Organism

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An organism is defined in a medical dictionary as any living thing that functions as an individual. [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 autonomous reproduction, growth, and metabolism. This would exclude viruses, despite the fact that they evolve like organisms. Other problematic cases include colonial organisms; a colony of eusocial insects is organised adaptively, and has germ-soma specialisation, with some insects reproducing, others not, like cells in an animal's body. The body of a siphonophore, a jelly-like marine animal, is composed of organism-like zooids, but the whole structure looks and functions much like an animal such as a jellyfish, the parts collaborating to provide the functions of the colonial organism.

Contents

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/alga 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 ὀργανισμός , derived from órganon, 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 1790 Critique of Judgment , Immanuel Kant defined an organism as "both an organized and a self-organizing being". [4] [5]

Whether criteria exist, or are needed

One criterion proposes that an organism cannot be divided without losing functionality. This basil plant cutting is however developing new adventitious roots from a small bit of stem, forming a new plant. Basilikumwurzling.jpg
One criterion proposes that an organism cannot be divided without losing functionality. This basil plant cutting is however developing new adventitious roots from a small bit of stem, forming a new plant.

Among the criteria that have been proposed for being an organism are:

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 include colonial organisms: for instance, a colony of eusocial insects fulfills criteria such as adaptive organisation and germ-soma specialisation. [17] If so, the same argument, or a criterion of high co-operation and low conflict, would include some mutualistic (e.g. lichens) and sexual partnerships (e.g. anglerfish) as organisms. [18] If group selection occurs, then a group could be viewed as a superorganism, optimized by group adaptation. [19]

Another view is that attributes like autonomy, genetic homogeneity and genetic uniqueness should be examined separately rather than demanding that an organism should have all of them; if so, there are multiple dimensions to biological individuality, resulting in several types of organism. [20]

Organisms at differing levels of biological organisation

A lichen consists of a body formed mainly by fungi, with unicellular algae or cyanobacteria (green) interspersed within the structure, and a bacterial microbiome. The species are mutually interdependent, like cells within a multicellular organism. Lichen cross section - heteromeric thallus unlabelled.svg
A lichen consists of a body formed mainly by fungi, with unicellular algae or cyanobacteria (green) interspersed within the structure, and a bacterial microbiome. The species are mutually interdependent, like cells within a multicellular organism.

A unicellular organism is a microorganism such as a protist, bacterium, or archaean, composed of a single cell, which may contain functional structures called organelles. [22]

A multicellular organism such as an animal, plant, fungus, or alga is composed of many cells, often specialised. [22]

A colonial organism such as a siphonophore is a being which functions as an individual but is composed of communicating individuals. [8]

A superorganism is a colony, such as of ants, consisting of many individuals working together as a single functional or social unit. [23] [17]

A mutualism is a partnership of two or more species which each provide some of the needs of the other. A lichen consists of fungi 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. [18] [21]

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. [18]

Queller and Strassmann's view of organisms as cooperating entities at differing levels of biological organisation [18]
LevelExampleCompositionMetabolism,
growth,
reproduction		Co-operation
Virus Tobacco mosaic virus Nucleic acid, protein NoNo metabolism, so not living, not an organism, say many biologists; [7] but they evolve, their genes collaborating to manipulate the host [18]
Unicellular organism Paramecium One cell, with organelles e.g. cilia for specific functionsYesInter-cellular (inter-organismal) signalling [22]
Swarming protistan Dictyostelium (cellular slime mould)Unicellular amoebae YesFree-living unicellular amoebae for most of lifetime; swarm and aggregate to a multicellular slug, cells specialising to form a dead stalk and a fruiting body [18]
Multicellular organism Mushroom-forming fungusCells, grouped into organs for specific functions (e.g. reproduction)YesCell specialisation, communication [22]
Permanent sexual partnership Anglerfish Male and female permanently fastened togetherYesMale provides male gametes; female provides all other functions [18]
Mutualism Lichen Organisms of different species Yes Fungus provides structure, absorbs water and minerals; alga photosynthesises [18]
Joined colony Siphonophore Zooids joined togetherYesOrganism specialisation; inter-organism signalling [8]
Superorganism Ant colonyIndividuals living togetherYesOrganism specialisation (many ants do not reproduce); inter-organism signalling [23]

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]

Boundary cases

Viruses

A virus such as tobacco mosaic virus is not a cell; it contains only its genetic material, and a protein coat. TMV structure simple.png
A virus such as tobacco 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 autonomous reproduction, growth, metabolism, or homeostasis. Although viruses have a few enzymes 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 own genes, and they evolve. 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. [7]

