryotes, approximately a billion years after their origin (at the latest).
== See also == Eukaryote hybrid genome List of sequenced eukaryotic genomes Parakaryon myojinensis
== References ==
== External links ==
"Eukaryotes" Archived 29 January 2012 at the Wayback Machine (Tree of Life Web Project) "Eukaryote". The Encyclopedia of Life.