non-cellular life

{{Further|Virus|Viral evolution|Virus classification}}

File:Type of viruses.jpg

Researchers initially described viruses as "poisons" or "toxins", then as "infectious proteins"; but they possess genetic material, a defined structure, and the ability to spontaneously assemble from their constituent parts. This has spurred extensive debate as to whether they should be regarded as fundamentally biotic or abiotic—as very small biological organisms or as very large biochemical molecules. Without their hosts, they are not able to perform any of the functions of life—such as metabolism, growth, or reproduction. Since the 1950s, many scientists have thought of viruses as existing at the border between chemistry and life; a gray area between living and nonliving.{{cite journal |url =http://www.scientificamerican.com/article.cfm?id=are-viruses-alive-2004 |title =Are Viruses Alive? |access-date =2013-04-27 |last =Villarreal |first =Luis P. |date=December 2004 |journal =Scientific American |volume =291 |issue =6 |pages =100–105 |doi =10.1038/scientificamerican1204-100 |pmid =15597986 |bibcode =2004SciAm.291f.100V |archive-date =22 December 2013 |archive-url =https://web.archive.org/web/20131222065841/http://www.scientificamerican.com/article.cfm?id=are-viruses-alive-2004 |url-status =live }}{{cite journal |title =Defining Life: The Virus Viewpoint |journal =Orig Life Evol Biosph |date =3 March 2010 |first =Patrick |last =Forterre |volume =40 |issue =2 |pages =151–160 |doi =10.1007/s11084-010-9194-1 |pmid =20198436 |pmc =2837877|bibcode =2010OLEB...40..151F }}{{cite journal|last =Lwoff|first =A.|date =1957-01-01|title =The Concept of Virus|journal =Microbiology|volume =17|issue =2|pages =239–253|doi =10.1099/00221287-17-2-239|pmid =13481308|doi-access =free}}

{{Main |Viroid}}

If viruses are borderline cases or nonliving, viroids are further from being living organisms. Viroids are some of the smallest infectious agents, consisting solely of short strands of circular, single-stranded RNA without protein coats. They are only known to infect flowering plants, of which some are of commercial importance. Viroid genomes are extremely small in size, ranging from 246 to 467 nucleobases. In comparison, the genome of the smallest viruses capable of causing an infection are around 2,000 nucleobases in size.{{cite journal |vauthors =Diener TO |title =Potato spindle tuber "virus". IV. A replicating, low molecular weight RNA |journal =Virology |volume =45 |issue =2 |pages =411–28 |date =August 1971 |pmid =5095900 |doi =10.1016/0042-6822(71)90342-4}}{{cite web |url =http://www.ars.usda.gov/is/timeline/viroid.htm |title =ARS Research Timeline – Tracking the Elusive Viroid |date =2006-03-02 |access-date =18 July 2007 |archive-date =6 July 2007 |archive-url =https://web.archive.org/web/20070706190644/http://www.ars.usda.gov/is/timeline/viroid.htm |url-status =live }} Viroid RNA does not code for any protein.{{cite journal |pmid =18764915 |year =2008 |last1 =Tsagris |first1 =E. M. |title =Viroids |journal =Cellular Microbiology |volume =10 |issue =11 |pages =2168–79 |last2 =Martínez De Alba |first2 =A. E. |last3 =Gozmanova |first3 =M |last4 =Kalantidis |first4 =K |doi =10.1111/j.1462-5822.2008.01231.x |doi-access =free}} Its replication mechanism hijacks RNA polymerase II, a host-cell enzyme normally associated with synthesis of messenger RNA from DNA, which instead catalyzes "rolling circle" synthesis of new RNA using the viroid's RNA as a template. Some viroids are ribozymes, having catalytic properties which allow self-cleavage and ligation of unit-size genomes from larger replication intermediates.{{cite journal |pages =593–598 |doi =10.1038/sj.embor.7400706 |pmid =16741503 |pmc =1479586 |year =2006 |last1 =Daròs |first1 =J. A. |title =Viroids: An Ariadne's thread into the RNA labyrinth |journal =EMBO Reports |volume =7 |issue =6 |last2 =Elena |first2 =S. F. |last3 =Flores }}

