# Pre-cell

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{{Short description|Hypothetical life before complete cells}}{{Not to be confused with|Protocell}}

The terms '''pre-cell''' (precell)''',''' [proto-cell](/source/Protocell) (protocell), etc. are frequently used to designate hypothetical ancestral entities precursing complete [cells](/source/Cell_(biology)). The meanings of these terms vary with the different hypotheses for the early evolution of life and, accordingly, with the corresponding publications.

There are different hypotheses attempting to explain the origin of the [three domains of life](/source/Domain_(biology)) ([Woese](/source/Carl_Woese) et al. 1990)<ref name="Woese_1990" /> from a [last universal common ancestor](/source/last_universal_common_ancestor) (LUCA).<ref name="Harold_2014" /><ref name="Brock_2015" /><ref name="Brock_2022" /> The nature of this ancestral entity remains a major subject of discussion. 

'''Under the [RNA world hypothesis](/source/RNA_world_hypothesis)''' (replication-first scenario),<ref name="Brock_2015" /><ref name="Brock_2022" /> over a precellular and early-cellular phase,<ref name="Brock_2015" /><ref name="Brock_2022">{{Cite book |last1=Madigan |first1=Michael T. |title=Brock Biology of Microorganisms |last2=Aiyer |first2=Jennifer |last3=Buckley |first3=Daniel H. |last4=Sattley |first4= Matthew |last5=Stahl |first5=David A. |publisher=Pearson Education Limited |year=2022 |isbn=978-1-292-40479-0 |edition=16 |location=Harlow |page=431}}</ref> the earliest self-replicating biological systems were based on [catalytic RNA](/source/catalytic_RNA) evolving stage by stage to a nearly complete ancestral cell, the last universal common ancestor ([LUCA](/source/LUCA))<ref name="Harold_2014">{{Cite book |last=Harold |first=Franklin M. |url=https://books.google.com/books?id=XjOOBAAAQBAJ&q=In+Search+of+Cell+History |title=In Search of Cell History: The Evolution of Life's Building Blocks |publisher=University of Chicago Press |year=2014 |isbn=978-0-226-17428-0 |location=Chicago, London}}</ref><ref name="Brock_2015">{{Cite book |last1=Madigan |first1=Michael T. |title=Brock Biology of Microorganisms |last2=Martinko |first2=John M. |last3=Bender |first3=Kelly S. |last4=Buckley |first4=Daniel H. |last5=Stahl |first5=David A. |publisher=Pearson Education Limited |year=2015 |isbn=978-1-292-01831-7 |edition=14 |location=Boston |pages=29; 374; 381}}</ref><ref name="Brock_2022">{{Cite book |last1=Madigan |first1=Michael T. |title=Brock Biology of Microorganisms |last2=Aiyer |first2=Jennifer |last3=Buckley |first3=Daniel H. |last4=Sattley |first4= Matthew |last5=Stahl |first5=David A. |publisher=Pearson Education Limited |year=2022 |isbn=978-1-292-40479-0 |edition=16 |location=Harlow |page=431}}</ref> from which the three domains of life<ref name="Woese_1990">{{cite journal |author-link=Carl Woese |author-link2=Otto Kandler |vauthors=Woese CR, Kandler O, Wheelis ML |date=June 1990 |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 |bibcode=1990PNAS...87.4576W |doi=10.1073/pnas.87.12.4576 |pmc=54159 |pmid=2112744 |doi-access=free}}</ref> emerged.

This ancestral cell (sometimes also called pre-cell or proto-cell), a hypothetical lipid-based structure, could have confined [RNA](/source/RNA) in ancient times. This structure allowed the RNA to remain in close proximity with other RNA molecules, keeping them concentrated and allowing for an increased [reaction rate](/source/reaction_rate) of [enzyme](/source/enzyme)s.<ref>{{cite journal|last=Black|first=S|title=Pre-cell evolution and the origin of enzymes.|journal=Nature|date=May 23, 1970|volume=226|issue=5247|pages=754–5|pmid=5443250|doi=10.1038/226754a0|bibcode=1970Natur.226..754B|s2cid=4271189}}</ref> It would have had [semi-permeable membrane](/source/semi-permeable_membrane)s, allowing only certain molecules to pass through. These enclosed structures may have facilitated [natural selection](/source/natural_selection) in RNA molecules.

