Eukaryote

{{Short description|Domain of life whose cells have nuclei}} {{good article}} {{Use dmy dates|date=October 2020}} {{Use American English|date=March 2017}} {{Automatic taxobox | name = Eukaryotes | fossil_range = Statherian–Present {{Long fossil range|1650|0 |earliest=2300}} | image = {{Multiple image |perrow = 2/2/2 |total_width = 270 |image1 = Rhodomonas salina CCMP 322 (cropped).jpg |caption1 = Cryptista |image2 = Ranunculus asiaticus4LEST.jpg |caption2 = Viridiplantae |image3 = Two Euglena.jpg |caption3 = Discoba  |image4 = Leocarpus fragilis 10649909.jpg |caption4 = Amoebozoa |image5 = Ammonia tepida.jpg |caption5 = Rhizaria |image6 = Mikrofoto.de-Glockentierchen-1.jpg |caption6 = Alveolata |image7 = Osmia rufa couple (aka).jpg |caption7 = Animalia |image8 = Boletus edulis (Tillegem).jpg |caption8 = Fungi |border = infobox }} | taxon = Eukaryota | authority = (Chatton, 1925) Whittaker & Margulis, 1978 | subdivision_ranks = Major subdivisions  | subdivision = <!-- Please don't add or change anything here that is not ALREADY reliably cited in the body of this article. Thanks.

-->* Amorphea ** Amoebozoa ** Obazoa (including animals and fungi) * Ancyromonadida * CRuMs * Discoba * Malawimonadida * Metamonada * Diaphoretickes ** Archaeplastida (including land plants) ** Pancryptista ** Haptista ** Telonemia **SAR *** Stramenopiles *** Alveolata *** Rhizaria **Disparia ***Provora ***Membrifera<!-- Please don't add or change anything here that is not ALREADY reliably cited in the body of this article. Thanks.

--> | synonyms = * Eucarya {{au|Woese et al. 1990}}<ref name="Woese 1990"/> * Eukarya {{au|Margulis 1996}}<ref name="Margulis 1996"/> }}

The '''eukaryotes''' ({{IPAc-en|j|uː|ˈ|k|ær|i|oʊ|t|s|,_|-|ə|t|s}})<ref>{{Cite Merriam-Webster|eukaryote|accessdate=2024-05-12}}</ref> are the domain '''Eukaryota''' or '''Eukarya''', organisms whose cells have a membrane-bound nucleus. All animals, plants, fungi, seaweeds, and many unicellular organisms are eukaryotes. They constitute a major group of life forms alongside the two groups of prokaryotes: 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 phylum Promethearchaeota. Ignoring mitochondrial DNA (which is bacterial rather than archaeal), this would imply only two domains of life, Bacteria and Archaea, with eukaryotes incorporated among the Archaea. Eukaryotes first emerged during the Paleoproterozoic, likely as flagellated cells. The leading evolutionary theory is they were created by symbiogenesis between an anaerobic Promethearchaeota archaeon and an aerobic proteobacterium, which formed the mitochondria. A second episode of symbiogenesis with a cyanobacterium created the plants, with chloroplasts.

Eukaryotic cells contain membrane-bound organelles such as the 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 protists. Eukaryotes can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion (fertilization). {{TOC limit|3}}

== Etymology ==

The word ''eukaryote'' is derived from the Greek words "eu" (εὖ) meaning "true" or "good" and "karyon" (κάρυον) meaning "nut" or "kernel", referring to the nucleus of a cell.<ref>{{cite web |title=eukaryotic (adj.) |url=https://www.etymonline.com/word/eukaryotic |publisher=Online Etymology Dictionary |access-date=7 January 2025}}</ref>

== Diversity ==

Eukaryotes are organisms that range from microscopic single cells, such as picozoans under 3 micrometers across,<ref name="Seenivasan 2013">{{cite journal |last1=Seenivasan |first1=Ramkumar |last2=Sausen |first2=Nicole |last3=Medlin |first3=Linda K. |last4=Melkonian |first4=Michael |name-list-style=vanc |title=Picomonas judraskeda Gen. Et Sp. Nov.: The First Identified Member of the Picozoa Phylum Nov., a Widespread Group of Picoeukaryotes, Formerly Known as 'Picobiliphytes' |journal=PLOS ONE |volume=8 |issue=3 |date=26 March 2013 |doi=10.1371/journal.pone.0059565 |article-number=e59565 |pmid=23555709 |pmc=3608682 |bibcode=2013PLoSO...859565S |doi-access=free }}</ref> to animals like the blue whale, weighing up to 190 tonnes and measuring up to {{convert|33.6|meter}} long,<ref name="Wood-1983">{{cite book |last=Wood |first=Gerald |name-list-style=vanc |title=The Guinness Book of Animal Facts and Feats |year=1983 |isbn=978-0-85112-235-9 |url=https://archive.org/details/guinnessbookofan00wood |location=Enfield, Middlesex |publisher=Guinness World Records }}</ref> or plants like the coast redwood, up to {{convert|120|meter}} tall.<ref>{{cite web |url=https://www.conifers.org/cu/Sequoia.php |title=Sequoia sempervirens |work=The Gymnosperm Database |editor=Earle CJ |date=2017 |access-date=2017-09-15 |archive-date=2016-04-01 |archive-url=https://web.archive.org/web/20160401041103/http://www.conifers.org/cu/Sequoia.php |url-status=live }}</ref> Many eukaryotes are unicellular; the informal grouping called protists includes many of these, with some multicellular forms like the giant kelp up to {{convert|200|ft}} long.<ref name="van den Hoek 1995">{{cite book |last1=van den Hoek |first1=C. |last2=Mann |first2=D.G. |last3=Jahns |first3=H.M. |name-list-style=vanc |year=1995 |url=https://books.google.com/books?id=xuUoiFesSHMC |title=Algae An Introduction to Phycology |publisher=Cambridge University Press |location=Cambridge |isbn=0-521-30419-9 |access-date=7 April 2023 |archive-date=10 February 2023 |archive-url=https://web.archive.org/web/20230210172546/https://books.google.com/books?id=xuUoiFesSHMC |url-status=live }}</ref> The multicellular eukaryotes include the animals, plants, and fungi, but again, these groups too contain many unicellular species.<ref name="Burki-2014">{{cite journal |vauthors=Burki F |title=The eukaryotic tree of life from a global phylogenomic perspective |journal=Cold Spring Harbor Perspectives in Biology |volume=6 |issue=5 |article-number=a016147 |date=May 2014 |pmid=24789819 |doi=10.1101/cshperspect.a016147 |pmc=3996474 }}</ref> Eukaryotic cells are typically much larger than those of prokaryotes—the bacteria and the archaea—having a volume of around 10,000 times greater.<ref name="DeRennaux 2001">{{cite book |last=DeRennaux |first=B. |name-list-style=vanc |title=Encyclopedia of Biodiversity |chapter=Eukaryotes, Origin of |publisher=Elsevier |year=2001 |volume=2 |doi=10.1016/b978-0-12-384719-5.00174-x |pages=329–332|isbn=978-0-12-384720-1 }}</ref><ref>{{cite journal |title=Deep-sea microorganisms and the origin of the eukaryotic cell |url=http://protistology.jp/journal/jjp47/JJP47YAMAGUCHI.pdf |vauthors=Yamaguchi M, Worman CO |journal=Japanese Journal of Protozoology |volume=47 |number=1,2 |date=2014 |pages=29–48 |archive-url=https://web.archive.org/web/20170809103456/http://protistology.jp/journal/jjp47/JJP47YAMAGUCHI.pdf|archive-date=9 August 2017 }}</ref> 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.<ref>{{cite journal |last1=Bar-On |first1=Yinon M. |last2=Phillips |first2=Rob |last3=Milo |first3=Ron |date=2018-05-17 |title=The biomass distribution on Earth |journal=Proceedings of the National Academy of Sciences |volume=115 |issue=25 |pages=6506–6511 |doi=10.1073/pnas.1711842115 |pmid=29784790 |pmc=6016768|bibcode=2018PNAS..115.6506B |doi-access=free}}</ref>

