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Eukaryogenesis, the process which created the eukaryotic cell and lineage, is a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. The process is widely agreed to have involved symbiogenesis, in which an archeon and a bacterium came together to create the first eukaryotic common ancestor (FECA). This cell had a new level of complexity and capability, with a nucleus, at least one centriole and cilium, facultatively aerobic mitochondria, sex (meiosis and syngamy), a dormant cyst with a cell wall of chitin and/or cellulose and peroxisomes. It evolved into a population of single-celled organisms that included the last eukaryotic common ancestor (LECA), gaining capabilities along the way, though the sequence of the steps involved has been disputed, and may not have started with symbiogenesis. In turn, the LECA gave rise to the eukaryotes' crown group, containing the ancestors of animals, fungi, plants, and a diverse range of single-celled organisms.

Context

Life arose on Earth once it had cooled enough for oceans to form. The last universal common ancestor (LUCA) was an organism which had ribosomes and the genetic code; it lived some 4 billion years ago. It gave rise to two main branches of prokaryotic life, the bacteria and the archaea. From among these small-celled, rapidly-dividing ancestors arose the Eukaryotes, with much larger cells, nuclei, and distinctive biochemistry.^[1]^[2] The eukaryotes form a domain that contains all complex cells and most types of multicellular organism, including the animals, plants, and fungi.^[3]^[4]

Symbiogenesis

According to the theory of symbiogenesis (also known as the endosymbiotic theory) championed by Lynn Margulis, a member of the archaea gained a bacterial cell as a component. The archaeal cell was a member of the Asgard group. The bacterium was one of the Alphaproteobacteria, which had the ability to use oxygen in its respiration. This enabled it – and the archaeal cells that included it – to survive in the presence of oxygen, which was poisonous to other organisms adapted to reducing conditions. The endosymbiotic bacteria became the eukaryotic cell's mitochondria, providing most of the energy of the cell.^[1]^[5] Lynn Margulis and colleagues have suggested that the cell also acquired a Spirochaete bacterium as a symbiont, providing the cell skeleton of microtubules and the ability to move, including the ability to pull chromosomes into two sets during mitosis, cell division.^[6] More recently, the Asgard archaean has been identified as belonging to the Heimdallarchaeota.^[7]

Last eukaryotic common ancestor (LECA)

The last eukaryotic common ancestor (LECA) is the hypothetical last common ancestor of all living eukaryotes, around 2 billion years ago,^[3]^[4] and was most likely a biological population.^[8] It is believed to have been a protist with a nucleus, at least one centriole and cilium, facultatively aerobic mitochondria, sex (meiosis and syngamy), a dormant cyst with a cell wall of chitin and/or cellulose, and peroxisomes.^[9]^[10]

It had been proposed that the LECA fed by phagocytosis, engulfing other organisms.^[9]^[10] However, in 2022, Nico Bremer and colleagues confirmed that the LECA had mitochondria, and stated that it had multiple nuclei, but disputed that it was phagotrophic. This would mean that the ability found in many eukaryotes to engulf materials developed later, rather than being acquired first and then used to engulf the alphaproteobacteria that became mitochondria.^[11]

The LECA has been described as having "spectacular cellular complexity".^[12] Its cell was divided into compartments.^[12] It appears to have inherited a set of endosomal sorting complex proteins that enable membranes to be remodelled, including pinching off vesicles to form endosomes.^[13] Its apparatuses for transcribing DNA into RNA, and then for translating the RNA into proteins, were separated, permitting extensive RNA processing and allowing the expression of genes to become more complex.^[14] It had mechanisms for reshuffling its genetic material, and possibly for manipulating its own evolvability. All of these gave the LECA "a compelling cohort of selective advantages".^[12]

Eukaryotic sex

Sex in eukaryotes is a composite process, consisting of meiosis and fertilisation, which can be coupled to reproduction.^[15] Dacks and Roger^[16] proposed on the basis of a phylogenetic analysis that facultative sex was likely present in the common ancestor of all eukaryotes. Early in eukaryotic evolution, about 2 billion years ago, organisms needed a solution to the major problem that oxidative metabolism releases reactive oxygen species that damage the genetic material, DNA.^[15] Eukaryotic sex provides a process, homologous recombination during meiosis, for using informational redundancy to repair such DNA damage.^[15]

Scenarios

Biologists have proposed multiple scenarios for the creation of the eukaryotes. While there is broad agreement that the LECA must have had a nucleus, mitochondria, and internal membranes, the order in which these were acquired has been disputed. In the syntrophic model, the first eukaryotic common ancestor (FECA, around 2.2 gya) gained mitochondria, then membranes, then a nucleus. In the phagotrophic model, it gained a nucleus, then membranes, then mitochondria. In a more complex process, it gained all three in short order, then other capabilities. Other models have been proposed. Whatever happened, many lineages must have been created, but the LECA either out-competed or came together with the other lineages to form a single point of origin for the eukaryotes.^[12] Nick Lane and William Martin have argued that mitochondria came first, on the grounds that energy had been the limiting factor on the size of the prokaryotic cell.^[17] The phagotrophic model presupposes the ability to engulf food, enabling the cell to engulf the aerobic bacterium that became the mitochondrion.^[12]

Eugene Koonin and others, noting that the archaea share many features with eukaryotes, argue that rudimentary eukaryotic traits such as membrane-lined compartments were acquired before endosymbiosis added mitochondria to the early eukaryotic cell, while the cell wall was lost. In the same way, mitochondrial acquisition must not be regarded as the end of the process, for still new complex families of genes had to be developed after or during the endosymbiotic exchange. In this way, from FECA to LECA, we can think of organisms that can be considered as protoeukaryotes. At the end of the process, LECA was already a complex organism with the presence of protein families involved in cellular compartmentalization.^[18]^[19]

