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Temporal range: StatherianPresent 1650–0 Ma
Scientific classification Edit this classification
Domain: Eukaryota
(Chatton, 1925) Whittaker & Margulis, 1978
Supergroups and kingdoms[2]

The eukaryotes (

global biomass
is much larger than that of prokaryotes.

The eukaryotes seemingly emerged within the

proteobacterium, which formed the mitochondria. A second episode of symbiogenesis with a cyanobacterium created the plants, with chloroplasts

Eukaryotic cells contain



Eukaryotes are organisms that range from microscopic single cells, such as picozoans under 3 micrometres across,[6] to animals like the blue whale, weighing up to 190 tonnes and measuring up to 33.6 metres (110 ft) long,[7] or plants like the coast redwood, up to 120 metres (390 ft) tall.[8] Many eukaryotes are unicellular; the informal grouping called protists includes many of these, with some multicellular forms like the giant kelp up to 200 feet (61 m) long.[9] The multicellular eukaryotes include the animals, plants, and fungi, but again, these groups too contain many unicellular species.[10] Eukaryotic cells are typically much larger than those of prokaryotes—the bacteria and the archaea—having a volume of around 10,000 times greater.[11][12] Eukaryotes represent a small minority of the number of organisms, but, as many of them are much larger, their collective global biomass (468 gigatons) is far larger than that of prokaryotes (77 gigatons), with plants alone accounting for over 81% of the total biomass of Earth.[13]

The eukaryotes are a diverse lineage, consisting mainly of

land plants.[17] Eukaryotes are grouped by genomic similarities, so that groups often lack visible shared characteristics.[14]

Distinguishing features


The defining feature of eukaryotes is that

nuclei. This gives them their name, from the Greek εὖ (eu, "well" or "good") and κάρυον (karyon, "nut" or "kernel", here meaning "nucleus").[18] Eukaryotic cells have a variety of internal membrane-bound structures, called organelles, and a cytoskeleton which defines the cell's organization and shape. The nucleus stores the cell's DNA, which is divided into linear bundles called chromosomes;[19] these are separated into two matching sets by a microtubular spindle during nuclear division, in the distinctively eukaryotic process of mitosis.[20]


Eukaryotes differ from prokaryotes in multiple ways, with unique biochemical pathways such as sterane synthesis.[21] The eukaryotic signature proteins have no homology to proteins in other domains of life, but appear to be universal among eukaryotes. They include the proteins of the cytoskeleton, the complex transcription machinery, the membrane-sorting systems, the nuclear pore, and some enzymes in the biochemical pathways.[22]

Internal membranes

membrane-bound organelles

Eukaryote cells include a variety of membrane-bound structures, together forming the

vesicles and vacuoles, can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis, where the outer membrane invaginates and then pinches off to form a vesicle.[24] Some cell products can leave in a vesicle through exocytosis.[25]

The nucleus is surrounded by a double membrane known as the

protein transport and maturation. It includes the rough endoplasmic reticulum, covered in ribosomes which synthesize proteins; these enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the smooth endoplasmic reticulum.[27] In most eukaryotes, these protein-carrying vesicles are released and further modified in stacks of flattened vesicles (cisternae), the Golgi apparatus.[28]

Vesicles may be specialized; for instance, lysosomes contain digestive enzymes that break down biomolecules in the cytoplasm.[29]


Mitochondria are essentially universal in the eukaryotes, and with their own DNA somewhat resemble prokaryotic cells.

aerobic respiration takes place.[33]

Mitochondria contain

tRNA genes that produce RNA which is closer in structure to bacterial RNA than to eukaryote RNA.[34]

Some eukaryotes, such as the metamonads Giardia and Trichomonas, and the amoebozoan Pelomyxa, appear to lack mitochondria, but all contain mitochondrion-derived organelles, like hydrogenosomes or mitosomes, having lost their mitochondria secondarily.[35] They obtain energy by enzymatic action in the cytoplasm.[36][35]


The most common type of plastid is the chloroplast, which contains chlorophyll and produces organic compounds by photosynthesis.

Plants and various groups of

secondary endosymbiosis or ingestion.[37] The capture and sequestering of photosynthetic cells and chloroplasts, kleptoplasty, occurs in many types of modern eukaryotic organisms.[38][39]

Cytoskeletal structures

in green. (The nucleus is in blue.)