The eukaryotes are a diverse lineage, consisting mainly of microscopic organisms.<ref name="Burki 2020">{{cite journal |last1=Burki |first1=Fabien |last2=Roger |first2=Andrew J. |last3=Brown |first3=Matthew W. |last4=Simpson |first4=Alastair G.B. |title=The New Tree of Eukaryotes |journal=Trends in Ecology & Evolution |volume=35 |issue=1 |date=2020 |doi=10.1016/j.tree.2019.08.008 |doi-access=free |pages=43–55 |pmid=31606140 |bibcode=2020TEcoE..35...43B |url=https://www.cell.com/article/S0169534719302575/pdf |access-date=16 October 2025}}</ref> Multicellularity in some form has evolved independently at least 25 times within the eukaryotes.<ref>{{cite journal |last1=Grosberg |first1=RK |last2=Strathmann |first2=RR |name-list-style=vanc |year=2007 |title=The evolution of multicellularity: A minor major transition? |journal=Annu Rev Ecol Evol Syst |volume=38 |pages=621–654 |doi=10.1146/annurev.ecolsys.36.102403.114735 |url=https://grosberglab.faculty.ucdavis.edu/wp-content/uploads/sites/453/2017/05/2007-Grosberg-R.-K.-and-R.-R.-Strathmann.pdf |access-date=8 April 2023 |archive-date=14 March 2023 |archive-url=https://web.archive.org/web/20230314222721/https://grosberglab.faculty.ucdavis.edu/wp-content/uploads/sites/453/2017/05/2007-Grosberg-R.-K.-and-R.-R.-Strathmann.pdf |url-status=live }}</ref><ref>{{cite journal |last1=Parfrey |first1=L.W. |last2=Lahr |first2=D.J.G. |name-list-style=vanc |year=2013 |title=Multicellularity arose several times in the evolution of eukaryotes |journal=BioEssays |volume=35 |issue=4 |pages=339–347 |doi=10.1002/bies.201200143 |pmid=23315654 |s2cid=13872783 |url=http://www.producao.usp.br/bitstream/handle/BDPI/45022/339_ftp.pdf?sequence=1&isAllowed=y |access-date=8 April 2023 |archive-date=25 July 2014 |archive-url=https://web.archive.org/web/20140725235332/http://www.producao.usp.br/bitstream/handle/BDPI/45022/339_ftp.pdf?sequence=1&isAllowed=y |url-status=live }}</ref> Complex multicellular organisms, not counting the aggregation of amoebae to form slime molds, have evolved within only six eukaryotic lineages: animals, symbiomycotan fungi, brown algae, red algae, green algae, and land plants.<ref>{{cite journal |last1=Popper |first1=Zoë A. |last2=Michel |first2=Gurvan |last3=Hervé |first3=Cécile |last4=Domozych |first4=David S. |last5=Willats |first5=William G.T. |last6=Tuohy |first6=Maria G. |last7=Kloareg |first7=Bernard |last8=Stengel |first8=Dagmar B. |name-list-style=vanc |year=2011 |title=Evolution and diversity of plant cell walls: From algae to flowering plants |journal=Annual Review of Plant Biology |volume=62 |issue=1 |pages=567–590 |pmid=21351878 |hdl=10379/6762 |hdl-access=free |s2cid=11961888 |doi=10.1146/annurev-arplant-042110-103809|bibcode=2011AnRPB..62..567P }}</ref> Eukaryotes are grouped by genomic similarities, so that groups often lack visible shared characteristics.<ref name="Burki 2020"/>

<gallery class="center" mode="nolines" widths="225" heights="225" caption="Eukaryotes range in size from single-celled organisms to huge trees and whales"> File:Gram-negative Bacteria and Paramecium forming cyst.jpg|Prokaryotes (small cylindrical cells, bacteria, on left) and a single-celled eukaryote, ''Paramecium'' File:California Redwood National Park (216450575).jpeg|Coast redwood File:Anim1754 - Flickr - NOAA Photo Library (rotated).jpg|Blue whale </gallery>

== Distinguishing features ==

=== Nucleus ===

The defining feature of eukaryotes is that their cells have a well-defined, membrane-bound nucleus, distinguishing them from prokaryotes that lack such a structure. 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;<ref>{{Cite journal |last1=Bonev |first1=B |last2=Cavalli |first2=G |name-list-style=vanc |date=14 October 2016 |title=Organization and function of the 3D genome |journal=Nature Reviews Genetics |volume=17 |issue=11 |pages=661–678 |doi=10.1038/nrg.2016.112 |pmid=27739532 |hdl=2027.42/151884 |s2cid=31259189 |hdl-access=free}}</ref> these are separated into two matching sets by a microtubular spindle during nuclear division, in the distinctively eukaryotic process of mitosis.<ref>{{Cite web |title=Chromosome Segregation: The Role of Centromeres |last=O'Connor |first=Clare |work=Nature Education |date= 2008|access-date=18 February 2024 |url= https://www.nature.com/scitable/topicpage/chromosome-segregation-in-mitosis-the-role-of-242/ |quote=eukar }}</ref>

=== Biochemistry ===

Eukaryotes differ from prokaryotes in multiple ways, with unique biochemical pathways such as sterane synthesis.<ref name="Brocks 1999"/> 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.<ref>{{cite journal |vauthors=Hartman H, Fedorov A |title=The origin of the eukaryotic cell: a genomic investigation |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=99 |issue=3 |pages=1420–5 |date=February 2002 |pmid=11805300 |pmc=122206 |doi=10.1073/pnas.032658599 |bibcode=2002PNAS...99.1420H |doi-access=free }}</ref>

=== Internal membranes ===

{{multiple image | image1 = Prokaryote cell.svg | width1 = 55 | caption1 = Prokaryote, to same scale | image2 = Endomembrane system diagram en (edit).svg | width2 = 330 | caption2 = Eukaryotic cell with endomembrane system | footer = Eukaryotic cells are some 10,000 times larger than prokaryotic cells by volume, and contain membrane-bound organelles. }}