Diversification: crown eukaryotes

In turn, the LECA gave rise to the eukaryotes' crown group, containing the ancestors of animals, fungi, plants, and a diverse range of single-celled organisms with the new capabilities and complexity of the eukaryotic cell.^[20]^[21] Single cells without cell walls are fragile and have a low probability of being fossilised. If fossilised, they have few features to distinguish them clearly from prokaryotes: size, morphological complexity, and (eventually) multicellularity. Early eukaryote fossils, from the late Paleoproterozoic, include acritarch microfossils with relatively robust ornate carbonaceous vesicles of Tappania from 1.63 gya and Shuiyousphaeridium from 1.8 gya.^[21]

References

^ ^a ^b ^c McGrath, Casey (31 May 2022). "Highlight: Unraveling the Origins of LUCA and LECA on the Tree of Life". Genome Biology and Evolution. 14 (6): evac072. doi:10.1093/gbe/evac072. PMC 9168435.
^ Weiss, Madeline C.; Sousa, F. L.; Mrnjavac, N.; et al. (2016). "The physiology and habitat of the last universal common ancestor" (PDF). Nature Microbiology. 1 (9): 16116. doi:10.1038/nmicrobiol.2016.116. PMID 27562259. S2CID 2997255.
^ ^a ^b Gabaldón, T. (October 2021). "Origin and Early Evolution of the Eukaryotic Cell". Annual Review of Microbiology. 75 (1): 631–647. doi:10.1146/annurev-micro-090817-062213. PMID 34343017. S2CID 236916203.
^ ^a ^b Woese, C.R.; Kandler, Otto; Wheelis, Mark L. (June 1990). "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya". Proceedings of the National Academy of Sciences of the United States of America. 87 (12): 4576–4579. Bibcode:1990PNAS...87.4576W. doi:10.1073/pnas.87.12.4576. PMC 54159. PMID 2112744.
^ ^a ^b Latorre, A.; Durban, A; Moya, A.; Pereto, J. (2011). "The role of symbiosis in eukaryotic evolution". In Gargaud, Muriel; López-Garcìa, Purificacion; Martin H. (eds.). Origins and Evolution of Life: An astrobiological perspective. Cambridge: Cambridge University Press. pp. 326–339. ISBN 978-0-521-76131-4. Archived from the original on 24 March 2019. Retrieved 27 August 2017.
^ Margulis, Lynn; Chapman, Michael; Guerrero, Ricardo; Hall, John (29 August 2006). "The last eukaryotic common ancestor (LECA): Acquisition of cytoskeletal motility from aerotolerant spirochetes in the Proterozoic Eon". Proceedings of the National Academy of Sciences. 103 (35): 13080–13085. Bibcode:2006PNAS..10313080M. doi:10.1073/pnas.0604985103. PMC 1559756. PMID 16938841.
^ Williams, Tom A.; Cox, Cymon J.; Foster, Peter G.; Szöllősi, Gergely J.; Embley, T. Martin (9 December 2019). "Phylogenomics provides robust support for a two-domains tree of life". Nature Ecology & Evolution. 4 (1): 138–147. Bibcode:2019NatEE...4..138W. doi:10.1038/s41559-019-1040-x. PMC 6942926. PMID 31819234.
^ O'Malley, Maureen A.; Leger, Michelle M.; Wideman, Jeremy G.; Ruiz-Trillo, Iñaki (18 February 2019). "Concepts of the last eukaryotic common ancestor". Nature Ecology & Evolution. 3 (3): 338–344. Bibcode:2019NatEE...3..338O. doi:10.1038/s41559-019-0796-3. hdl:10261/201794. PMID 30778187. S2CID 256718457.
^ ^a ^b Leander, B. S. (May 2020). "Predatory protists". Current Biology. 30 (10): R510–R516. doi:10.1016/j.cub.2020.03.052. PMID 32428491. S2CID 218710816.
^ ^a ^b Strassert, Jürgen F. H.; Irisarri, Iker; Williams, Tom A.; Burki, Fabien (25 March 2021). "A molecular timescale for eukaryote evolution with implications for the origin of red algal-derived plastids". Nature Communications. 12 (1): 1879. Bibcode:2021NatCo..12.1879S. doi:10.1038/s41467-021-22044-z. PMC 7994803. PMID 33767194.
^ Bremer, Nico; Tria, Fernando D. K.; Skejo, Josip; Garg, Sriram G.; Martin, William F. (31 May 2022). "Ancestral State Reconstructions Trace Mitochondria But Not Phagocytosis to the Last Eukaryotic Common Ancestor". Genome Biology and Evolution. 14 (6). doi:10.1093/gbe/evac079. PMC 9185374. PMID 35642316.
^ ^a ^b ^c ^d ^e Koumandou, V. Lila; Wickstead, Bill; Ginger, Michael L.; van der Giezen, Mark; Dacks, Joel B.; Field, Mark C. (2013). "Molecular paleontology and complexity in the last eukaryotic common ancestor". Critical Reviews in Biochemistry and Molecular Biology. 48 (4): 373–396. doi:10.3109/10409238.2013.821444. PMC 3791482. PMID 23895660.
^ Makarova, Kira S.; Yutin, Natalya; Bell, Stephen D.; Koonin, Eugene V. (6 September 2010). "Evolution of diverse cell division and vesicle formation systems in Archaea". Nature Reviews Microbiology. 8 (10): 731–741. doi:10.1038/nrmicro2406. PMC 3293450. PMID 20818414.
^ Martin, William; Koonin, Eugene V. (2006). "Introns and the origin of nucleus–cytosol compartmentalization". Nature. 440 (7080): 41–45. doi:10.1038/nature04531. ISSN 0028-0836.
^ ^a ^b ^c Horandl, E.; Speijer, D. (7 February 2018). "How oxygen gave rise to eukaryotic sex". Proceedings of the Royal Society B: Biological Sciences. 285 (1872). The Royal Society. doi:10.1098/rspb.2017.2706. PMC 5829205. PMID 29436502.
^ Dacks, J.; Roger, A. J. (1999). "The first sexual lineage and the relevance of facultative sex". Journal of Molecular Evolution. 48 (6): 779–783. Bibcode:1999JMolE..48..779D. doi:10.1007/pl00013156. PMID 10229582. S2CID 9441768.
^ Lane, Nick; Martin, William F. (2010). "The energetics of genome complexity". Nature. 467 (7318): 929–934. Bibcode:2010Natur.467..929L. doi:10.1038/nature09486. PMID 20962839. S2CID 17086117.
^ Koonin, Eugene V. (March 2005). "The incredible expanding ancestor of eukaryotes". Cell. 140 (5): 606–608. doi:10.1016/j.cell.2010.02.022. PMC 3293451. PMID 20211127.
^ Martijn, J.; Ettema, T.J.G. (February 2013). "From archaeon to eukaryote: the evolutionary dark ages of the eukaryotic cell". Biochem Soc Trans. 41 (1): 451–7. doi:10.1042/BST20120292. PMID 23356327.
^ Van de Peer, Yves; Baldaufrid, Sandra L.; Doolittle, W. Ford; Meyerid, Axel (2000). "An Updated and Comprehensive rRNA Phylogeny of (Crown) Eukaryotes Based on Rate-Calibrated Evolutionary Distances". Journal of Molecular Evolution. 51 (6): 565–576. Bibcode:2000JMolE..51..565V. doi:10.1007/s002390010120. PMID 11116330. S2CID 9400485.
^ ^a ^b Butterfield, N.J. (2015). "Early evolution of the Eukaryota". Palaeontology. 58 (1): 5–17. Bibcode:2015Palgy..58....5B. doi:10.1111/pala.12139.