'''Under the pre-cell theory''' ([Kandler](/source/Otto_Kandler) 1994ff)'''<ref name="Kandler_1994">{{Cite book |last=Kandler |first=Otto |title=Early Life on Earth. Nobel Symposium 84 |date=1994 |publisher=Columbia U.P. |editor=Stefan Bengtson |location=New York |pages=152–160 |chapter=The early diversification of life |author-link=Otto Kandler}}</ref><ref name="Kandler_1995">{{cite journal |last=Kandler |first=Otto |author-link=Otto Kandler |year=1995 |title=Cell Wall Biochemistry in Archaea and its Phylogenetic Implications |journal=[Journal of Biological Physics](/source/Journal_of_Biological_Physics) |volume=20 |issue=1–4 |pages=165–169 |doi=10.1007/BF00700433 |s2cid=83906865}}</ref><ref name="Kandler_1998">{{Cite book |last=Kandler |first=Otto |title=Thermophiles: The keys to molecular evolution and the origin of life? |publisher=Taylor and Francis Ltd. |year=1998 |isbn=978-0-203-48420-3 |editor1=Jürgen Wiegel |location=London |pages=19–31 |chapter=The early diversification of life and the origin of the three domains: A proposal |author-link=Otto Kandler |editor2=Michael W.W. Adams |chapter-url=https://books.google.com/books?id=FtSzl4iastsC}}</ref>''', based on the '''[Iron-Sulfur world hypothesis](/source/Iron%E2%80%93sulfur_world_hypothesis)''' (metabolism-first scenario),<ref>{{Cite journal |last=Wächtershäuser |first=Günter |author-link=Günter Wächtershäuser |date=1988 |title=Before enzymes and templates: theory of surface metabolism |journal=Microbiological Reviews |volume=52 |issue=4 |pages=452–484 |doi=10.1128/mr.52.4.452-484.1988 |pmc=373159 |pmid=3070320}}</ref> primordial metabolism led to the early diversification of life through the evolution of a '''multiphenotypical''' '''population of pre-cells''', defined by Kandler as metabolizing, replicating loose entities exhibiting many of the basic properties of a cell but no proper cytoplasmic membrane and no stable chromosome, thus allowing frequent mutual exchange of genetic information. 
[[File:Kandler 1998 Early diversification of life and pre-cell theory.svg|thumb|Early diversification of life with Kandler's pre-cell theory ([Kandler](/source/Otto_Kandler) 1998, p. 22)<ref name="Kandler_1998" />]]

From this pre-cell population the three founder groups A, B, C and then, from them, the precursor cells (here named proto-cells) of the three domains of life<ref name="Woese_1990" /> emerged successively, leading first to the domain Bacteria, then to the domain Archea and finally to the domain Eucarya.

Thus, under this scenario there was no almost complete ancestral “first cell“ or cell stage. Instead, the three domains originated from a population of evolving pre-cells. The emergence of cells was a process of successive evolutionary improvements, for which Kandler introduced the term [cellularization](/source/cellularization).<ref name="Kandler_1995" /><ref name="Kandler_1998" />

A scheme of the pre-cell scenario is presented in the adjacent figure, where essential evolutionary improvements are indicated by numbers:

"(1) Reductive formation of organic compounds from CO or CO2 by Me-sulfur coordinative chemistry; (2) tapping of various redox energy sources and formation of primitive enzymes and templates; (3) elements of a transcription and translation apparatus and loose associations; (4) formation of pre-cells; (5) stabilized circular or linear genomes; (6) cytoplasmic membranes; (7) rigid murein cell walls; (8) various non-murein rigid cell walls; (9) glycoproteinaceous cell envelope or glycokalyx; (10) cytoskeleton; (11) complex chromosomes and nuclear membrane; (12) cell organelles via endosymbiosis".<ref name="Kandler_1998" /><sup>: 22 </sup>

This scenario may explain the quasi-random distribution of evolutionarily important features among the three domains and, at the same time, the existence of the most basic biochemical features (genetic code, set of protein amino acids etc.) in all three domains (unity of life), as well as the close relationship between the Archaea and the Eucarya. 

Kandler's pre-cell theory is supported by [Wächtershäuser](/source/G%C3%BCnter_W%C3%A4chtersh%C3%A4user).<ref name="Wächtershäuser_2003">{{cite journal |last=Wächtershäuser |first=Günter |author-link=Günter Wächtershäuser |year=2003 |title=From pre-cells to Eukarya – a tale of two lipids |journal=Molecular Microbiology |volume=47 |issue=1 |pages=13–22 |doi=10.1046/j.1365-2958.2003.03267.x |pmid=12492850 |doi-access=free |s2cid=37944519}}</ref> According to Wächtershäuser, pre-cells had a [membrane](/source/cell_membrane) composed of mixed-[enantiomer](/source/enantiomer) lipid molecules. As natural selection proceeded, pre-cells may have developed [stereospecific](/source/stereospecificity) lipid membranes through frequent [fission](/source/fission_(biology)) and [fusion](/source/cell_fusion) of [racemic](/source/racemic) pre-cells.<ref name="Wächtershäuser_2003" />

For more theories on the evolution of cells see main article [History of life](/source/History_of_life) (examples under chapter “Replication first”).

== See also ==

* [First universal common ancestor](/source/First_universal_common_ancestor)

==References==
{{reflist}}

RNA

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Adapted from the Wikipedia article [Pre-cell](https://en.wikipedia.org/wiki/Pre-cell) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Pre-cell?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.