Eukaryote cells include a variety of membrane-bound structures, together forming the endomembrane system.<ref>{{cite book |vauthors=Linka M, Weber AP |chapter=Evolutionary Integration of Chloroplast Metabolism with the Metabolic Networks of the Cells |veditors=Burnap RL, Vermaas WF |title=Functional Genomics and Evolution of Photosynthetic Systems |publisher=Springer |year=2011 |isbn=978-94-007-1533-2 |page=215 |chapter-url=https://books.google.com/books?id=WfzEgaLibuwC&pg=PA215 |access-date=27 October 2015 |archive-date=29 May 2016 |archive-url=https://web.archive.org/web/20160529130011/https://books.google.com/books?id=WfzEgaLibuwC&pg=PA215 |url-status=live }}</ref> Simple compartments, called 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.<ref>{{cite book |vauthors=Marsh M |title=Endocytosis |publisher=Oxford University Press |year=2001 |page=vii |isbn=978-0-19-963851-2}}</ref> Some cell products can leave in a vesicle through exocytosis.<ref>{{Cite journal |last1=Stalder |first1=Danièle |last2=Gershlick |first2=David C. |name-list-style=vanc |date=November 2020 |title=Direct trafficking pathways from the Golgi apparatus to the plasma membrane |journal=Seminars in Cell & Developmental Biology |volume=107 |pages=112–125 |doi=10.1016/j.semcdb.2020.04.001 |pmc=7152905 |pmid=32317144}}</ref>

The nucleus is surrounded by a double membrane known as the nuclear envelope, with nuclear pores that allow material to move in and out.<ref>{{cite journal |vauthors=Hetzer MW |title=The nuclear envelope |journal=Cold Spring Harbor Perspectives in Biology |volume=2 |issue=3 |article-number=a000539 |date=March 2010 |pmid=20300205 |pmc=2829960 |doi=10.1101/cshperspect.a000539 }}</ref> 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 ribosomes which synthesize proteins; these enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the smooth endoplasmic reticulum.<ref>{{cite web |title=Endoplasmic Reticulum (Rough and Smooth) |url=http://bscb.org/learning-resources/softcell-e-learning/endoplasmic-reticulum-rough-and-smooth/ |publisher=British Society for Cell Biology |access-date=12 November 2017 |archive-date=24 March 2019 |archive-url=https://web.archive.org/web/20190324055727/https://bscb.org/learning-resources/softcell-e-learning/endoplasmic-reticulum-rough-and-smooth/ |url-status=live }}</ref> In most eukaryotes, these protein-carrying vesicles are released and their contents further modified in stacks of flattened vesicles (cisternae), the Golgi apparatus.<ref>{{cite web |title=Golgi Apparatus |url=http://bscb.org/learning-resources/softcell-e-learning/golgi-apparatus/ |publisher=British Society for Cell Biology |access-date=12 November 2017 |archive-url=https://web.archive.org/web/20171113060218/http://bscb.org/learning-resources/softcell-e-learning/golgi-apparatus/ |archive-date=13 November 2017 }}</ref>

Vesicles may be specialized; for instance, lysosomes contain digestive enzymes that break down biomolecules in the cytoplasm.<ref>{{cite web |title=Lysosome |url=http://bscb.org/learning-resources/softcell-e-learning/lysosome/ |publisher=British Society for Cell Biology |access-date=12 November 2017 |archive-url=https://web.archive.org/web/20171113060214/http://bscb.org/learning-resources/softcell-e-learning/lysosome/ |archive-date=13 November 2017 }}</ref>

=== Mitochondria ===

[[File:Mitochondrion structure.svg|thumb |upright=1.4 |Mitochondria are essentially universal in the eukaryotes, and with their own DNA somewhat resemble prokaryotic cells.]]

Mitochondria are organelles in eukaryotic cells. The mitochondrion is commonly called "the powerhouse of the cell",<ref>{{Cite journal | vauthors = Siekevitz P | title = Powerhouse of the Cell | journal = Scientific American | volume = 197 | issue = 1 | pages = 131–144 | date = July 1957 | doi = 10.1038/scientificamerican0757-131 | url = https://www.scientificamerican.com/article/powerhouse-of-the-cell | bibcode = 1957SciAm.197a.131S | issn = 0036-8733 | url-access = subscription }}</ref> for its function providing energy by oxidizing sugars or fats to produce the energy-storing molecule ATP.<ref name="Voet-2006">{{cite book |vauthors=Voet D, Voet JC, Pratt CW |name-list-style=vanc |title=Fundamentals of Biochemistry |edition=2nd |publisher=John Wiley and Sons |year=2006 |pages=[https://archive.org/details/fundamentalsofbi00voet_0/page/547 547, 556] |isbn=978-0-471-21495-3 |url=https://archive.org/details/fundamentalsofbi00voet_0/page/547 }}</ref><ref>{{cite web |url=http://www.madsci.org/posts/archives/2006-05/1146679455.Ev.r.html |title=Re: Are there eukaryotic cells without mitochondria? |date=1 May 2006 |work=madsci.org |vauthors=Mack S |access-date=24 April 2014 |archive-date=24 April 2014 |archive-url=https://web.archive.org/web/20140424224700/http://www.madsci.org/posts/archives/2006-05/1146679455.Ev.r.html |url-status=live }}</ref> Mitochondria have two surrounding membranes, each a phospholipid bilayer, the inner of which is folded into invaginations called cristae where aerobic respiration takes place.<ref>{{cite journal |last1=Zick |first1=M |last2=Rabl |first2=R |last3=Reichert |first3=AS |name-list-style=vanc |title=Cristae formation-linking ultrastructure and function of mitochondria. |journal=Biochimica et Biophysica Acta (BBA) - Molecular Cell Research |date=January 2009 |volume=1793 |issue=1 |pages=5–19 |pmid=18620004 |doi=10.1016/j.bbamcr.2008.06.013 |doi-access=}}</ref>

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.<ref>{{cite book |vauthors=Watson J, Hopkins N, Roberts J, Steitz JA, Weiner A |title=Molecular Biology of the Gene |date=1988 |publisher=Benjamin Cummings |location=Menlo Park, California |isbn=978-0-8053-9614-0 |page=[https://archive.org/details/molecularbiology0004unse/page/1154 1154] |edition=Fourth |chapter=28: The Origins of Life |chapter-url=https://archive.org/details/molecularbiology0004unse/page/1154 }}</ref>

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.<ref name="Karnkowska 2016"/> They obtain energy by enzymatic action in the cytoplasm.<ref>{{cite web |url=http://www.iflscience.com/plants-and-animals/first-eukaryote-found-lack-mitochondria |title=Scientists Shocked To Discover Eukaryote With NO Mitochondria |date=13 May 2016 |vauthors=Davis JL |website=IFL Science |access-date=2016-05-13 |archive-url=https://web.archive.org/web/20190217214255/https://www.iflscience.com/plants-and-animals/first-eukaryote-found-lack-mitochondria/ |archive-date=17 February 2019 }}</ref><ref name="Karnkowska 2016">{{cite journal |vauthors=Karnkowska A, Vacek V, Zubáčová Z, Treitli SC, Petrželková R, Eme L, Novák L, Žárský V, Barlow LD, Herman EK, Soukal P, Hroudová M, Doležal P, Stairs CW, Roger AJ, Eliáš M, Dacks JB, Vlček Č, Hampl V |display-authors=3 |title=A Eukaryote without a Mitochondrial Organelle |journal=Current Biology |volume=26 |issue=10 |pages=1274–1284 |date=May 2016 |pmid=27185558 |doi=10.1016/j.cub.2016.03.053 |doi-access=free |bibcode=2016CBio...26.1274K |hdl=11104/0281161 |hdl-access=free }}</ref> It is thought that mitochondria developed from prokaryotic cells which became endosymbionts living inside eukaryotes.<ref>{{cite journal |vauthors = McCutcheon JP |title = The Genomics and Cell Biology of Host-Beneficial Intracellular Infections |journal = Annual Review of Cell and Developmental Biology |volume = 37 |issue = 1 |pages = 115–142 |date = October 2021 |pmid = 34242059 |doi = 10.1146/annurev-cellbio-120219-024122 |doi-access = free }}</ref>

=== Plastids ===

[[File:Chloroplast II (cropped).svg|thumb|upright=1.2|The most common type of plastid is the chloroplast, which contains chlorophyll and produces organic compounds by photosynthesis.]]