External links

Attraction and sex among our microbial Last Eukaryotic Common Ancestors, The Atlantic, November 11, 2020

[McGrath_2022-1] McGrath, Casey (31 May 2022). "Highlight: Unraveling the Origins of LUCA and LECA on the Tree of Life". Genome Biology and Evolution. 14 (6): evac072. doi:10.1093/gbe/evac072. PMC 9168435.

[Weiss_et_al_2016-2] Weiss, Madeline C.; Sousa, F. L.; Mrnjavac, N.; et al. (2016). "The physiology and habitat of the last universal common ancestor" (PDF). Nature Microbiology. 1 (9): 16116. doi:10.1038/nmicrobiol.2016.116. PMID 27562259. S2CID 2997255.

[Gabaldón-3] Gabaldón, T. (October 2021). "Origin and Early Evolution of the Eukaryotic Cell". Annual Review of Microbiology. 75 (1): 631–647. doi:10.1146/annurev-micro-090817-062213. PMID 34343017. S2CID 236916203.

[w1990-4] Woese, C.R.; Kandler, Otto; Wheelis, Mark L. (June 1990). "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya". Proceedings of the National Academy of Sciences of the United States of America. 87 (12): 4576–4579. Bibcode:1990PNAS...87.4576W. doi:10.1073/pnas.87.12.4576. PMC 54159. PMID 2112744.

[latorre-5] Latorre, A.; Durban, A; Moya, A.; Pereto, J. (2011). "The role of symbiosis in eukaryotic evolution". In Gargaud, Muriel; López-Garcìa, Purificacion; Martin H. (eds.). Origins and Evolution of Life: An astrobiological perspective. Cambridge: Cambridge University Press. pp. 326–339. ISBN 978-0-521-76131-4. Archived from the original on 24 March 2019. Retrieved 27 August 2017.

[Margulis_Chapman_Guerrero_Hall_2006-6] Margulis, Lynn; Chapman, Michael; Guerrero, Ricardo; Hall, John (29 August 2006). "The last eukaryotic common ancestor (LECA): Acquisition of cytoskeletal motility from aerotolerant spirochetes in the Proterozoic Eon". Proceedings of the National Academy of Sciences. 103 (35): 13080–13085. Bibcode:2006PNAS..10313080M. doi:10.1073/pnas.0604985103. PMC 1559756. PMID 16938841.

[Williams_Cox_Foster_Szöllősi_2019_pp._138–147-7] Williams, Tom A.; Cox, Cymon J.; Foster, Peter G.; Szöllősi, Gergely J.; Embley, T. Martin (9 December 2019). "Phylogenomics provides robust support for a two-domains tree of life". Nature Ecology & Evolution. 4 (1): 138–147. Bibcode:2019NatEE...4..138W. doi:10.1038/s41559-019-1040-x. PMC 6942926. PMID 31819234.

[O'Malley_Leger_Wideman_Ruiz-Trillo_2019-8] O'Malley, Maureen A.; Leger, Michelle M.; Wideman, Jeremy G.; Ruiz-Trillo, Iñaki (18 February 2019). "Concepts of the last eukaryotic common ancestor". Nature Ecology & Evolution. 3 (3): 338–344. Bibcode:2019NatEE...3..338O. doi:10.1038/s41559-019-0796-3. hdl:10261/201794. PMID 30778187. S2CID 256718457.

[Leander_R510–R516-9] Leander, B. S. (May 2020). "Predatory protists". Current Biology. 30 (10): R510–R516. doi:10.1016/j.cub.2020.03.052. PMID 32428491. S2CID 218710816.

[Strassert_Irisarri_Williams_Burki_2021-10] Strassert, Jürgen F. H.; Irisarri, Iker; Williams, Tom A.; Burki, Fabien (25 March 2021). "A molecular timescale for eukaryote evolution with implications for the origin of red algal-derived plastids". Nature Communications. 12 (1): 1879. Bibcode:2021NatCo..12.1879S. doi:10.1038/s41467-021-22044-z. PMC 7994803. PMID 33767194.

[Bremer_Tria_Skejo_Garg_2022-11] Bremer, Nico; Tria, Fernando D. K.; Skejo, Josip; Garg, Sriram G.; Martin, William F. (31 May 2022). "Ancestral State Reconstructions Trace Mitochondria But Not Phagocytosis to the Last Eukaryotic Common Ancestor". Genome Biology and Evolution. 14 (6). doi:10.1093/gbe/evac079. PMC 9185374. PMID 35642316.