A possible explanation of the origin of viroids sees them as "living relics" from a hypothetical, ancient, and non-cellular RNA world before the evolution of DNA or of protein.{{cite journal |pmid =2480600 |pmc =298497 |year =1989 |last1 =Diener |first1 =T. O. |title =Circular RNAs: Relics of precellular evolution? |journal =Proceedings of the National Academy of Sciences of the United States of America |volume =86 |issue =23 |pages =9370–4 |doi =10.1073/pnas.86.23.9370 |bibcode=1989PNAS...86.9370D |doi-access =free}}{{cite book |last1 =Villarreal |first1 =Luis P. |title =Viruses and the evolution of life |url =https://archive.org/details/virusesevolution0000vill |url-access =registration |date =2005 |publisher =ASM Press |location =Washington, D.C. |isbn =1-55581-309-7 |page =[https://archive.org/details/virusesevolution0000vill/page/31 31] }} This view, first proposed in the 1980s, regained popularity in the 2010s to explain crucial intermediate steps in the evolution of life from inanimate matter (abiogenesis).{{cite journal |pmid =25002087 |year =2014 |last1 =Flores |first1 =R |title =Viroids: Survivors from the RNA world? |journal =Annual Review of Microbiology |volume =68 |pages =395–414 |last2 =Gago-Zachert |first2 =S. |last3 =Serra |first3 =P. |last4 =Sanjuán |first4 =R. |last5 =Elena |first5 =S. F. |doi =10.1146/annurev-micro-091313-103416 |url =https://digital.csic.es/bitstream/10261/107724/1/Annu.%20Rev.%20Microbiol.%20Flores%20et%20al%202014.pdf |hdl =10261/107724 |hdl-access =free |access-date =15 December 2018 |archive-date =22 December 2018 |archive-url =https://web.archive.org/web/20181222183118/http://digital.csic.es/bitstream/10261/107724/1/Annu.%20Rev.%20Microbiol.%20Flores%20et%20al%202014.pdf |url-status =live}}{{cite news |last =Zimmer |first =Carl |author-link =Carl Zimmer |title =A Tiny Emissary From the Ancient Past |url =https://www.nytimes.com/2014/09/25/science/a-tiny-emissary-from-the-ancient-past.html |date =25 September 2014 |work =The New York Times |access-date =22 November 2014 |archive-date =27 September 2014 |archive-url =https://web.archive.org/web/20140927022738/http://www.nytimes.com/2014/09/25/science/a-tiny-emissary-from-the-ancient-past.html |url-status =live }}

In 2024, researchers announced the possible discovery of viroid-like, but distinct, RNA-based elements dubbed obelisks. Obelisks, found in sequence databases of the human microbiome, are possibly hosted in gut bacteria. They differ from viroids in that they code for two distinct proteins, dubbed "oblins", and for the predicted rod-like secondary structure of their RNA.{{cite journal |last1 =Sidik |first1 =Saima |title ='Wildly weird' RNA bits discovered infesting the microbes in our guts |url =https://www.nature.com/articles/d41586-024-00266-7 |journal =Nature |access-date =31 January 2024 |doi =10.1038/d41586-024-00266-7 |date =29 January 2024 |pmid =38291328 |archive-date =30 January 2024 |archive-url =https://web.archive.org/web/20240130213825/https://www.nature.com/articles/d41586-024-00266-7 |url-status =live }}

{{cite journal |last1 =Pennisi |first1 =Elizabeth |title ='It's insane': New viruslike entities found in human gut microbes |url =https://www.science.org/content/article/it-s-insane-new-viruslike-entities-found-human-gut-microbes |website =Science |access-date =31 January 2024 |doi =10.1126/science.znxt3dk |date =26 January 2024 |archive-date =30 January 2024 |archive-url =https://web.archive.org/web/20240130203652/https://www.science.org/content/article/it-s-insane-new-viruslike-entities-found-human-gut-microbes |url-status =live }}

The first universal common ancestor (FUCA) is an example of a proposed non-cellular lifeform, as it is the earliest ancestor of the last universal common ancestor, its sister lineages, and every currently living cell.{{Citation |last1=Prosdocimi |first1=Francisco |title=The First Universal Common Ancestor (FUCA) as the Earliest Ancestor of LUCA's (Last UCA) Lineage |date=2019 |url=https://doi.org/10.1007/978-3-030-30363-1_3 |work=Evolution, Origin of Life, Concepts and Methods |pages=43–54 |editor-last=Pontarotti |editor-first=Pierre |access-date=2023-11-02 |place=Cham |publisher=Springer International Publishing |doi=10.1007/978-3-030-30363-1_3 |isbn=978-3-030-30363-1 |s2cid=199534387 |last2=José |first2=Marco V. |last3=de Farias |first3=Sávio Torres |archive-date=8 March 2024 |archive-url=https://web.archive.org/web/20240308093423/https://link.springer.com/chapter/10.1007/978-3-030-30363-1_3 |url-status=live }}

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• Carbon chauvinism
• Hypothetical types of biochemistry
• Nanobe
• Plasmid
• Pre-cell
• Prion
• Protocell

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{{Reflist|colwidth=30em}}

{{Organisms et al.}}

{{Self-replicating organic structures}}

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Category:Biological classification

Category:Life

Category:Virology

Category:Viruses