Plants and various groups of algae have plastids as well as mitochondria. Plastids, like mitochondria, have their own DNA and are developed from endosymbionts, in this case cyanobacteria. They usually take the form of chloroplasts which, like cyanobacteria, contain chlorophyll and produce organic compounds (such as glucose) through photosynthesis. Others are involved in storing food. Although plastids probably had a single origin, not all plastid-containing groups are closely related. Instead, some eukaryotes have obtained them from other eukaryotes through secondary endosymbiosis or ingestion.<ref>{{cite book |vauthors=Sato N |year=2006 |pages= 75–102 |title=The Structure and Function of Plastids |volume=23 |veditors=Wise RR, Hoober JK |publisher=Springer Netherlands |chapter=Origin and Evolution of Plastids: Genomic View on the Unification and Diversity of Plastids |isbn=978-1-4020-4060-3 |doi=10.1007/978-1-4020-4061-0_4 |series=Advances in Photosynthesis and Respiration}}</ref> The capture and sequestering of photosynthetic cells and chloroplasts, kleptoplasty, occurs in many types of modern eukaryotic organisms.<ref>{{cite journal |vauthors=Minnhagen S, Carvalho WF, Salomon PS, Janson S |title=Chloroplast DNA content in Dinophysis (Dinophyceae) from different cell cycle stages is consistent with kleptoplasty |journal=Environ. Microbiol. |volume=10 |issue=9 |pages=2411–7 |date=September 2008 |pmid=18518896 |doi=10.1111/j.1462-2920.2008.01666.x |bibcode=2008EnvMi..10.2411M }}</ref><ref name="Bodył 2018">{{cite journal |vauthors=Bodył A |s2cid=24613863 |title=Did some red alga-derived plastids evolve via kleptoplastidy? A hypothesis |journal=Biological Reviews of the Cambridge Philosophical Society |volume=93 |issue=1 |pages=201–222 |date=February 2018 |pmid=28544184 |doi=10.1111/brv.12340 }}</ref>

===Cytoskeletal structures===

[[File:FluorescentCells.jpg|thumb|The cytoskeleton. Actin filaments are shown in red, microtubules 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. These include α-actinin, fimbrin, and filamin in submembranous cortical layers and bundles. Motor proteins of microtubules, dynein and kinesin, and myosin of actin filaments, make the network dynamic.<ref>{{cite book |chapter=Molecular Motors |title=Molecular Biology of the Cell |edition=4th |url=https://www.ncbi.nlm.nih.gov/books/NBK26888/ |date=2002-01-01 |first1=Bruce |last1=Alberts |first2=Alexander |last2=Johnson |first3=Julian |last3=Lewis |first4=Martin |last4=Raff |first5=Keith |last5=Roberts |first6=Peter |last6=Walter |name-list-style=vanc |publisher=Garland Science |location=New York |isbn=978-0-8153-3218-3 |access-date=6 April 2023 |archive-date=8 March 2019 |archive-url=https://web.archive.org/web/20190308094109/https://www.ncbi.nlm.nih.gov/books/NBK26888/ |url-status=live }}</ref><ref name="Sweeney-2018">{{cite journal |vauthors=Sweeney HL, Holzbaur EL |title=Motor Proteins |journal=Cold Spring Harbor Perspectives in Biology |volume=10 |issue=5 |article-number= a021931|date=May 2018 |pmid=29716949 |pmc=5932582 |doi=10.1101/cshperspect.a021931 |url=}}</ref>

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.<ref name="Bardy-2003">{{cite journal |vauthors=Bardy SL, Ng SY, Jarrell KF |title=Prokaryotic motility structures |journal=Microbiology |volume=149 |issue=Pt 2 |pages=295–304 |date=February 2003 |pmid=12624192 |doi=10.1099/mic.0.25948-0 |doi-access=free }}</ref><ref name="Silflow-2001">{{cite journal |vauthors=Silflow CD, Lefebvre PA |title=Assembly and motility of eukaryotic cilia and flagella. Lessons from Chlamydomonas reinhardtii |journal=Plant Physiology |volume=127 |issue=4 |pages=1500–7 |date=December 2001 |pmid=11743094 |pmc=1540183 |doi=10.1104/pp.010807 |bibcode=2001PlanP.127.1500S }}</ref>

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.<ref>{{cite book |vauthors=Vorobjev IA, Nadezhdina ES |title=The Centrosome and Its Role in the Organization of Microtubules |volume=106 |pages=227–293 |year=1987 |pmid=3294718 |doi=10.1016/S0074-7696(08)61714-3 |isbn=978-0-12-364506-7 |series=International Review of Cytology }}</ref>

=== 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.<ref>{{cite book |vauthors=Howland JL |year=2000 |title=The Surprising Archaea: Discovering Another Domain of Life |pages=69–71 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-511183-5}}</ref>

The major polysaccharides making up the primary cell wall of 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.<ref>{{cite journal |vauthors=Fry SC |title=The Structure and Functions of Xyloglucan |journal=Journal of Experimental Botany |volume=40 |issue=1 |year=1989 |pages=1–11 |doi=10.1093/jxb/40.1.1}}</ref>

=== Sexual reproduction ===

[[File:Sexual cycle N-2N.svg|thumb|Sexual reproduction requires a life cycle that alternates between a haploid phase, with one copy of each chromosome in the cell, and a 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 sexual reproduction, alternating between a haploid phase, where only one copy of each chromosome is present in each cell, and a diploid phase, with two copies of each chromosome in each cell. The diploid phase is formed by fusion of two haploid gametes, such as eggs and 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.<ref>{{cite book |last=Hamilton |first=Matthew B. |name-list-style=vanc |title=Population genetics |url=https://archive.org/details/populationgeneti00hami |url-access=limited |year=2009 |publisher=Wiley-Blackwell |isbn=978-1-4051-3277-0 |page=[https://archive.org/details/populationgeneti00hami/page/n69 55]}}</ref> There is considerable variation in this pattern. Plants have both haploid and diploid multicellular phases.<ref>{{cite journal |last1=Taylor |first1=TN |last2=Kerp |first2=H |last3=Hass |first3=H |name-list-style=vanc |year=2005 |title=Life history biology of early land plants: Deciphering the gametophyte phase |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=102 |issue=16 |pages=5892–5897 |doi=10.1073/pnas.0501985102 |pmid=15809414 |pmc=556298 |doi-access=free |bibcode=2005PNAS..102.5892T }}</ref> 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.<ref name="Lane-2011">{{cite journal |vauthors=Lane N |author-link=Nick Lane |title=Energetics and genetics across the prokaryote-eukaryote divide |journal=Biology Direct |volume=6 |issue=1 |page=35 |date=June 2011 |pmid=21714941 |pmc=3152533 |doi=10.1186/1745-6150-6-35 |bibcode=2011BiDir...6...35L |doi-access=free }}</ref>

