"Living creatures" redirects here. For the class of heavenly beings in Jewish mythology, seeliving creatures (Bible).
Anorganism is anyliving thing that functions as anindividual.[1] Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have been proposed to define what an organism is. Among the most common is that an organism has autonomousreproduction,growth, andmetabolism. This would excludeviruses, despite the fact that theyevolve like organisms. Other problematic cases includecolonial organisms; a colony ofeusocial insects is organised adaptively, and hasgerm-soma specialisation, with some insects reproducing, others not, like cells in an animal's body. The body of asiphonophore, a jelly-like marine animal, is composed of organism-likezooids, but the whole structure looks and functions much like an animal such as ajellyfish, the parts collaborating to provide the functions of the colonial organism.
The evolutionary biologistsDavid Queller andJoan Strassmann state that "organismality", the qualities or attributes that define an entity as an organism, has evolved socially as groups of simpler units (from cells upwards) came to cooperate without conflicts. They propose that cooperation should be used as the "defining trait" of an organism. This would treat many types of collaboration, including thefungus/alga partnership of different species in alichen, or the permanent sexual partnership of ananglerfish, as an organism.
The term "organism" (from theAncient Greekὀργανισμός, derived fromórganon, meaning'instrument, implement, tool','organ of sense', or'apprehension')[2][3] first appeared in the English language in the 1660s with the now-obsolete meaning of an organic structure or organization.[3] It is related to the verb "organize".[3] In his 1790Critique of Judgment,Immanuel Kant defined an organism as "both an organized and a self-organizing being".[4][5]
One criterion proposes that an organism cannot be divided without losing functionality.[6] Thisbasil plantcutting is however developing new adventitious roots from a small bit ofstem, forming a new plant.
Among the criteria that have been proposed for being an organism are:
noncompartmentability – structure cannot be divided without losing functionality.[6]Richard Dawkins stated this as "the quality of being sufficiently heterogeneous in form to be rendered non-functional if cut in half".[8] However, many organisms can be cut into pieces which then grow into whole organisms.[8]
individuality – the entity has simultaneous holdings of genetic uniqueness, genetic homogeneity andautonomy[9]
Other scientists think that the concept of the organism is inadequate in biology;[13] that the concept of individuality is problematic;[14] and from a philosophical point of view, question whether such a definition is necessary.[15][16][8]
Problematic cases includecolonial organisms: for instance, a colony ofeusocial insects fulfills criteria such as adaptive organisation andgerm-soma specialisation.[17] If so, the same argument, or a criterion of high co-operation and low conflict, would include somemutualistic (e.g. lichens) and sexual partnerships (e.g.anglerfish) as organisms.[18] Ifgroup selection occurs, then a group could be viewed as asuperorganism, optimized by groupadaptation.[19]
Another view is that attributes like autonomy, genetic homogeneity and genetic uniqueness should be examined separately, rather than requiring that an organism possess all of them. On this view, there are multiple dimensions to biological individuality, resulting in several types of organism.[20]
Organisms at differing levels of biological organisation
Alichen consists of a body formed mainly byfungi, with unicellularalgae orcyanobacteria (green) interspersed within the structure, and a bacterialmicrobiome. Thespecies are mutually interdependent, like cells within a multicellular organism.[21]
Differing levels of biological organisation give rise to potentially different understandings of the nature of organisms. Aunicellular organism is amicroorganism such as aprotist,bacterium, orarchaean, composed of a singlecell, which may contain functional structures calledorganelles.[22] Amulticellular organism such as ananimal,plant,fungus, oralga is composed of many cells, often specialised.[22] Acolonial organism such as asiphonophore is a being which functions as an individual but is composed of communicating individuals.[8] Asuperorganism is a colony, such as ofants, consisting of many individuals working together as a single functional orsocial unit.[23][17] Amutualism is a partnership of two or morespecies which each provide some of the needs of the other. Alichen consists offungi andalgae orcyanobacteria, with a bacterialmicrobiome; together, they are able to flourish as a kind of organism, the components having different functions, in habitats such as dry rocks where neither could grow alone.[18][21] The evolutionary biologistsDavid Queller andJoan Strassmann state that "organismality" has evolved socially, as groups of simpler units (from cells upwards) came to cooperate without conflicts. They propose that cooperation should be used as the "defining trait" of an organism.[18]
Queller and Strassmann's view of organisms as cooperating entities at differing levels of biological organisation[18]
Free-living unicellular amoebae for most of lifetime; swarm and aggregate to a multicellular slug, cells specialising to form a dead stalk anda fruiting body[18]
Samuel Díaz‐Muñoz and colleagues (2016) accept Queller and Strassmann's view that organismality can be measured wholly by degrees of cooperation and of conflict. They state that this situates organisms in evolutionary time, so that organismality is context dependent. They suggest that highly integrated life forms, which are not context dependent, may evolve through context-dependent stages towards complete unification.[24]
A virus such astobacco mosaic virus is not a cell; it contains only its genetic material, and a protein coat.
