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The eukaryotes ( ) comprise the domain of Eukaryota or Eukarya, whose cells have a membrane-bound cell nucleus. All , , Fungus, , and many unicellular organisms are eukaryotes. They constitute a major group of life forms alongside the two groups of : 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 eukaryotes emerged within the archaeal kingdom Promethearchaeati and its sole phylum Promethearchaeota. This implies that there are only two domains of life, Bacteria and Archaea, with eukaryotes incorporated among the Archaea. Eukaryotes first emerged during the Paleoproterozoic, likely as Flagellated cell cells. The leading evolutionary theory is they were created by symbiogenesis between an anaerobic Promethearchaeati archaean and an aerobic proteobacterium, which formed the Mitochondrion. A second episode of symbiogenesis with a Cyanobacteria created the plants, with .
Eukaryotic cells contain organelle such as the cell nucleus, the endoplasmic reticulum, and the Golgi apparatus. Eukaryotes may be either unicellular or multicellular. In comparison, prokaryotes are typically unicellular. Unicellular eukaryotes are sometimes called . Eukaryotes can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion (fertilization).
The eukaryotes are a diverse lineage, consisting mainly of Microorganism. Multicellularity in some form has evolved independently at least 25 times within the eukaryotes. Complex multicellular organisms, not counting the aggregation of amoebae to form , have evolved within only six eukaryotic lineages: animals, symbiomycotan fungi, brown algae, red algae, green algae, and Embryophyte. Eukaryotes are grouped by genomic similarities, so that groups often lack visible shared characteristics.
The nucleus is surrounded by a double membrane known as the nuclear envelope, with that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form the endoplasmic reticulum, which is involved in protein transport and maturation. It includes the rough endoplasmic reticulum, covered in which synthesize proteins; these enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the smooth endoplasmic reticulum. In most eukaryotes, these protein-carrying vesicles are released and their contents further modified in stacks of flattened vesicles (), the Golgi apparatus.
Vesicles may be specialized; for instance, contain that break down in the cytoplasm.
Mitochondria contain their own DNA, which has close structural similarities to bacterial DNA, from which it originated, and which encodes rRNA and tRNA genes that produce RNA which is closer in structure to bacterial RNA than to eukaryote RNA. (1988). 9780805396140, Benjamin Cummings. ISBN 9780805396140
Some eukaryotes, such as the Giardia and Trichomonas, and the amoebozoan Pelomyxa, appear to lack mitochondria, but all contain mitochondrion-derived organelles, like or , having lost their mitochondria secondarily. They obtain energy by enzymatic action in the cytoplasm. It is thought that mitochondria developed from Prokaryote cells which became living inside eukaryotes.
Many eukaryotes have long slender motile cytoplasmic projections, called flagellum, or multiple shorter structures called cilium. Undulipodium are variously involved in movement, feeding, and sensation. They are composed mainly of tubulin, and are entirely distinct from prokaryotic flagella. They are supported by a bundle of arising from a centriole, characteristically arranged as nine doublets surrounding two singlets. Flagella may have hairs (), as in many . Their interior is continuous with the cell's cytoplasm.
Centrioles are often present, even in cells and groups that do not have flagella, but and have neither. They generally occur in groups that give rise to various microtubular roots. These form a primary component of the cytoskeleton, and are often assembled over the course of several cell divisions, with one flagellum retained from the parent and the other derived from it. Centrioles produce the spindle during nuclear division. (1987). 9780123645067 ISBN 9780123645067
The major polysaccharides making up the primary cell wall of Embryophyte are cellulose, hemicellulose, and pectin. The cellulose are linked together with hemicellulose, embedded in a pectin matrix. The most common hemicellulose in the primary cell wall is xyloglucan.