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.<ref>{{cite journal |vauthors=Dacks J, Roger AJ |s2cid=9441768 |title=The first sexual lineage and the relevance of facultative sex |journal=Journal of Molecular Evolution |volume=48 |issue=6 |pages=779–783 |date=June 1999 |pmid=10229582 |doi=10.1007/PL00013156 |bibcode=1999JMolE..48..779D }}</ref> 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.<ref name="Ramesh-2005">{{cite journal |vauthors=Ramesh MA, Malik SB, Logsdon JM |title=A phylogenomic inventory of meiotic genes; evidence for sex in Giardia and an early eukaryotic origin of meiosis |journal=Current Biology |volume=15 |issue=2 |pages=185–191 |date=January 2005 |pmid=15668177 |doi=10.1016/j.cub.2005.01.003 |s2cid=17013247 |doi-access=free |bibcode=2005CBio...15..185R }}</ref><ref name="Malik-2007">{{cite journal |vauthors=Malik SB, Pightling AW, Stefaniak LM, Schurko AM, Logsdon JM |title=An expanded inventory of conserved meiotic genes provides evidence for sex in Trichomonas vaginalis |journal=PLOS One |volume=3 |issue=8 |article-number=e2879 |date=August 2007 |pmid=18663385 |pmc=2488364 |doi=10.1371/journal.pone.0002879 |veditors=Hahn MW |bibcode=2008PLoSO...3.2879M |doi-access=free }}</ref> 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.<ref name="Ramesh-2005"/><ref name="Malik-2007"/> Species once thought to be asexual, such as ''Leishmania'' parasites, have a sexual cycle.<ref>{{cite journal |vauthors=Akopyants NS, Kimblin N, Secundino N, Patrick R, Peters N, Lawyer P, Dobson DE, Beverley SM, Sacks DL |title=Demonstration of genetic exchange during cyclical development of Leishmania in the sand fly vector |journal=Science |volume=324 |issue=5924 |pages=265–268 |date=April 2009 |pmid=19359589 |pmc=2729066 |doi=10.1126/science.1169464 |bibcode=2009Sci...324..265A }}</ref> Amoebae, previously regarded as asexual, may be anciently sexual; while present-day asexual groups could have arisen recently.<ref>{{cite journal |vauthors=Lahr DJ, Parfrey LW, Mitchell EA, Katz LA, Lara E |title=The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms |journal=Proceedings: Biological Sciences |volume=278 |issue=1715 |pages=2081–2090 |date=July 2011 |pmid=21429931 |pmc=3107637 |doi=10.1098/rspb.2011.0289 |bibcode=2011PBioS.278.2081L }}</ref>

== Evolution ==

=== History of classification ===

In antiquity, the two lineages of animals and plants were recognized by Aristotle and Theophrastus. The lineages were given the taxonomic rank of kingdom by 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.<ref name="Moore-1980">{{cite journal |vauthors=Moore RT |year=1980 |title=Taxonomic proposals for the classification of marine yeasts and other yeast-like fungi including the smuts |journal=Botanica Marina |volume=23 |issue=6 |pages=361–373|doi=10.1515/bot-1980-230605 |bibcode=1980BoMar..23..361M }}</ref> 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 ciliates,<ref>{{cite journal |author=Goldfuß |title=Ueber die Classification der Zoophyten |journal=Isis, Oder, Encyclopädische Zeitung von Oken |date=1818 |volume=2 |issue=6 |pages=1008–1019 |url=https://www.biodiversitylibrary.org/item/47614#page/530/mode/1up |trans-title=On the classification of zoophytes |language=de |access-date=15 March 2019 |archive-date=24 March 2019 |archive-url=https://web.archive.org/web/20190324105702/https://www.biodiversitylibrary.org/item/47614#page/530/mode/1up |url-status=live }} From p. 1008: ''"Erste Klasse. Urthiere. Protozoa."'' (First class. Primordial animals. Protozoa.) [Note: each column of each page of this journal is numbered; there are two columns per page.]</ref> and this group was expanded until Ernst Haeckel made it a kingdom encompassing all single-celled eukaryotes, the Protista, in 1866.<ref name="Scamardella-1999">{{cite journal |vauthors=Scamardella JM |title=Not plants or animals: a brief history of the origin of Kingdoms Protozoa, Protista and Protoctista |year=1999 |journal=International Microbiology |volume=2 |issue=4 |pages=207–221 |pmid=10943416 |url=http://www.im.microbios.org/08december99/03%20Scamardella.pdf |archive-url=https://web.archive.org/web/20110614000656/http://www.im.microbios.org/08december99/03%20Scamardella.pdf |archive-date=14 June 2011 }}</ref><ref name="Rothschild-1989">{{cite journal |vauthors=Rothschild LJ |title=Protozoa, Protista, Protoctista: what's in a name? |journal=Journal of the History of Biology |volume=22 |issue=2 |pages=277–305 |year=1989 |pmid=11542176 |doi=10.1007/BF00139515 |s2cid=32462158 |author-link=Lynn J. Rothschild |url=https://zenodo.org/record/1232387 |access-date=4 February 2020 |archive-date=4 February 2020 |archive-url=https://web.archive.org/web/20200204233203/https://zenodo.org/record/1232387 |url-status=live }}</ref><ref name="Whittaker-1969">{{cite journal |vauthors=Whittaker RH |title=New concepts of kingdoms or organisms. Evolutionary relations are better represented by new classifications than by the traditional two kingdoms |journal=Science |volume=163 |issue=3863 |pages=150–60 |date=January 1969 |pmid=5762760 |doi=10.1126/science.163.3863.150 |citeseerx=10.1.1.403.5430 |bibcode=1969Sci...163..150W }}</ref> The eukaryotes thus came to be seen as four kingdoms:

* Kingdom Protista * Kingdom Plantae * Kingdom Fungi * Kingdom Animalia

The protists were at that time thought to be "primitive forms", and thus an evolutionary grade, united by their primitive unicellular nature.<ref name="Rothschild-1989"/> 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".<ref name="Woese 1990">{{cite journal |vauthors=Woese CR, Kandler O, Wheelis ML |author1-link=Carl Woese |author2-link=Otto Kandler |author3-link=Mark Wheelis |title=Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=87 |issue=12 |pages=4576–4579 |date=June 1990 |pmid=2112744 |pmc=54159 |doi=10.1073/pnas.87.12.4576 |bibcode=1990PNAS...87.4576W |doi-access=free }}</ref><ref name="Knoll-1992">{{cite journal |last=Knoll |first=Andrew H. |name-list-style=vanc |author-link=Andrew H. Knoll |title=The Early Evolution of Eukaryotes: A Geological Perspective |journal=Science |volume=256 |issue=5057 |year=1992 |doi=10.1126/science.1585174 |pages=622–627 |pmid=1585174 |bibcode=1992Sci...256..622K |quote=Eucarya, or eukaryotes}}</ref> 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)".<ref name="Margulis 1996">{{cite journal |last=Margulis |first=Lynn |author-link=Lynn Margulis |name-list-style=vanc |title=Archaeal-eubacterial mergers in the origin of Eukarya: phylogenetic classification of life |journal=Proceedings of the National Academy of Sciences |volume=93 |issue=3 |date=6 February 1996 |doi=10.1073/pnas.93.3.1071 |pages=1071–1076 |pmid=8577716 |pmc=40032 |bibcode=1996PNAS...93.1071M |doi-access=free }}</ref>