[Koumandou_Wickstead_Ginger_van_der_Giezen_2013-12] Koumandou, V. Lila; Wickstead, Bill; Ginger, Michael L.; van der Giezen, Mark; Dacks, Joel B.; Field, Mark C. (2013). "Molecular paleontology and complexity in the last eukaryotic common ancestor". Critical Reviews in Biochemistry and Molecular Biology. 48 (4): 373–396. doi:10.3109/10409238.2013.821444. PMC 3791482. PMID 23895660.

[Makarova_Yutin_Bell_Koonin_2010-13] Makarova, Kira S.; Yutin, Natalya; Bell, Stephen D.; Koonin, Eugene V. (6 September 2010). "Evolution of diverse cell division and vesicle formation systems in Archaea". Nature Reviews Microbiology. 8 (10): 731–741. doi:10.1038/nrmicro2406. PMC 3293450. PMID 20818414.

[Martin_Koonin_2006_pp._41–45-14] Martin, William; Koonin, Eugene V. (2006). "Introns and the origin of nucleus–cytosol compartmentalization". Nature. 440 (7080): 41–45. doi:10.1038/nature04531. ISSN 0028-0836.

[Horandl_2018-15] Horandl, E.; Speijer, D. (7 February 2018). "How oxygen gave rise to eukaryotic sex". Proceedings of the Royal Society B: Biological Sciences. 285 (1872). The Royal Society. doi:10.1098/rspb.2017.2706. PMC 5829205. PMID 29436502.

[16] Dacks, J.; Roger, A. J. (1999). "The first sexual lineage and the relevance of facultative sex". Journal of Molecular Evolution. 48 (6): 779–783. Bibcode:1999JMolE..48..779D. doi:10.1007/pl00013156. PMID 10229582. S2CID 9441768.

[Lane_Martin_2010-17] Lane, Nick; Martin, William F. (2010). "The energetics of genome complexity". Nature. 467 (7318): 929–934. Bibcode:2010Natur.467..929L. doi:10.1038/nature09486. PMID 20962839. S2CID 17086117.

[Koonin_2005-18] Koonin, Eugene V. (March 2005). "The incredible expanding ancestor of eukaryotes". Cell. 140 (5): 606–608. doi:10.1016/j.cell.2010.02.022. PMC 3293451. PMID 20211127.

[19] Martijn, J.; Ettema, T.J.G. (February 2013). "From archaeon to eukaryote: the evolutionary dark ages of the eukaryotic cell". Biochem Soc Trans. 41 (1): 451–7. doi:10.1042/BST20120292. PMID 23356327.

[Van_de_Peer_Baldaufrid_Doolittle_Meyerid_2000-20] Van de Peer, Yves; Baldaufrid, Sandra L.; Doolittle, W. Ford; Meyerid, Axel (2000). "An Updated and Comprehensive rRNA Phylogeny of (Crown) Eukaryotes Based on Rate-Calibrated Evolutionary Distances". Journal of Molecular Evolution. 51 (6): 565–576. Bibcode:2000JMolE..51..565V. doi:10.1007/s002390010120. PMID 11116330. S2CID 9400485.

[Butterfield_2015-21] Butterfield, N.J. (2015). "Early evolution of the Eukaryota". Palaeontology. 58 (1): 5–17. Bibcode:2015Palgy..58....5B. doi:10.1111/pala.12139.