Viruses are not typically considered to be organisms, because they are incapable of autonomousreproduction,growth,metabolism, orhomeostasis. Although viruses have a fewenzymes and molecules like those in living organisms, they have no metabolism of their own; they cannot synthesize the organic compounds from which they are formed. In this sense, they are similar to inanimate matter.[7] Viruses have their owngenes, and theyevolve. Thus, an argument that viruses should be classed as living organisms is their ability to undergo evolution and replicate through self-assembly. However, some scientists argue that viruses neither evolve nor self-reproduce. Instead, viruses are evolved by their host cells, meaning that there was co-evolution of viruses and host cells. If host cells did not exist, viral evolution would be impossible. As for reproduction, viruses rely on hosts' machinery to replicate. The discovery of viruses with genes coding for energy metabolism and protein synthesis fuelled the debate about whether viruses are living organisms, but the genes have a cellular origin. Most likely, they were acquired throughhorizontal gene transfer from viral hosts.[7]
There is an argument for viewing viruses as cellular organisms. Some researchers perceive viruses not as virions alone, which they believe are justspores of an organism, but as a virocell - an ontologically mature viral organism that has cellular structure.[25] Such virus is a result of infection of a cell and shows all major physiological properties of other organisms:metabolism, growth, andreproduction, therefore, life in its effective presence.[12][26]
TheRNA world is a hypothetical stage in the evolutionary history of life on Earth during which self-replicating RNA molecules reproduced before the evolution of DNA and proteins.[27] According to this hypothesis “organisms” emerged when RNA chains began to self-replicate, initiating the three mechanisms of Darwinian selection:heritability, variation of type and differential reproductive output. The fitness of an RNA replicator (its per capita rate of increase) would presumably have been a function of its intrinsic adaptive capacities, determined by itsnucleotide sequence, and the availability of external resources.[28][29] The three primary adaptive capacities of these early "organisms" may have been: (1) replication with moderate fidelity, giving rise to both heritability while allowing variation of type, (2) resistance to decay, and (3) acquisition of and processing of resources[28][29] The capacities of these "organisms" would have functioned by means of the folded configurations of the RNA replicators resulting from their nucleotide sequences.
The philosopher Jack A. Wilson examines some boundary cases to demonstrate that the concept of organism is not sharply defined.[8] In his view,sponges,lichens,siphonophores,slime moulds, andeusocial colonies such as those ofants ornaked molerats, all lie in the boundary zone between being definite colonies and definite organisms (or superorganisms).[8]
Jack A. Wilson's analysis of the similar organism-like nature of siphonophores and jellyfish[8]
Scientists and bio-engineers are experimenting with different types ofsynthetic organism, fromchimaeras composed of cells from two or more species,cyborgs includingelectromechanical limbs,hybrots containing both electronic and biological elements, and other combinations of systems that have variously evolved and been designed.[30]
An evolved organism takes its form by the partially understood mechanisms ofevolutionary developmental biology, in which thegenome directs an elaborated series of interactions to produce successively more elaborate structures. The existence of chimaeras and hybrids demonstrates that these mechanisms are "intelligently" robust in the face of radically altered circumstances at all levels from molecular to organismal.[30]
Synthetic organisms already take diverse forms, and their diversity will increase. What they all have in common is ateleonomic or goal-seeking behaviour that enables them to correct errors of many kinds so as to achieve whatever result they are designed for. Such behaviour is reminiscent of intelligent action by organisms; intelligence is seen as an embodied form ofcognition.[30]
^Huneman, Philippe (2017). "Kant's Concept of Organism Revisited: A Framework for a Possible Synthesis between Developmentalism and Adaptationism?".The Monist.100 (3):373–390.doi:10.1093/monist/onx016.JSTOR26370801.
^abFolse, H.J., III; Roughgarden, J. (December 2010). "What is an individual organism? A multilevel selection perspective".The Quarterly Review of Biology.85 (4):447–472.doi:10.1086/656905.PMID21243964.S2CID19816447.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^Johnson, Mark (9 March 2024). "'Monumental' experiment suggests how life on Earth may have started". The Washington Post. Archived from the original on 9 March 2024. Retrieved 10 March 2024
^abBernstein, H., Byerly, H. C., Hopf, F. A., Michod, R. A., & Vemulapalli, G. K. (1983). The Darwinian Dynamic. The Quarterly Review of Biology, 58(2), 185–207.http://www.jstor.org/stable/2828805
^abMichod, R.E. Darwinian Dynamics: Evolutionary transitions in fitness and individuality. Copyright 1999 Princeton University Press, Princeton, New Jersey ISBN 0-691-02699-8