=== Phylogeny ===

By the early 21st century, a rough consensus started to emerge from phylogenomic studies.<ref name="Burki-2014"/><ref name="Burki-2016">{{cite journal |vauthors=Burki F, Kaplan M, Tikhonenkov DV, Zlatogursky V, Minh BQ, Radaykina LV, Smirnov A, Mylnikov AP, Keeling PJ |display-authors=3 |title=Untangling the early diversification of eukaryotes: a phylogenomic study of the evolutionary origins of Centrohelida, Haptophyta and Cryptista |journal=Proceedings: Biological Sciences |volume=283 |issue=1823 |article-number=20152802 |date=January 2016 |pmid=26817772 |pmc=4795036 |doi=10.1098/rspb.2015.2802 |bibcode=2016PBioS.28352802B }}</ref><ref name="Keeling-2023">{{cite journal |last1=Keeling |first1=Patrick J. |last2=Eglit |first2=Yana |title=Openly available illustrations as tools to describe eukaryotic microbial diversity |journal=PLOS Biology |volume=21 |issue=11 |date=21 November 2023 |pmid=37988341 |pmc=10662721 |doi=10.1371/journal.pbio.3002395 |doi-access=free |article-number=e3002395}}</ref> 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.<ref name="Adl-2019">{{cite journal |vauthors=Adl SM, Bass D, Lane CE, Lukeš J, Schoch CL, Smirnov A, Agatha S, Berney C, Brown MW, Burki F, Cárdenas P, Čepička I, Chistyakova L, Del Campo J, Dunthorn M, Edvardsen B, Eglit Y, Guillou L, Hampl V, Heiss AA, Hoppenrath M, James TY, Karnkowska A, Karpov S, Kim E, Kolisko M, Kudryavtsev A, Lahr DJ, Lara E, Le Gall L, Lynn DH, Mann DG, Massana R, Mitchell EA, Morrow C, Park JS, Pawlowski JW, Powell MJ, Richter DJ, Rueckert S, Shadwick L, Shimano S, Spiegel FW, Torruella G, Youssef N, Zlatogursky V, Zhang Q |display-authors=3 |title=Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes |journal=The Journal of Eukaryotic Microbiology |volume=66 |issue=1 |pages=4–119 |date=January 2019 |pmid=30257078 |pmc=6492006 |doi=10.1111/jeu.12691 }}</ref> The proposed phylogeny below includes two groups of excavates (Discoba and Metamonada),<ref name="Brown-2018">{{Cite journal |last1=Brown |first1=Matthew W. |last2=Heiss |first2=Aaron A. |last3=Kamikawa |first3=Ryoma |last4=Inagaki |first4=Yuji |last5=Yabuki |first5=Akinori |last6=Tice |first6=Alexander K |last7=Shiratori |first7=Takashi |last8=Ishida |first8=Ken-Ichiro |last9=Hashimoto |first9=Tetsuo |last10=Simpson |first10=Alastair |last11=Roger |first11=Andrew |name-list-style=vanc |date=2018-01-19 |title=Phylogenomics Places Orphan Protistan Lineages in a Novel Eukaryotic Super-Group|journal=Genome Biology and Evolution |volume=10 |issue=2 |pages=427–433 |doi=10.1093/gbe/evy014 |pmc=5793813|pmid=29360967}}</ref> and incorporates the 2021 proposal that picozoans are close relatives of rhodophytes.<ref name="Schön-2021">{{cite journal |vauthors=Schön ME, Zlatogursky VV, Singh RP, Poirier C, Wilken S, Mathur V, Strassert JF, Pinhassi J, Worden AZ, Keeling PJ, Ettema TJ |display-authors=3 |title=Picozoa are archaeplastids without plastid |journal=Nature Communications |year=2021 |volume=12 |issue=1 |page=6651 |doi=10.1038/s41467-021-26918-0 |pmid=34789758 |pmc=8599508 |biorxiv=10.1101/2021.04.14.439778 |s2cid=233328713 |url=http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-189959 |access-date=20 December 2021 |archive-date=2 February 2024 |archive-url=https://web.archive.org/web/20240202091441/https://umu.diva-portal.org/smash/record.jsf?pid=diva2%3A1614928&dswid=-3028 |url-status=live }}</ref> The Provora are a group of microbial predators discovered in 2022.<ref name="Tikhonenkov-2022">{{cite journal |vauthors=Tikhonenkov DV, Mikhailov KV, Gawryluk RM, Belyaev AO, Mathur V, Karpov SA, Zagumyonnyi DG, Borodina AS, Prokina KI, Mylnikov AP, Aleoshin VV, Keeling PJ |display-authors=3 |title=Microbial predators form a new supergroup of eukaryotes |journal=Nature |date=December 2022 |volume=612 |issue=7941 |pages=714–719 |pmid=36477531 |doi=10.1038/s41586-022-05511-5 |bibcode=2022Natur.612..714T |s2cid=254436650 }}</ref> TSAR is a possible clade that would contain Telonemia and the SAR supergroup.<ref>{{cite journal |vauthors=Strassert JF, Jamy M, Mylnikov AP, Tikhonenkov DV, Burki F |title=New Phylogenomic Analysis of the Enigmatic Phylum Telonemia Further Resolves the Eukaryote Tree of Life |journal=Molecular Biology and Evolution |volume=36 |issue=4 |pages=757–765 |date=April 2019 |pmid=30668767 |pmc=6844682 |doi=10.1093/molbev/msz012 |veditors=Shapiro B }}</ref><ref>{{cite journal |last1=Yazaki |first1=Euki |last2=Yabuki |first2=Akinori |last3=Imaizumi |first3=Ayaka |last4=Kume |first4=Keitaro |last5=Hashimoto |first5=Tetsuo |last6=Inagaki |first6=Yuji|date=2022|title=The closest lineage of Archaeplastida is revealed by phylogenomics analyses that include Microheliella maris|journal=Open Biology |volume=12 |issue=4 |article-number=210376|doi=10.1098/rsob.210376|doi-access=free|pmid=35414259 |pmc=9006020}}</ref><ref>{{cite journal |last1=Torruella |first1=Guifré |last2=Galindo |first2=Luis Javier |last3=Moreira |first3=David |last4=López-García |first4=Purificación |title=Phylogenomics of neglected flagellated protists supports a revised eukaryotic tree of life |journal=Current Biology |date=27 August 2024 |volume=35 |issue=1 |pages=198–207.e4 |biorxiv=10.1101/2024.05.15.594285 |doi=10.1016/j.cub.2024.10.075 |pmid=39642877 |hdl=10481/102343 |hdl-access=free }}</ref>

=== Origin of eukaryotes ===

{{anchor|Eukaryogenesis|Origin of eukaryotes|Last common ancestor}} {{main|Eukaryogenesis}}

[[File:Symbiogenesis 2 mergers.svg|thumb|upright=1.5|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.<ref name="Latorre-2011"/>]]