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+{{Short description|Process of forming the first eukaryotic cell}}
-[[File:LUCA and LECA McGrath 2022.jpg|thumb|upright=2|LUCA and LECA: the origins of the [[eukaryote]]s.<ref name="McGrath 2022"/> The point of fusion (marked "?") below LECA is the FECA, the first eukaryotic common ancestor, some 2.2 billion years ago. Much earlier, some 4 billion years ago, the [[Last universal common ancestor|LUCA]] gave rise to the two domains of [[prokaryote]]s, the [[bacteria]] and the [[archaea]].]]
+{{Use British English|date=April 2023}}
+{{Use dmy dates|date=April 2023}}
+[[File:LUCA and LECA McGrath 2022.jpg|thumb|upright=2|LUCA and LECA: the origins of the [[eukaryote]]s.<ref name="McGrath 2022"/> The point of fusion (marked "?") below LECA is the FECA, the first eukaryotic common ancestor, some 2.2 billion years ago. Much earlier, some 4 billion years ago, the [[Last universal common ancestor|LUCA]] gave rise to the two domains of [[prokaryote]]s, the [[bacteria]] and the [[archaea]]. After the LECA, some 2 billion years ago, the eukaryotes diversified into a crown group, which gave rise to animals, plants, fungi, and protists.]]
-'''Eukaryogenesis''', the process which created the [[Eukaryote|eukaryotic]] cell and lineage, is a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. The process is widely agreed to have involved [[symbiogenesis]], in which [[archaea]] and [[bacteria]] came together to create the '''first eukaryotic common ancestor''' ('''FECA'''), a cell with a new level of complexity and capability. It evolved into a population of single-celled organisms that included the '''last eukaryotic common ancestor''' ('''LECA'''), gaining capabilities along the way, though the sequence of the steps involved has been disputed.
+'''Eukaryogenesis''', the process which created the [[Eukaryote|eukaryotic]] cell and lineage, is a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. The process is widely agreed to have involved [[symbiogenesis]], in which an [[archaea|archeon]] and a [[bacteria|bacterium]] came together to create the '''first eukaryotic common ancestor''' ('''FECA'''). This cell had a new level of complexity and capability, with a nucleus, at least one [[centriole]] and [[cilium]], facultatively aerobic [[Mitochondrion|mitochondria]], sex ([[meiosis]] and [[syngamy]]), a dormant [[cyst]] with a cell wall of [[chitin]] and/or [[cellulose]] and [[peroxisome]]s. It evolved into a population of single-celled organisms that included the '''last eukaryotic common ancestor''' ('''LECA'''), gaining capabilities along the way, though the sequence of the steps involved has been disputed, and may not have started with symbiogenesis. In turn, the LECA gave rise to the eukaryotes' [[crown group]], containing the ancestors of [[animal]]s, [[Fungus|fungi]], [[plant]]s, and a diverse range of single-celled organisms.
 == Context ==
@@ Line 7: / Line 10: @@
 {{further|Abiogenesis|Last universal common ancestor|last eukaryotic common ancestor}}
-[[Abiogenesis|Life arose on Earth]] once it had cooled enough for oceans to form. The [[last universal common ancestor]] (LUCA) was an [[organism]] which had [[ribosome]]s and the [[genetic code]]; it lived some 4 billion years ago. It gave rise to two main branches of [[Prokaryote|prokaryotic]] life, the [[bacteria]] and the [[archaea]]. From among these small-celled, rapidly-dividing ancestors arose the Eukaryotes, with much larger cells, nuclei, and distinctive biochemistry.<ref name="McGrath 2022">{{cite journal | last=McGrath | first=Casey | title=Highlight: Unraveling the Origins of LUCA and LECA on the Tree of Life | journal=Genome Biology and Evolution | volume=14 | issue=6 | date=31 May 2022 | doi=10.1093/gbe/evac072}}</ref><ref name="Weiss et al 2016">{{cite journal |last1=Weiss |first1=Madeline C. |last2=Sousa |first2=F. L. |last3=Mrnjavac |first3=N. |last4=Neukirchen |first4=S. |last5=Roettger |first5=M. |last6=Nelson-Sathi |first6=S. |last7=Martin |first7=William F. |author7-link=William F. Martin |display-authors=3 |s2cid=2997255 |year=2016 |title=The physiology and habitat of the last universal common ancestor |journal=Nature Microbiology |volume=1 |issue=9 |page=16116 |doi=10.1038/nmicrobiol.2016.116 |pmid=27562259 |url=http://complexityexplorer.s3.amazonaws.com/supplemental_materials/3.6+Early+Metabolisms/Weiss_et_al_Nat_Microbiol_2016.pdf }}</ref> The eukaryotes form a [[Domain (biology)|domain]] that contains all complex cells and most types of [[multicellular organism]], including the [[animal]]s, [[plant]]s, and [[Fungus|fungi]].<ref name="Gabaldón">{{cite journal |last=Gabaldón |first=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><ref name="w1990">{{cite journal |last1=Woese |first1=C.R. |author1-link=Carl Woese |last2=Kandler |first2=O. |last3=Wheelis |first3=M.L. |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>
+[[Abiogenesis|Life arose on Earth]] once it had cooled enough for oceans to form. The [[last universal common ancestor]] (LUCA) was an [[organism]] which had [[ribosome]]s and the [[genetic code]]; it lived some 4 billion years ago. It gave rise to two main branches of [[Prokaryote|prokaryotic]] life, the [[bacteria]] and the [[archaea]]. From among these small-celled, rapidly-dividing ancestors arose the Eukaryotes, with much larger cells, nuclei, and distinctive biochemistry.<ref name="McGrath 2022">{{cite journal |last=McGrath |first=Casey |title=Highlight: Unraveling the Origins of LUCA and LECA on the Tree of Life |journal=Genome Biology and Evolution |volume=14 |issue=6 |date=31 May 2022 |pages=evac072 |doi=10.1093/gbe/evac072|pmc=9168435 }}</ref><ref name="Weiss et al 2016">{{cite journal |last1=Weiss |first1=Madeline C. |last2=Sousa |first2=F. L. |last3=Mrnjavac |first3=N. |last4=Neukirchen |first4=S. |last5=Roettger |first5=M. |last6=Nelson-Sathi |first6=S. |last7=Martin |first7=William F. |author7-link=William F. Martin |display-authors=3 |s2cid=2997255 |year=2016 |title=The physiology and habitat of the last universal common ancestor |journal=Nature Microbiology |volume=1 |issue=9 |page=16116 |doi=10.1038/nmicrobiol.2016.116 |pmid=27562259 |url=http://complexityexplorer.s3.amazonaws.com/supplemental_materials/3.6+Early+Metabolisms/Weiss_et_al_Nat_Microbiol_2016.pdf }}</ref> The eukaryotes form a [[Domain (biology)|domain]] that contains all complex cells and most types of [[multicellular organism]], including the [[animal]]s, [[plant]]s, and [[Fungus|fungi]].<ref name="Gabaldón">{{cite journal |last=Gabaldón |first=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><ref name="w1990">{{cite journal |last1=Woese |first1=C.R. |author1-link=Carl Woese |last2=Kandler |first2=Otto |author2-link=Otto Kandler |last3=Wheelis |first3=Mark L. |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>