The cytoskeleton provides stiffening structure and points of attachment for motor structures that enable the cell to move, change shape, or transport materials. The motor structures are microfilaments of actin and actin-binding proteins, including α-actinin, fimbrin, and filamin are present in submembranous cortical layers and bundles. Motor proteins of microtubules, dynein and kinesin, and myosin of actin filaments, provide dynamic character of the network.[40][41]

Many eukaryotes have long slender motile cytoplasmic projections, called flagella, or multiple shorter structures called cilia. These organelles 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 microtubules arising from a centriole, characteristically arranged as nine doublets surrounding two singlets. Flagella may have hairs (mastigonemes), as in many Stramenopiles. Their interior is continuous with the cell's cytoplasm.[42][43]

Centrioles are often present, even in cells and groups that do not have flagella, but conifers and flowering plants 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.[44]

Cell wall

The cells of plants, algae, fungi and most chromalveolates, but not animals, are surrounded by a cell wall. This is a layer outside the cell membrane, providing the cell with structural support, protection, and a filtering mechanism. The cell wall also prevents over-expansion when water enters the cell.[45]

The major

land plants are cellulose, hemicellulose, and pectin. The cellulose microfibrils are linked together with hemicellulose, embedded in a pectin matrix. The most common hemicellulose in the primary cell wall is xyloglucan.[46]

Sexual reproduction

diploid phase, with two copies. In eukaryotes, haploid gametes are produced by meiosis; two gametes fuse to form a diploid zygote

Eukaryotes have a life cycle that involves

spermatozoa, to form a zygote; this may grow into a body, with its cells dividing by mitosis, and at some stage produce haploid gametes through meiosis, a division that reduces the number of chromosomes and creates genetic variability.[47] There is considerable variation in this pattern. Plants have both haploid and diploid multicellular phases.[48] Eukaryotes have lower metabolic rates and longer generation times than prokaryotes, because they are larger and therefore have a smaller surface area to volume ratio.[49]


Giardia intestinalis, two organisms previously thought to be asexual.[51][52] 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.[51][52] Species once thought to be asexual, such as Leishmania parasites, have a sexual cycle.[53] Amoebae, previously regarded as asexual, are anciently sexual; present-day asexual groups likely arose recently.[54]


Tree of eukaryotes showing major subgroups and thumbnail diagrams of representative members of each group. Updated synthesis based on recent (as of 2023) phylogenomic reconstructions.[55]

History of classification


Protista, in 1866.[58][59][60]
The eukaryotes thus came to be seen as four kingdoms:

  • Kingdom
  • Kingdom
  • Kingdom
  • Kingdom

The protists were at that time thought to be "primitive forms", and thus an evolutionary grade, united by their primitive unicellular nature.[59] 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".[3][61] 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)".[4]


By 2014, a rough consensus started to emerge from the phylogenomic studies of the previous two decades.

Discoba),[63] and incorporates the 2021 proposal that picozoans are close relatives of rhodophytes.[64] The Provora are a group of microbial predators discovered in 2022.[1]











Holomycota (inc. fungi)

Holozoa (inc. animals)

1300 mya
1500 mya







Rhodophyta (red algae)



1100 mya

Viridiplantae (plants)

1000 mya
1600 mya








550 mya



2200 mya

One view of the great kingdoms and their stem groups.

Origin of eukaryotes

In the theory of symbiogenesis, a merger of an archaean and an aerobic bacterium created the eukaryotes, with aerobic mitochondria; a second merger added chloroplasts, creating the green plants.[67]

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

syngamy), a dormant cyst with a cell wall of chitin or cellulose, and peroxisomes.[70][71][72]

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.[67]

The presence of eukaryotic biomarkers in archaea points towards an archaeal origin. The genomes of

cryo-electron tomography demonstrated that Asgard archaea have a complex actin-based cytoskeleton, providing the first direct visual evidence of the archaeal ancestry of eukaryotes.[73]


The timing of the origin of eukaryotes is hard to determine but the discovery of Qingshania magnificia, the earliest multicelluar eukaryote from North China which lived during 1.635 billion years ago, suggests that the crown group eukaryotes would have originated from the late Paleoproterozoic (Statherian); the earliest unequivocal unicellular eukaryotes which lived during approximately 1.65 billion years ago are also discovered from North China: Tappania plana, Shuiyousphaeridium macroreticulatum, Dictyosphaera macroreticulata, Germinosphaera alveolata, and Valeria lophostriata.[74]

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.[75][76] The "problematic"[77] fossil Diskagma has been found in paleosols 2.2 billion years old.[77]

Diskagma buttonii
, a terrestrial fossil less than 1mm high, from rocks around 2.2 billion years old

Structures proposed to represent "large colonial organisms" have been found in the

Vindhya basin dating back perhaps to 1.6 to 1.7 billion years ago.[82]

The presence of

biomarkers, in Australian shales previously indicated that eukaryotes were present in these rocks dated at 2.7 billion years old,[21][83] but these Archaean biomarkers have been rebutted as later contaminants.[84] The oldest valid biomarker records are only around 800 million years old.[85] In contrast, a molecular clock analysis suggests the emergence of sterol biosynthesis as early as 2.3 billion years ago.[86] The nature of steranes as eukaryotic biomarkers is further complicated by the production of sterols by some bacteria.[87][88]

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).[89]

See also


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External links