An organism is any life thing that functions as an individual.
The evolutionary biologists David Queller and Joan 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 the fungus/Algae partnership of different species in a lichen, or the permanent sexual partnership of an anglerfish, as an organism.
Etymology
The term "organism" (from the
Ancient Greek , derived from , meaning , , or )
first appeared in the English language in the 1660s with the now-obsolete meaning of an organic structure or organization.
It is related to the verb "organize".
In his 1790
Critique of Judgment,
Immanuel Kant defined an organism as "both an organized and a self-organizing being".
Whether criteria exist, or are needed
Among the criteria that have been proposed for being an organism are:
-
autonomous reproduction, Cell growth, and metabolism
-
noncompartmentability – structure cannot be divided without losing functionality.
-
individuality – the entity has simultaneous holdings of genetic uniqueness, genetic homogeneity and autonomy
-
an immune response, separating self from foreign
-
"anti-entropy", the ability to maintain order, a concept first proposed by Erwin Schrödinger;
Other scientists think that the concept of the organism is inadequate in biology;
Problematic cases include colonial organisms: for instance, a colony of eusocial insects fulfills criteria such as adaptive organisation and germ-soma specialisation.
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.
Organisms at differing levels of biological organisation
Differing levels of biological organisation give rise to potentially different understandings of the nature of organisms. A unicellular organism is a
microorganism such as a
protist,
bacterium, or
, composed of a single cell, which may contain functional structures called
.
A multicellular organism such as an
animal,
plant,
fungus, or
alga is composed of many cells, often specialised.
A colonial organism such as a
siphonophore is a being which functions as an individual but is composed of communicating individuals.
A
superorganism is a colony, such as of
, consisting of many individuals working together as a single functional or
Social group.
A mutualism is a partnership of two or more
species which each provide some of the needs of the other. A
lichen consists of
Fungus and
algae or
cyanobacteria, with a bacterial
microbiome; 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.
The evolutionary biologists David Queller and Joan 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.
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.
Boundary cases
Viruses
are not typically considered to be organisms, because they are incapable of autonomous
reproduction,
Cell growth,
metabolism, or
homeostasis. Although viruses have a few
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.
Viruses have their own
, and they
evolution. 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 through horizontal gene transfer from viral hosts.
Evolutionary emergence of organisms
The
RNA 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.
[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] 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 its nucleotide sequence, and the availability of external resources.
[Bernstein, 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][Michod, R.E. Darwinian Dynamics: Evolutionary transitions in fitness and individuality. Copyright 1999 Princeton University Press, Princeton, New Jersey ISBN 0-691-02699-8] 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
[ The capacities of these "organisms" would have functioned by means of the folded configurations of the RNA replicators resulting from their nucleotide sequences.
]
Organism-like colonies
The philosopher Jack A. Wilson examines some boundary cases to demonstrate that the concept of organism is not sharply defined.
Synthetic organisms
Scientists and bio-engineers are experimenting with different types of synthetic organism, from chimaeras composed of cells from two or more species, including electromechanics limbs, containing both electronic and biological elements, and other combinations of systems that have variously evolved and been designed.
An evolved organism takes its form by the partially understood mechanisms of evolutionary developmental biology, in which the genome 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.
Synthetic organisms already take diverse forms, and their diversity will increase. What they all have in common is a Teleonomy 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 of cognition.
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