 == Symbiogenesis ==
-{{main|Symbiogenesis}}
+{{further|Symbiogenesis}}
+[[File:Symbiogenesis 2 mergers.svg|thumb|upright=1.8|In the theory of [[symbiogenesis]], a merger of an [[archaea]]n and an aerobic bacterium created the eukaryotes, with aerobic [[Mitochondrion|mitochondria]], some 2.2 billion years ago. A second merger, 1.6 billion years ago, added [[chloroplast]]s, creating the green plants.<ref name=latorre/>]]
-According to the theory of [[symbiogenesis]], championed by [[Lynn Margulis]], a union between a motile [[anaerobic organism|anaerobic]] bacterium (like ''Spirochaeta'') and a thermoacidophilic [[thermoproteota]]n (like ''Thermoplasma'' which is sulfidogenic in nature) gave rise to eukaryotes. This union established a motile organism capable of living in what were then acidic and sulfurous waters. Oxygen kills organisms that lack the required [[Metabolism|metabolic]] machinery. Thus, the archaeon provided the bacterium with a highly beneficial reducing environment. Endosymbiosis with alphaproteobacteria and cyanobacteria led to the origin of [[Mitochondrion|mitochondria]] and [[plastid]]s respectively.<ref name=latorre>{{cite book |last1=Latorre |first1=A. |last2=Durban |first2=A |last3=Moya |first3=A. |last4=Pereto |first4=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 |editor1=Gargaud, Muriel |editor2=López-Garcìa, Purificacion |editor3=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>
+According to the theory of [[symbiogenesis]] (also known as the ''endosymbiotic theory'') championed by [[Lynn Margulis]], a member of the archaea gained a bacterial cell as a component. The archaeal cell was a member of the [[Asgard (archaea)|Asgard]] group. The bacterium was one of the [[Alphaproteobacteria]], which had the ability to use oxygen in its respiration. This enabled it – and the archaeal cells that included it – to survive in the presence of oxygen, which was poisonous to other organisms adapted to [[Redox|reducing]] conditions. The endosymbiotic bacteria became the eukaryotic cell's [[Mitochondrion|mitochondria]], providing most of the energy of the cell.<ref name="McGrath 2022"/><ref name=latorre>{{cite book |last1=Latorre |first1=A. |last2=Durban |first2=A |last3=Moya |first3=A. |last4=Pereto |first4=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 |editor1=Gargaud, Muriel |editor2=López-Garcìa, Purificacion |editor3=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> [[Lynn Margulis]] and colleagues have suggested that the cell also acquired a [[Spirochaete]] bacterium as a symbiont, providing the [[Cytoskeleton|cell skeleton]] of [[microtubule]]s and the ability to move, including the ability to pull [[chromosome]]s into two sets during [[mitosis]], cell division.<ref name="Margulis Chapman Guerrero Hall 2006">{{cite journal |last1=Margulis |first1=Lynn |author-link=Lynn Margulis |last2=Chapman |first2=Michael |last3=Guerrero |first3=Ricardo |last4=Hall |first4=John |title=The last eukaryotic common ancestor (LECA): Acquisition of cytoskeletal motility from aerotolerant spirochetes in the Proterozoic Eon |journal=Proceedings of the National Academy of Sciences |volume=103 |issue=35 |date=29 August 2006 |doi=10.1073/pnas.0604985103 |pages=13080–13085|pmid=16938841 |pmc=1559756 |bibcode=2006PNAS..10313080M |doi-access=free }}</ref> More recently, the Asgard archaean has been identified as belonging to the [[Heimdallarchaeota]].<ref name="Williams Cox Foster Szöllősi 2019 pp. 138–147">{{cite journal | last1=Williams | first1=Tom A. | last2=Cox | first2=Cymon J. | last3=Foster | first3=Peter G. | last4=Szöllősi | first4=Gergely J. | last5=Embley | first5=T. Martin | title=Phylogenomics provides robust support for a two-domains tree of life | journal=Nature Ecology & Evolution | volume=4 | issue=1 | date=2019-12-09 | doi=10.1038/s41559-019-1040-x | pages=138–147| pmid=31819234 | pmc=6942926 | bibcode=2019NatEE...4..138W }}</ref>
 == Last eukaryotic common ancestor (LECA) ==
+The last eukaryotic common ancestor (LECA) is the hypothetical [[Most recent common ancestor|last common ancestor]] of all living eukaryotes, around 2 billion years ago,<ref name="Gabaldón"/><ref name="w1990"/> and was most likely a [[Population|biological population]].<ref name="O'Malley Leger Wideman Ruiz-Trillo 2019">{{cite journal |last1=O'Malley |first1=Maureen A. |last2=Leger |first2=Michelle M. |last3=Wideman |first3=Jeremy G. |last4=Ruiz-Trillo |first4=Iñaki |title=Concepts of the last eukaryotic common ancestor |journal=Nature Ecology & Evolution |volume=3 |issue=3 |date=18 February 2019 |doi=10.1038/s41559-019-0796-3 |pages=338–344|pmid=30778187 |bibcode=2019NatEE...3..338O |hdl=10261/201794 |s2cid=256718457 |hdl-access=free }}</ref> It is believed to have been a [[protist]] with a nucleus, at least one [[centriole]] and [[cilium]], facultatively aerobic mitochondria, sex ([[meiosis]] and [[syngamy]]), a dormant [[cyst]] with a cell wall of [[chitin]] and/or [[cellulose]], and [[peroxisome]]s.<ref name="Leander R510–R516">{{cite journal |last=Leander |first=B. S. |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 }}</ref><ref name="Strassert Irisarri Williams Burki 2021">{{cite journal |last1=Strassert |first1=Jürgen F. H. |last2=Irisarri |first2=Iker |last3=Williams |first3=Tom A. |last4=Burki |first4=Fabien |title=A molecular timescale for eukaryote evolution with implications for the origin of red algal-derived plastids |journal=Nature Communications |volume=12 |issue=1 |date=25 March 2021 |page=1879 |doi=10.1038/s41467-021-22044-z |pmid=33767194 |pmc=7994803 |bibcode=2021NatCo..12.1879S }}</ref>