The evolution of sexual reproduction may be a primordial characteristic of eukaryotes. Based on a phylogenetic analysis, Dacks and Roger have proposed that facultative sex was present in the group's common ancestor. A core set of genes that function in meiosis is present in both Trichomonas vaginalis and Giardia intestinalis, two organisms previously thought to be asexual. Since these two species are descendants of lineages that diverged early from the eukaryotic evolutionary tree, core meiotic genes, and hence sex, were likely present in the common ancestor of eukaryotes. Species once thought to be asexual, such as Leishmania parasites, have a sexual cycle. Amoebae, previously regarded as asexual, may be anciently sexual; while present-day asexual groups could have arisen recently.
The protists were at that time thought to be "primitive forms", and thus an evolutionary grade, united by their primitive unicellular nature. Understanding of the oldest branchings in the tree of life only developed substantially with DNA sequencing, leading to a system of domains rather than kingdoms as top level rank being put forward by Carl Woese, Otto Kandler, and Mark Wheelis in 1990, uniting all the eukaryote kingdoms in the domain "Eucarya", stating, however, that eukaryotes' will continue to be an acceptable common synonym". In 1996, the evolutionary biologist Lynn Margulis proposed to replace kingdoms and domains with "inclusive" names to create a "symbiosis-based phylogeny", giving the description "Eukarya (symbiosis-derived nucleated organisms)".
An Symbiogenesis between a motile anaerobic archaean and an aerobic alphaproteobacterium gave rise to the LECA and all eukaryotes with mitochondrion. A second, much later endosymbiosis with a cyanobacterium gave rise to the ancestor of plants, with .
The presence of eukaryotic biomarkers in archaea points towards an archaeal origin. The genomes of Promethearchaeati 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 Promethearchaeati archaea have a complex actin-based cytoskeleton, providing the first direct visual evidence of the archaeal ancestry of eukaryotes.
Some 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 Incertae sedis fossil Diskagma has been found in 2.2 billion years old.
The Neoarchean fossil Thuchomyces shares similarities with eukaryotes, specifically 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 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 . The oldest fossils that can unambiguously be assigned to eukaryotes are from the Ruyang Group of China, dating to approximately 1.8-1.6 billion years ago. Fossils that are clearly related to modern groups start appearing an estimated 1.2 billion years ago, in the form of red algae, though recent work suggests the existence of fossilized filamentous algae in the Vindhya basin dating back perhaps to 1.6 to 1.7 billion years ago.
The presence of , eukaryotic-specific biomarkers, in 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 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 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).
Evolution
History of classification
In Ancient history, the two lineages of and were recognized by Aristotle and Theophrastus. The lineages were given the taxonomic rank of kingdom by Carl Linnaeus in the 18th century. Though he included the fungi with plants with some reservations, it was later realized that they are quite distinct and warrant a separate kingdom. The various single-cell eukaryotes were originally placed with plants or animals when they became known. In 1818, the German biologist Georg A. Goldfuss coined the word Protozoa to refer to organisms such as , From p. 1008: "Erste Klasse. Urthiere. Protozoa." (First class. Primordial animals. Protozoa.) Note: and this group was expanded until Ernst Haeckel made it a kingdom encompassing all single-celled eukaryotes, the Protista, in 1866. The eukaryotes thus came to be seen as four kingdoms:
Phylogeny
By 2014, a rough consensus started to emerge from the phylogenomic studies of the previous two decades. 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 Metamonad), and incorporates the 2021 proposal that are close relatives of rhodophytes. The Provora are a group of microbial predators discovered in 2022.
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 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 microbial cyst with a cell wall of chitin or cellulose, and .
(2025). 9780521761314, Cambridge University Press. ISBN 9780521761314
Fossils
The timing of the origin of eukaryotes is hard to determine, but the discovery of Qingshania magnificia, the earliest multicellular eukaryote from North China which lived 1.635 billion years ago, suggests that the crown group eukaryotes originated from the late Paleoproterozoic (Statherian). The earliest unequivocal unicellular eukaryotes, Tappania, Shuiyousphaeridium macroreticulatum, Dictyosphaera macroreticulata, Germinosphaera alveolata, and Valeria lophostriata from North China, lived approximately 1.65 billion years ago.
(2025). 9780128130124 ISBN 9780128130124
See also
External links
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