Comparison of cellular organisms and viruses [7]
Capability Cellular organism Virus
Metabolism YesNo, rely entirely on host cell
Growth YesNo, just self-assembly
Reproduction YesNo, rely entirely on host cell
Store genetic information about themselves DNA DNA or RNA
Able to evolve Yes: mutation, recombination, natural selection Yes: high mutation rate, natural selection

There is an argument for viewing viruses as cellular organisms. Some researchers perceive viruses not as virions alone, which they believe are just spores 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, and reproduction, therefore, life in its effective presence. [12] [26]

Organism-like colonies

Apolemia, a colonial siphonophore that functions as a single individual Apolemia sp.jpg
Apolemia , a colonial siphonophore that functions as a single individual

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, and eusocial colonies such as those of ants or naked 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]
FunctionColonial siphonophore Jellyfish
BuoyancyTop of colony is gas-filledJelly
Propulsion Nectophores co-ordinate to pump waterBody pulsates to pump water
Feeding Palpons and gastrozooids ingest prey, feed other zooidsTentacles trap prey, pass it to mouth
Functional structureSingle functional individualSingle functional individual
CompositionMany zooids, possibly individualsMany cells

Synthetic organisms

Insect cyborg Insect cyborg.jpg
Insect cyborg

Scientists and bio-engineers are experimenting with different types of synthetic organism, from chimaeras composed of cells from two or more species, cyborgs including electromechanical limbs, hybrots containing both electronic and biological elements, and other combinations of systems that have variously evolved and been designed. [27]

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. [27]

Synthetic organisms already take diverse forms, and their diversity will increase. What they all have in common is a teleonomic 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. [27]

Early evolution of organisms

All organisms that exist today possess a self-replicating informational molecule (genome), and such an informational molecule is likely intrinsic to life. Thus, the earliest organisms also presumably possessed a self-replicating informational molecule (genome), perhaps RNA [28] [29] or an informational molecule more primitive than RNA. The specific nucleotide sequences in all currently extant organisms contain information that functions to promote survival, reproduction, and the ability to acquire resources necessary for reproduction, and sequences with such functions probably emerged early in the evolution of life. It is also likely that survival sequences present early in the evolution of organisms included sequences that facilitate the avoidance of damage to the self-replicating molecule and promote the capability to repair such damages that do occur. Repair of some of the genome damages in these early organisms may have involved the capacity to use undamaged information from another similar genome by a process of recombination (a primitive form of sexual interaction). [30]

Related Research Articles

<span class="mw-page-title-main">Cell (biology)</span> Basic unit of many life forms

The cell is the basic structural and functional unit of all forms of life. Every cell consists of cytoplasm enclosed within a membrane; many cells contain organelles, each with a specific function. The term comes from the Latin word cellula meaning 'small room'. Most cells are only visible under a microscope. Cells emerged on Earth about 4 billion years ago. All cells are capable of replication, protein synthesis, and motility.

<span class="mw-page-title-main">Life</span> Matter with biological processes

Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from matter that does not. It is defined descriptively by the capacity for homeostasis, organisation, metabolism, growth, adaptation, response to stimuli, and reproduction. All life over time eventually reaches a state of death and none is immortal. Many philosophical definitions of living systems have been proposed, such as self-organizing systems. Viruses in particular make definition difficult as they replicate only in host cells. Life exists all over the Earth in air, water, and soil, with many ecosystems forming the biosphere. Some of these are harsh environments occupied only by extremophiles.

<span class="mw-page-title-main">RNA world</span> Hypothetical stage in the early evolutionary history of life on Earth

The RNA world is a hypothetical stage in the evolutionary history of life on Earth in which self-replicating RNA molecules proliferated before the evolution of DNA and proteins. The term also refers to the hypothesis that posits the existence of this stage.

<span class="mw-page-title-main">Reproduction</span> Biological process by which new organisms are generated from one or more parent organisms

Reproduction is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. There are two forms of reproduction: asexual and sexual.

<span class="mw-page-title-main">Superorganism</span> Group of synergistic organisms

A superorganism, or supraorganism, is a group of synergetically-interacting organisms of the same species. A community of synergetically-interacting organisms of different species is called a holobiont.

<span class="mw-page-title-main">Multicellular organism</span> Organism that consists of more than one cell

A multicellular organism is an organism that consists of more than one cell, unlike unicellular organisms. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- and partially multicellular, like slime molds and social amoebae such as the genus Dictyostelium.