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,<ref name="Gabaldón-2021">{{cite journal |vauthors=Gabaldón T |title=Origin and Early Evolution of the Eukaryotic Cell |journal=Annual Review of Microbiology |volume=75 |issue=1 |pages=631–647 |date=October 2021 |pmid=34343017 |doi=10.1146/annurev-micro-090817-062213 |s2cid=236916203 }}</ref> and was most likely a biological population, not a single individual.<ref name="O'Malley-2019">{{cite journal |vauthors=O'Malley MA, Leger MM, Wideman JG, Ruiz-Trillo I |title=Concepts of the last eukaryotic common ancestor |journal=Nature Ecology & Evolution |volume=3 |issue=3 |pages=338–344 |date=March 2019 |pmid=30778187 |doi=10.1038/s41559-019-0796-3 |bibcode=2019NatEE...3..338O |hdl-access=free |s2cid=67790751 |hdl=10261/201794 }}</ref> 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.<ref>{{cite journal |vauthors=Leander BS |title=Predatory protists |journal=Current Biology |volume=30 |issue=10 |pages=R510–R516 |date=May 2020 |pmid=32428491 |doi=10.1016/j.cub.2020.03.052 |s2cid=218710816 |doi-access=free |bibcode=2020CBio...30.R510L }}</ref><ref name="Strassert-2021">{{cite journal |vauthors=Strassert JF, Irisarri I, Williams TA, Burki F |title=A molecular timescale for eukaryote evolution with implications for the origin of red algal-derived plastids |journal=Nature Communications |volume=12 |issue=1 |article-number=1879 |date=March 2021 |pmid=33767194 |pmc=7994803 |doi=10.1038/s41467-021-22044-z |bibcode=2021NatCo..12.1879S |doi-access=free }}</ref><ref name="Koumandou-2013">{{cite journal |last1=Koumandou |first1=V. Lila |last2=Wickstead |first2=Bill |last3=Ginger |first3=Michael L. |last4=van der Giezen |first4=Mark |last5=Dacks |first5=Joel B. |last6=Field |first6=Mark C. |name-list-style=vanc |title=Molecular paleontology and complexity in the last eukaryotic common ancestor |journal=Critical Reviews in Biochemistry and Molecular Biology |volume=48 |issue=4 |year=2013 |doi=10.3109/10409238.2013.821444 |pages=373–396|pmid=23895660 |pmc=3791482 }}</ref>

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.<ref name="Latorre-2011">{{cite book |vauthors=Latorre A, Durban A, Moya A, Pereto J |chapter-url=https://books.google.com/books?id=m3oFebknu1cC&pg=PA326 |chapter=The role of symbiosis in eukaryotic evolution |title=Origins and Evolution of Life: An astrobiological perspective |veditors=Gargaud M, López-Garcìa P, Martin H |year=2011 |location=Cambridge |publisher=Cambridge University Press |pages=326–339 |isbn=978-0-521-76131-4 |access-date=27 August 2017 |archive-date=24 March 2019 |archive-url=https://web.archive.org/web/20190324055723/https://books.google.com/books?id=m3oFebknu1cC&pg=PA326 |url-status=live }}</ref>

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.<ref name="Rodrigues-Oliveira-2023">{{cite journal |vauthors=Rodrigues-Oliveira T, Wollweber F, Ponce-Toledo RI, etal. |title=Actin cytoskeleton and complex cell architecture in an Asgard archaean |journal=Nature |volume=613 |pages=332–339 |date=2023 |issue=7943 |doi=10.1038/s41586-022-05550-y |pmid=36544020 |pmc=9834061 |bibcode=2023Natur.613..332R |hdl=20.500.11850/589210 |hdl-access=free }}</ref>

=== 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.<ref>{{cite journal |vauthors=Bengtson S, Belivanova V, Rasmussen B, Whitehouse M |title=The controversial "Cambrian" fossils of the Vindhyan are real but more than a billion years older |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=106 |issue=19 |pages=7729–7734 |date=May 2009 |pmid=19416859 |pmc=2683128 |doi=10.1073/pnas.0812460106 |bibcode=2009PNAS..106.7729B |doi-access=free}}</ref>

Fossils from the Ruyang Group of China, dating to approximately 1.8-1.6 billion years ago, may be the oldest known eukaryotes.<ref>{{cite journal |last1=Fakhraee |first1=Mojtaba |last2=Tarhan |first2=Lidya G. |last3=Reinhard |first3=Christopher T. |last4=Crowe |first4=Sean A. |last5=Lyons |first5=Timothy W. |last6=Planavsky |first6=Noah J. |date=May 2023 |title=Earth's surface oxygenation and the rise of eukaryotic life: Relationships to the Lomagundi positive carbon isotope excursion revisited |journal=Earth-Science Reviews |volume=240 |article-number=104398 |doi=10.1016/j.earscirev.2023.104398 |bibcode=2023ESRv..24004398F |s2cid=257761993 |doi-access=free }}</ref> 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.<ref>{{cite journal |last1=Miao |first1=L. |last2=Yin |first2=Z. |last3=Knoll |first3=A. H. |last4=Qu |first4=Y. |last5=Zhu |first5=M. |title=1.63-billion-year-old multicellular eukaryotes from the Chuanlinggou Formation in North China |year=2024 |journal=Science Advances |volume=10 |issue=4 |article-number=eadk3208 |doi=10.1126/sciadv.adk3208 |doi-access=free |pmid=38266082 |pmc=10807817 |bibcode=2024SciA...10K3208M }}</ref>

[[File:Diskagma butonii.jpg|thumb|left|Reconstruction of the problematic<ref name="Retallack-2013"/> ''Diskagma buttonii'', a terrestrial fossil less than 1mm high, from rocks around 2.2 billion years old]]

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.<ref name="Han-1992">{{cite journal |vauthors=Han TM, Runnegar B |title=Megascopic eukaryotic algae from the 2.1-billion-year-old negaunee iron-formation, Michigan |journal=Science |volume=257 |issue=5067 |pages=232–5 |date=July 1992 |pmid=1631544 |doi=10.1126/science.1631544 |bibcode=1992Sci...257..232H}}</ref><ref>{{cite journal |vauthors=Knoll AH, Javaux EJ, Hewitt D, Cohen P |title=Eukaryotic organisms in Proterozoic oceans |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |volume=361 |issue=1470 |pages=1023–1038 |date=June 2006 |pmid=16754612 |pmc=1578724 |doi=10.1098/rstb.2006.1843 }}</ref> The "problematic"<ref name="Retallack-2013"/> fossil ''Diskagma'' has been found in paleosols 2.2 billion years old.<ref name="Retallack-2013">{{cite journal |vauthors=Retallack GJ, Krull ES, Thackray GD, Parkinson DH |title= Problematic urn-shaped fossils from a Paleoproterozoic (2.2 Ga) paleosol in South Africa. |journal=Precambrian Research |year=2013 |volume=235 |pages=71–87 |doi=10.1016/j.precamres.2013.05.015 |bibcode=2013PreR..235...71R }}</ref> The Neoarchean fossil ''Thuchomyces'' shares some similarities with fungi. It especially resembles the problematic fossil ''Diskagma'',<ref name="Retallack-2013"/> with hyphae and multiple differentiated layers.<ref>{{cite journal |last1=Hallbauer |first1=D. K. |last2=Jahns |first2=H. M. |last3=Beltmann |first3=H. A. |title=Morphological and anatomical observations on some precambrian plants from the Witwatersrand, South Africa |journal=Geologische Rundschau |date=December 1977 |volume=66 |issue=1 |pages=477–491 |doi=10.1007/BF01989590|bibcode=1977GeoRu..66..477H }}</ref> 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.<ref>{{cite journal |last1=Lücking |first1=Robert |last2=Nelsen |first2=Matthew P. |title=Ediacarans, Protolichens, and Lichen-Derived Penicillium |journal=Transformative Paleobotany |date=2018 |pages=551–590 |doi=10.1016/B978-0-12-813012-4.00023-1|isbn=978-0-12-813012-4 }}</ref>

