Eukaryogenesis, the process which created theeukaryotic 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 involvedsymbiogenesis, in which anarchaeon and one or morebacteria came together to create thefirst eukaryotic common ancestor (FECA). This cell had a new level of complexity and capability, with a nucleus, at least onecentriole andcilium, facultatively aerobicmitochondria, sex (meiosis andsyngamy), a dormantcyst with a cell wall ofchitin and/orcellulose andperoxisomes. It evolved into a population of single-celled organisms that included thelast eukaryotic common ancestor (LECA), gaining capabilities along the way, though the sequence of 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 ofanimals,fungi,plants, and a diverse range of single-celled organisms.
Life arose on Earth once it had cooled enough for oceans to form. That developed into thelast universal common ancestor (LUCA), anorganism which hadribosomes and thegenetic code, some 4 billion years ago. It gave rise to two main branches ofprokaryotic 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 adomain that contains all complex cells and most types ofmulticellular organism, including theanimals,plants, andfungi.[3][4]
According to the theory ofsymbiogenesis (theendosymbiotic theory) championed byLynn Margulis, a member of the archaea gained a bacterial cell as a component. The archaeal cell was a member of thePromethearchaeatikingdom. The bacterium was one of thealphaproteobacteria, 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 toreducing conditions. The endosymbiotic bacteria became the eukaryotic cell'smitochondria, providing most of the energy of the cell.[1][5]Lynn Margulis and colleagues have suggested that the cell also acquired aSpirochaete bacterium as a symbiont, providing thecell skeleton ofmicrotubules and the ability to move, including the ability to pullchromosomes into two sets duringmitosis, cell division.[6] More recently, the archaean has been identified as belonging to the unrankedtaxonHeimdallarchaeia of thephylumPromethearchaeota.[7]
The last eukaryotic common ancestor (LECA) is the hypotheticallast common ancestor of all living eukaryotes, around 2 billion years ago,[3][4] and was most likely a biologicalpopulation.[8] It is believed to have been aprotist with a nucleus, at least onecentriole andcilium, facultatively aerobic mitochondria, sex (meiosis andsyngamy), a dormantcyst with a cell wall ofchitin and/orcellulose, andperoxisomes.[9][10]
It had been proposed that the LECA fed byphagocytosis, 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 ofendosomal sorting complex proteins that enable membranes to be remodelled, including pinching offvesicles to formendosomes.[13] Its apparatuses fortranscribing DNA into RNA, and then fortranslating 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 ownevolvability. All of these gave the LECA "a compelling cohort of selective advantages".[12]
Sex in eukaryotes is a composite process, consisting ofmeiosis andfertilisation, which can be coupled toreproduction.[15] Dacks and Roger[16] proposed on the basis of aphylogenetic 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 releasesreactive oxygen species that damage the genetic material,DNA.[15] Eukaryotic sex provides a process,homologous recombination during meiosis, for using informational redundancy torepair such DNA damage.[15]
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.[12] In the syntrophic model, the first eukaryotic common ancestor (FECA, around 2.2gya) gained mitochondria, then membranes, then a nucleus.[12] In the phagotrophic model, it gained a nucleus, then membranes, then mitochondria.[12] 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 andWilliam 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] Enrique M. Muro et al. have argued, however, that the genetic system needed to reach a critical point that led to a new regulatory system (withintrons and thespliceosome), which enabled coordination betweengenetic networks.[18] 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 asmembrane-lined compartments were acquired before endosymbiosis added mitochondria to the early eukaryotic cell, while thecell 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, the organisms can be considered as proto-eukaryotes. At the end of the process, LECA was already a complex organism with protein families involved in cellular compartmentalization.[19][20]
Another scenario isviral eukaryogenesis, which proposes that the eukaryotes arose as an emergent superorganism, with the nucleus deriving from a "viral factory" alongside the alphaproteobacterium mitochondrion, hosted by an archaeal cell. In this scenario, eukaryogenesis began when a virus colonised an archaeal cell, making it support the production of viruses. The virus may later have assisted the bacterium's entry into the reprogrammed cell.[21] Eukaryotes share genes for several DNA synthesis and transcription enzymes with DNA viruses (Nucleocytoviricota). Those viruses may thus be older than the LECA and may have exchanged DNA with proto-eukaryotes.[22]
In turn, the LECA gave rise to the eukaryotes'crown group, containing the ancestors ofanimals,fungi,plants, and a diverserange of single-celled organisms with the new capabilities and complexity of the eukaryotic cell.[23][24] Single cells without cell walls are fragile and have a low probability ofbeing fossilised. If fossilised, they have few features to distinguish them clearly from prokaryotes: size, morphological complexity, and (eventually)multicellularity. Early eukaryote fossils, from the latePaleoproterozoic, includeacritarch microfossils with relatively robust ornate carbonaceous vesicles ofTappania from 1.63gya andShuiyousphaeridium from 1.8 gya.[24]
The position of the LECA on the eukaryotic tree of life remains controversial. Some studies believe that the first split after the LECA happened between theUnikonta and theBikonta (Stechmann and Cavalier-Smith 2003), or betweenAmorphea and all other eukaryotes (Adl et al. 2012; Derelle and Lang 2012). Some believe that the first split happened withinExcavata (al Jewari and Baldauf 2023).[25] Yet others believe in a first split between theOpisthokonta and all others (Cerón-Romero et al. 2024).[26]