+It had been proposed that the LECA fed by [[phagocytosis]], engulfing other organisms.<ref name="Leander R510–R516"/><ref name="Strassert Irisarri Williams Burki 2021"/> However, in 2022, Nico Bremer and colleagues confirmed that the LECA had mitochondria, and stated that it had multiple nuclei, but disputed that it was phagotrophic. This would mean that the ability found in many eukaryotes to engulf materials developed later, rather than being acquired first and then used to engulf the alphaproteobacteria that became mitochondria.<ref name="Bremer Tria Skejo Garg 2022">{{cite journal |last1=Bremer |first1=Nico |last2=Tria |first2=Fernando D. K. |last3=Skejo |first3=Josip |last4=Garg |first4=Sriram G. |last5=Martin |first5=William F. |title=Ancestral State Reconstructions Trace Mitochondria But Not Phagocytosis to the Last Eukaryotic Common Ancestor |journal=Genome Biology and Evolution |volume=14 |issue=6 |date=31 May 2022 |doi=10.1093/gbe/evac079 |pmid=35642316 |pmc=9185374 }}</ref>
-The [[last eukaryotic common ancestor]] (LECA) is the hypothetical [[Most recent common ancestor|last common ancestor]] of all living eukaryotes,<ref name="Gabaldón">{{cite journal |last=Gabaldón |first=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><ref name="w1990"/> and was most likely a [[Population|biological population]].<ref name="O’Malley Leger Wideman Ruiz-Trillo 2019">{{cite journal |last1=O'Malley |first1=Maureen A. |last2=Leger |first2=Michelle M. |last3=Wideman |first3=Jeremy G. |last4=Ruiz-Trillo |first4=Iñaki |title=Concepts of the last eukaryotic common ancestor |journal=Nature Ecology & Evolution |volume=3 |issue=3 |date=18 February 2019 |doi=10.1038/s41559-019-0796-3 |pages=338–344|pmid=30778187 |hdl=10261/201794 |s2cid=256718457 }}</ref> It is believed to have been a protist with a nucleus, at least one [[centriole]] and [[cilium]], facultatively aerobic mitochondria, sex ([[meiosis]] and [[syngamy]]), a dormant [[cyst]] with a cell wall of [[chitin]] and/or [[cellulose]], and [[peroxisome]]s.<ref>{{cite journal |last=Leander |first=B. S. |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 }}</ref><ref name="Strassert Irisarri Williams Burki 2021">{{cite journal |last1=Strassert |first1=Jürgen F. H. |last2=Irisarri |first2=Iker |last3=Williams |first3=Tom A. |last4=Burki |first4=Fabien |title=A molecular timescale for eukaryote evolution with implications for the origin of red algal-derived plastids |journal=Nature Communications |volume=12 |issue=1 |date=25 March 2021 |page=1879 |doi=10.1038/s41467-021-22044-z|pmid=33767194 |pmc=7994803 |bibcode=2021NatCo..12.1879S }}</ref>
+The LECA has been described as having "spectacular cellular complexity".<ref name="Koumandou Wickstead Ginger van der Giezen 2013"/> Its cell was divided into compartments.<ref name="Koumandou Wickstead Ginger van der Giezen 2013"/> It appears to have inherited a set of [[ESCRT|endosomal sorting complex]] proteins that enable membranes to be remodelled, including pinching off [[Vesicle (biology and chemistry)|vesicles]] to form [[endosome]]s.<ref name="Makarova Yutin Bell Koonin 2010">{{cite journal |last1=Makarova |first1=Kira S. |last2=Yutin |first2=Natalya |last3=Bell |first3=Stephen D. |last4=Koonin |first4=Eugene V. |author-link4=Eugene Koonin |title=Evolution of diverse cell division and vesicle formation systems in Archaea |journal=Nature Reviews Microbiology |volume=8 |issue=10 |date=6 September 2010 |doi=10.1038/nrmicro2406 |pages=731–741 |pmid=20818414 |pmc=3293450 }}</ref>
+Its apparatuses for [[Eukaryotic transcription|transcribing DNA into RNA]], and then for [[Translation (biology)|translating the RNA]] into proteins, were separated, permitting extensive RNA processing and allowing the expression of genes to become more complex.<ref name="Martin Koonin 2006 pp. 41–45">{{cite journal |last=Martin |first=William |last2=Koonin |first2=Eugene V. |title=Introns and the origin of nucleus–cytosol compartmentalization |journal=Nature |volume=440 |issue=7080 |date=2006 |issn=0028-0836 |doi=10.1038/nature04531 |pages=41–45}}</ref> It had mechanisms for reshuffling its genetic material, and possibly for manipulating its own [[evolvability]]. All of these gave the LECA "a compelling cohort of selective advantages".<ref name="Koumandou Wickstead Ginger van der Giezen 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. |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>
+===Eukaryotic sex===
+Sex in eukaryotes is a composite process, consisting of [[meiosis]] and [[fertilisation]], which can be coupled to [[reproduction]].<ref name="Horandl 2018">{{cite journal |last1=Horandl |first1=E. |last2=Speijer |first2=D. |title=How oxygen gave rise to eukaryotic sex |journal=Proceedings of the Royal Society B: Biological Sciences |publisher=The Royal Society |volume=285 |issue=1872 |date=7 February 2018 |doi=10.1098/rspb.2017.2706 |pmid=29436502 |pmc=5829205}}</ref>  Dacks and Roger<ref>{{cite journal |last1=Dacks |first1=J. |last2=Roger |first2=A. J. |title=The first sexual lineage and the relevance of facultative sex |journal=Journal of Molecular Evolution |year=1999 |volume=48 |issue=6 |pages=779–783 |doi=10.1007/pl00013156 |pmid=10229582|bibcode=1999JMolE..48..779D |s2cid=9441768 }}</ref> proposed on the basis of a [[phylogenetics|phylogenetic]] analysis that facultative sex was likely present in the common ancestor of all eukaryotes. Early in eukaryotic evolution, about 2 billion years ago, organisms needed a solution to the major problem that oxidative metabolism releases [[reactive oxygen species]] that damage the genetic material, [[DNA]].<ref name="Horandl 2018"/>  Eukaryotic sex provides a process, [[homologous recombination]] during meiosis, for using informational redundancy to [[DNA repair|repair such DNA damage]].<ref name="Horandl 2018"/>
 == Scenarios ==