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.<ref name="El Albani-2010">{{cite journal |vauthors=El Albani A, Bengtson S, Canfield DE, Bekker A, Macchiarelli R, Mazurier A, Hammarlund EU, Boulvais P, Dupuy JJ, Fontaine C, Fürsich FT, Gauthier-Lafaye F, Janvier P, Javaux E, Ossa FO, Pierson-Wickmann AC, Riboulleau A, Sardini P, Vachard D, Whitehouse M, Meunier A |display-authors=3 |s2cid=4331375 |title=Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago |journal=Nature |volume=466 |issue=7302 |pages=100–104 |date=July 2010 |pmid=20596019 |doi=10.1038/nature09166 |bibcode=2010Natur.466..100A }}</ref><ref name="El Albani-2023">{{cite journal |title=A search for life in Palaeoproterozoic marine sediments using Zn isotopes and geochemistry |last=El Albani |first=Abderrazak |journal=Earth and Planetary Science Letters |year=2023 |volume=623 |article-number=118169 |doi=10.1016/j.epsl.2023.118169|bibcode=2023E&PSL.61218169E |s2cid=258360867 |doi-access=free |url=https://hal.science/hal-04095643/file/El%20Albani%20et%20al._EPSL_2023.pdf }}</ref> However, the status of these structures as fossils is contested, with other authors suggesting that they might represent pseudofossils.<ref name="Ossa Ossa-2023">{{cite journal |last1=Ossa Ossa |first1=Frantz |last2=Pons |first2=Marie-Laure |last3=Bekker |first3=Andrey |last4=Hofmann |first4=Axel |last5=Poulton |first5=Simon W. |last6=Andersen |first6=Morten B. |last7=Agangi |first7=Andrea |last8=Gregory |first8=Daniel |last9=Reinke |first9=Christian |last10=Steinhilber |first10=Bernd |last11=Marin-Carbonne |first11=Johanna |last12=Schoenberg |first12=Ronny |display-authors=5 |title=Zinc enrichment and isotopic fractionation in a marine habitat of the c. 2.1 Ga Francevillian Group: A signature of zinc utilization by eukaryotes? |journal=Earth and Planetary Science Letters |volume=611 |date=2023 |doi=10.1016/j.epsl.2023.118147 |article-number=118147 |doi-access=free |bibcode=2023E&PSL.61118147O |url=https://eprints.whiterose.ac.uk/197720/8/1-s2.0-S0012821X23001607-main.pdf }}</ref>

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,<ref name="Brocks 1999">{{cite journal |vauthors=Brocks JJ, Logan GA, Buick R, Summons RE |title=Archean molecular fossils and the early rise of eukaryotes |journal=Science |volume=285 |issue=5430 |pages=1033–1036 |date=August 1999 |pmid=10446042 |doi=10.1126/science.285.5430.1033 |bibcode=1999Sci...285.1033B |citeseerx=10.1.1.516.9123 }}</ref><ref>{{cite magazine |vauthors=Ward P |title=Mass extinctions: the microbes strike back |magazine=New Scientist |pages=40–43 |date=9 February 2008 |url=https://www.newscientist.com/channel/life/mg19726421.900-mass-extinctions-the-microbes-strike-back.html |author-link=Peter Ward (paleontologist) |access-date=27 August 2017 |archive-date=8 July 2008 |archive-url=https://web.archive.org/web/20080708222803/http://www.newscientist.com/channel/life/mg19726421.900-mass-extinctions-the-microbes-strike-back.html |url-status=live }}</ref> but these Archaean biomarkers have been rebutted as later contaminants.<ref>{{cite journal |vauthors=French KL, Hallmann C, Hope JM, Schoon PL, Zumberge JA, Hoshino Y, Peters CA, George SC, Love GD, Brocks JJ, Buick R, Summons RE |title=Reappraisal of hydrocarbon biomarkers in Archean rocks |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=112 |issue=19 |pages=5915–5920 |date=May 2015 |pmid=25918387 |doi=10.1073/pnas.1419563112 |pmc=4434754 |bibcode=2015PNAS..112.5915F |doi-access=free }}</ref> The oldest valid biomarker records are only around 800 million years old.<ref>{{cite journal |vauthors=Brocks JJ, Jarrett AJ, Sirantoine E, Hallmann C, Hoshino Y, Liyanage T |title=The rise of algae in Cryogenian oceans and the emergence of animals |journal=Nature |volume=548 |issue=7669 |pages=578–581 |date=August 2017 |pmid=28813409 |doi=10.1038/nature23457 |s2cid=205258987 |bibcode=2017Natur.548..578B }}</ref> In contrast, a molecular clock analysis suggests the emergence of sterol biosynthesis as early as 2.3 billion years ago.<ref>{{cite journal |vauthors=Gold DA, Caron A, Fournier GP, Summons RE |title=Paleoproterozoic sterol biosynthesis and the rise of oxygen |journal=Nature |volume=543 |issue=7645 |pages=420–423 |date=March 2017 |pmid=28264195 |doi=10.1038/nature21412 |hdl-access=free |s2cid=205254122 |bibcode=2017Natur.543..420G |hdl=1721.1/128450 |url=https://resolver.caltech.edu/CaltechAUTHORS:20170407-083556533 }}</ref> The nature of steranes as eukaryotic biomarkers is further complicated by the production of sterols by some bacteria.<ref>{{cite journal |vauthors=Wei JH, Yin X, Welander PV |title=Sterol Synthesis in Diverse Bacteria |journal=Frontiers in Microbiology |volume=7 |page=990 |date=2016-06-24 |pmid=27446030 |pmc=4919349 |doi=10.3389/fmicb.2016.00990 |bibcode=2016FrMic...700990W |doi-access=free |author-link3=Paula V. Welander }}</ref><ref>{{cite journal |vauthors=Hoshino Y, Gaucher EA |title=Evolution of bacterial steroid biosynthesis and its impact on eukaryogenesis |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=118 |issue=25 |article-number=e2101276118 |date=June 2021 |pmid=34131078 |pmc=8237579 |doi=10.1073/pnas.2101276118 |bibcode=2021PNAS..11801276H |doi-access=free }}</ref>

Whenever their origins, eukaryotes may not have become ecologically dominant until much later; a massive increase in the zinc composition of marine sediments {{Ma|800}} 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).<ref name="Isson-2018">{{cite journal |vauthors=Isson TT, Love GD, Dupont CL, Reinhard CT, Zumberge AJ, Asael D, Gueguen B, McCrow J, Gill BC, Owens J, Rainbird RH, Rooney AD, Zhao MY, Stueeken EE, Konhauser KO, John SG, Lyons TW, Planavsky NJ |display-authors=3 |title=Tracking the rise of eukaryotes to ecological dominance with zinc isotopes |journal=Geobiology |volume=16|issue=4|pages=341–352|date=June 2018 |pmid=29869832 |doi=10.1111/gbi.12289 |bibcode=2018Gbio...16..341I |doi-access=free }}</ref>

== References ==

== External links ==

* [http://www.tolweb.org/Eukaryotes/3 "Eukaryotes"] {{Webarchive|url=https://web.archive.org/web/20120129074456/http://www.tolweb.org/Eukaryotes/3 |date=29 January 2012 }} (Tree of Life Web Project) * {{EOL}}

Category:Eukaryotes Category:Articles containing video clips Eukaryote Category:Biology terminology