<span class="mw-page-title-main">Evolution of sexual reproduction</span>

Evolution of sexual reproduction describes how sexually reproducing animals, plants, fungi and protists could have evolved from a common ancestor that was a single-celled eukaryotic species. Sexual reproduction is widespread in eukaryotes, though a few eukaryotic species have secondarily lost the ability to reproduce sexually, such as Bdelloidea, and some plants and animals routinely reproduce asexually without entirely having lost sex. The evolution of sexual reproduction contains two related yet distinct themes: its origin and its maintenance. Bacteria and Archaea (prokaryotes) have processes that can transfer DNA from one cell to another, but it is unclear if these processes are evolutionarily related to sexual reproduction in Eukaryotes. In eukaryotes, true sexual reproduction by meiosis and cell fusion is thought to have arisen in the last eukaryotic common ancestor, possibly via several processes of varying success, and then to have persisted.

<span class="mw-page-title-main">Unit of selection</span> Biological entity within the hierarchy of biological organization

A unit of selection is a biological entity within the hierarchy of biological organization that is subject to natural selection. There is debate among evolutionary biologists about the extent to which evolution has been shaped by selective pressures acting at these different levels.

The philosophy of biology is a subfield of philosophy of science, which deals with epistemological, metaphysical, and ethical issues in the biological and biomedical sciences. Although philosophers of science and philosophers generally have long been interested in biology, philosophy of biology only emerged as an independent field of philosophy in the 1960s and 1970s, associated with the research of David Hull. Philosophers of science then began paying increasing attention to biology, from the rise of Neodarwinism in the 1930s and 1940s to the discovery of the structure of DNA in 1953 to more recent advances in genetic engineering. Other key ideas include the reduction of all life processes to biochemical reactions, and the incorporation of psychology into a broader neuroscience.

Non-cellular life, also known as acellular life, is life that exists without a cellular structure for at least part of its life cycle. Historically, most definitions of life postulated that an organism must be composed of one or more cells, but, for some, this is no longer considered necessary, and modern criteria allow for forms of life based on other structural arrangements.

<span class="mw-page-title-main">Biological organisation</span> Hierarchy of complex structures and systems within biological sciences

Biological organisation is the organisation of complex biological structures and systems that define life using a reductionistic approach. The traditional hierarchy, as detailed below, extends from atoms to biospheres. The higher levels of this scheme are often referred to as an ecological organisation concept, or as the field, hierarchical ecology.

<span class="mw-page-title-main">Biology</span> Science that studies life

Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary information encoded in genes, which can be transmitted to future generations. Another major theme is evolution, which explains the unity and diversity of life. Energy processing is also important to life as it allows organisms to move, grow, and reproduce. Finally, all organisms are able to regulate their own internal environments.

The evolution of biological complexity is one important outcome of the process of evolution. Evolution has produced some remarkably complex organisms – although the actual level of complexity is very hard to define or measure accurately in biology, with properties such as gene content, the number of cell types or morphology all proposed as possible metrics.

<span class="mw-page-title-main">Living systems</span> Multiple interactions and regulation of life forms with their environment

Living systems are life forms treated as a system. They are said to be open self-organizing and said to interact with their environment. These systems are maintained by flows of information, energy and matter. Multiple theories of living systems have been proposed. Such theories attempt to map general principles for how all living systems work.

<span class="mw-page-title-main">Virus</span> Infectious agent that replicates in cells

A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity. Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, more than 11,000 of the millions of virus species have been described in detail. The study of viruses is known as virology, a subspeciality of microbiology.

A protocell is a self-organized, endogenously ordered, spherical collection of lipids proposed as a rudimentary precursor to cells during the origin of life. A central question in evolution is how simple protocells first arose and how their progeny could diversify, thus enabling the accumulation of novel biological emergences over time. Although a functional protocell has not yet been achieved in a laboratory setting, the goal to understand the process appears well within reach.

Evolution of cells refers to the evolutionary origin and subsequent evolutionary development of cells. Cells first emerged at least 3.8 billion years ago approximately 750 million years after Earth was formed.

<span class="mw-page-title-main">Allorecognition</span>

Allorecognition is the ability of an individual organism to distinguish its own tissues from those of another. It manifests itself in the recognition of antigens expressed on the surface of cells of non-self origin. Allorecognition has been described in nearly all multicellular phyla.

<span class="mw-page-title-main">Minimal genome</span> Concept in genetics

The minimal genome is a concept which can be defined as the set of genes sufficient for life to exist and propagate under nutrient-rich and stress-free conditions. Alternatively, it can also be defined as the gene set supporting life on an axenic cell culture in rich media, and it is thought what makes up the minimal genome will depend on the environmental conditions that the organism inhabits.

The first universal common ancestor (FUCA) is a proposed non-cellular entity that was the earliest organism with a genetic code capable of biological translation of RNA molecules into peptides to produce proteins. Its descendents include the last universal common ancestor (LUCA) and every modern cell. FUCA would also be the ancestor of ancient sister lineages of LUCA, none of which have modern descendants, but which are thought to have horizontally transferred some of their genes into the genome of early descendants of LUCA.

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