-Biologists have proposed multiple scenarios for the creation of the eukaryotes. While there is broad agreement that the LECA must have had a nucleus, mitochondria, and internal membranes, the order in which these were acquired has been disputed. In the syntrophic model, the first eukaryotic common ancestor (FECA) gained mitochondria, then membranes, then a nucleus. In the phagotrophic model, it gained a nucleus, then membranes, then mitochondria. In a more complex process, it gained all three in short order, then other capabilities. Other models have been proposed. Whatever happened, many lineages must have been created, but the LECA either out-competed or came together with the other lineages to form a single point of origin for the eukaryotes.<ref name="Koumandou Wickstead Ginger van der Giezen 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. |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> [[Nick Lane]] and William Martin have argued that mitochondria came first, on the grounds that energy had been the limiting factor on the size of the prokaryotic cell.<ref name="Lane Martin 2010">{{cite journal |last1=Lane |first1=Nick |author-link=Nick Lane |last2=Martin |first2=William |title=The energetics of genome complexity |journal=Nature |volume=467 |issue=7318 |year=2010 |doi=10.1038/nature09486 |pages=929–934|pmid=20962839 |bibcode=2010Natur.467..929L |s2cid=17086117 }}</ref> The phagotrophic model presupposes the ability to engulf food, enabling the cell to engulf the aerobic bacterium that became the mitochondrion.<ref name="Koumandou Wickstead Ginger van der Giezen 2013"/>
+Biologists have proposed multiple scenarios for the creation of the eukaryotes. While there is broad agreement that the LECA must have had a nucleus, mitochondria, and internal membranes, the order in which these were acquired has been disputed. In the syntrophic model, the first eukaryotic common ancestor (FECA, around 2.2 [[gya]]) gained mitochondria, then membranes, then a nucleus. In the phagotrophic model, it gained a nucleus, then membranes, then mitochondria. In a more complex process, it gained all three in short order, then other capabilities. Other models have been proposed. Whatever happened, many lineages must have been created, but the LECA either out-competed or came together with the other lineages to form a single point of origin for the eukaryotes.<ref name="Koumandou Wickstead Ginger van der Giezen 2013"/> [[Nick Lane]] and [[William F. Martin|William Martin]] have argued that mitochondria came first, on the grounds that energy had been the limiting factor on the size of the prokaryotic cell.<ref name="Lane Martin 2010">{{cite journal |last1=Lane |first1=Nick |author-link=Nick Lane |last2=Martin |first2=William F. |author-link2=William F. Martin |title=The energetics of genome complexity |journal=Nature |volume=467 |issue=7318 |year=2010 |doi=10.1038/nature09486 |pages=929–934 |pmid=20962839 |bibcode=2010Natur.467..929L |s2cid=17086117 }}</ref> The phagotrophic model presupposes the ability to engulf food, enabling the cell to engulf the aerobic bacterium that became the mitochondrion.<ref name="Koumandou Wickstead Ginger van der Giezen 2013"/>
+[[Eugene Koonin]] and others, noting that the archaea share many features with eukaryotes, argue that rudimentary eukaryotic traits such as [[Organelle|membrane-lined compartments]] were acquired before endosymbiosis added mitochondria to the early eukaryotic cell, while the [[cell wall]] was lost. In the same way, mitochondrial acquisition must not be regarded as the end of the process, for still new complex families of genes had to be developed after or during the endosymbiotic exchange. In this way, from FECA to LECA, we can think of organisms that can be considered as protoeukaryotes. At the end of the process, LECA was already a complex organism with the presence of protein families involved in cellular compartmentalization.<ref name="Koonin 2005">{{Cite journal |last=Koonin |first=Eugene V. |author-link=Eugene Koonin |date=March 2005 |title=The incredible expanding ancestor of eukaryotes |journal=Cell |volume=140 |issue=5 |pages=606–608 |doi=10.1016/j.cell.2010.02.022 |pmid=20211127 |pmc=3293451}}</ref><ref>{{Cite journal |last1=Martijn |first1=J. |last2=Ettema |first2=T.J.G. |date=February 2013 |title=From archaeon to eukaryote: the evolutionary dark ages of the eukaryotic cell |url=http://www.biochemsoctrans.org/content/41/1/451.long |journal=Biochem Soc Trans |volume=41 |issue=1 |pages=451–7 |doi=10.1042/BST20120292 |pmid=23356327}}</ref>
+{{anchor|Crown eukaryotes}}
+== Diversification: crown eukaryotes ==
+In turn, the LECA gave rise to the eukaryotes' [[crown group]], containing the ancestors of [[animal]]s, [[Fungus|fungi]], [[plant]]s, and a diverse [[protist|range of single-celled organisms]] with the new capabilities and complexity of the eukaryotic cell.<ref name="Van de Peer Baldaufrid Doolittle Meyerid 2000">{{cite journal |last1=Van de Peer |first1=Yves |last2=Baldaufrid |first2=Sandra L. |last3=Doolittle |first3=W. Ford |last4=Meyerid |first4=Axel |title=An Updated and Comprehensive rRNA Phylogeny of (Crown) Eukaryotes Based on Rate-Calibrated Evolutionary Distances |journal=Journal of Molecular Evolution |volume=51 |issue=6 |year=2000 |doi=10.1007/s002390010120 |pages=565–576|pmid=11116330 |bibcode=2000JMolE..51..565V |s2cid=9400485 |url=http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-35255 }}</ref><ref name="Butterfield 2015">{{Cite journal |last=Butterfield |first=N.J. |date=2015 |title=Early evolution of the Eukaryota |journal=Palaeontology |volume=58 |issue=1 |pages=5–17 |doi=10.1111/pala.12139 |bibcode=2015Palgy..58....5B |doi-access=free}}</ref> Single cells without cell walls are fragile and have a low probability of [[taphonomy|being fossilised]]. If fossilised, they have few features to distinguish them clearly from prokaryotes: size, morphological complexity, and (eventually) [[multicellularity]]. Early eukaryote fossils, from the late [[Paleoproterozoic]], include [[acritarch]] microfossils with relatively robust ornate carbonaceous vesicles of ''[[Tappania]]'' from 1.63 [[gya]] and ''[[Shuiyousphaeridium]]'' from 1.8 gya.<ref name="Butterfield 2015"/>
 == References ==
 {{reflist|30em}}
+== External links ==
+* [https://www.theatlantic.com/science/archive/2020/11/how-do-single-celled-organisms-have-sex/617072/ Attraction and sex among our microbial Last Eukaryotic Common Ancestors], ''The Atlantic'', November 11, 2020
+{{Eukaryota}}
 [[Category:Eukaryote biology]]
 [[Category:Evolution by taxon]]
+[[Category:Most recent common ancestors]]