Eukaryogenesis
Eukaryogenesis, the process which created the
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
Last eukaryotic common ancestor (LECA)
The last eukaryotic common ancestor (LECA) is the hypothetical
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
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'
References
- ^ PMC 9168435.
- S2CID 2997255.
- ^ S2CID 236916203.
- ^ PMID 2112744.
- ^ ISBN 978-0-521-76131-4. Archivedfrom the original on 24 March 2019. Retrieved 27 August 2017.
- PMID 16938841.
- PMID 31819234.
- S2CID 256718457.
- ^ S2CID 218710816.
- ^ PMID 33767194.
- PMID 35642316.
- ^ PMID 23895660.
- PMID 20818414.
- ISSN 0028-0836.
- ^ PMID 29436502.
- S2CID 9441768.
- S2CID 17086117.
- PMID 20211127.
- PMID 23356327.
- S2CID 9400485.
- ^ .
External links
- Attraction and sex among our microbial Last Eukaryotic Common Ancestors, The Atlantic, November 11, 2020