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Amoeba (Amobe) 01.jpg
An amoeba is a single-celled eukaryote.
Cyanobacteria are multicellular prokaryotes.
Vliegenzwam (Amanita muscaria). Locatie De Famberhorst. 27-09-2020 (d.j.b.).jpg
Fungi are multicellular eukaryotes.

An organism (from Ancient Greek ὄργανον (órganon) 'instrument, implement, tool',and -ισμός (-ismós)) is any biological living system that functions as an individual life form. [1] All organisms are composed of cells. [1] The idea of organism is based on the concept of minimal functional unit of life. Three traits have been proposed to play the main role in qualification as an organism:


Organisms include multicellular animals, plants, and fungi; or unicellular microorganisms such as protists, bacteria, and archaea. [5] All types of organisms are capable of reproduction, growth and development, maintenance, and some degree of response to stimuli. Most multicellular organisms differentiate into specialized tissues and organs during their development.

In 2016, a set of 355 genes from the last universal common ancestor (LUCA) of all organisms from Earth was identified.


The term "organism" (from Greek ὀργανισμός, organismos, from ὄργανον, organon, i.e. "instrument, implement, tool, organ of sense or apprehension") [6] [7] first appeared in the English language in 1703 and took on its current definition by 1834 ( Oxford English Dictionary ). It is directly related to the term "organization". There is a long tradition of defining organisms as self-organizing beings, going back at least to Immanuel Kant's 1790 Critique of Judgment . [8]


An organism may be defined as an assembly of molecules functioning as a more or less stable whole that exhibits the properties of life. Dictionary definitions can be broad, using phrases such as "any living structure, such as a plant, animal, fungus or bacterium, capable of growth and reproduction". [9] Many definitions exclude viruses and possible synthetic non-organic life forms, as viruses are dependent on the biochemical machinery of a host cell for reproduction. [10] A superorganism is an organism consisting of many individuals working together as a single functional or social unit. [11]

There has been controversy about the best way to define the organism, [12] and from a philosophical point of view, whether such a definition is necessary. [13] [14] [15] Problematic cases include colonial organisms: for instance, a colony of eusocial insects fulfils criteria such as adaptive organisation and germ-soma specialisation. [16] If so, the same argument would include some mutualistic and sexual partnerships as organisms. [17] If group selection occurs, then a group could be viewed as a superorganism, optimized by group adaptation. [18] 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. [19]

Other views include the idea that an individual is distinguished by its immune response, separating self from foreign; [20] that "anti-entropy", the ability to maintain order, is what distinguishes an organism; [21] or that Shannon's information theory can be used to identify organisms as capable of self-maintaining their information content. [4] Finally, it may be that the concept of the organism is inadequate in biology. [22]


Viruses are not typically considered to be organisms because they are incapable of autonomous reproduction, growth or metabolism. 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. [23] 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. [23]


Precambrian stromatolites in the Siyeh Formation, Glacier National Park. In 2002, a paper in the scientific journal Nature suggested that these 3.5 Gya (billion years old) geological formations contain fossilized cyanobacteria microbes. This suggests they are evidence of one of the earliest known life forms on Earth. Stromatolites.jpg
Precambrian stromatolites in the Siyeh Formation, Glacier National Park. In 2002, a paper in the scientific journal Nature suggested that these 3.5 Gya (billion years old) geological formations contain fossilized cyanobacteria microbes. This suggests they are evidence of one of the earliest known life forms on Earth.

There is strong evidence from genetics that all organisms have a common ancestor. In particular, every living cell makes use of nucleic acids as its genetic material, and uses the same twenty amino acids as the building blocks for proteins. All organisms use the same genetic code (with some extremely rare and minor deviations) to translate nucleic acid sequences into proteins. The universality of these traits strongly suggests common ancestry, because the selection of many of these traits seems arbitrary. Horizontal gene transfer makes it more difficult to study the last universal ancestor. [24] However, the universal use of the same genetic code, same nucleotides, and same amino acids makes the existence of such an ancestor overwhelmingly likely. [25] The first organisms were possibly anaerobic and thermophilic chemolithoautotrophs that evolved within inorganic compartments at geothermal environments. [26] [27]

The last universal common ancestor is the most recent organism from which all organisms now living on Earth descend. [25] Thus, it is the most recent common ancestor of all current life on Earth. The last universal common ancestor lived some 3.5 to 3.8 billion years ago, in the Paleoarchean era. [28] [29] In 2016, a set of 355 genes considered likely to derive directly from the last universal common ancestor was identified. [30] [31]

Human intervention

Modern biotechnology is challenging traditional concepts of organisms and species. Cloning is the process of creating a new multicellular organism, genetically identical to another, with the potential of creating entirely new species of organisms. Cloning is the subject of ethical debate. [32] [33] [34]

In 2008, the J. Craig Venter Institute assembled a synthetic bacterial genome, Mycoplasma genitalium , by using recombination in yeast of 25 overlapping DNA fragments in a single step. The use of yeast recombination greatly simplifies the assembly of large DNA molecules from both synthetic and natural fragments. [35]

See also

Related Research Articles

<span class="mw-page-title-main">Asexual reproduction</span> Reproduction without a sexual process

Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the full set of genes of their single parent and thus the newly created individual is genetically and physically similar to the parent or an exact clone of the parent. Asexual reproduction is the primary form of reproduction for single-celled organisms such as archaea and bacteria. Many eukaryotic organisms including plants, animals, and fungi can also reproduce asexually. In vertebrates, the most common form of asexual reproduction is parthenogenesis, which is typically used as an alternative to sexual reproduction in times when reproductive opportunities are limited. Komodo dragons and some monitor lizards can reproduce asexually.

<span class="mw-page-title-main">Evolution</span> Change in the heritable characteristics of biological populations

Evolution is the change in the heritable characteristics of biological populations over successive generations. Evolution occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, resulting in certain characteristics becoming more or less common within a population over successive generations. The process of evolution has given rise to biodiversity at every level of biological organisation.

<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, and is defined descriptively by the capacity for homeostasis, organisation, metabolism, growth, adaptation, response to stimuli, and reproduction. 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">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">Colony (biology)</span> Living things grouping together, usually for common benefit

In biology, a colony is composed of two or more conspecific individuals living in close association with, or connected to, one another. This association is usually for mutual benefit such as stronger defense or the ability to attack bigger prey.

In cellular biology, a somatic cell, or vegetal cell, is any biological cell forming the body of a multicellular organism other than a gamete, germ cell, gametocyte or undifferentiated stem cell. Somatic cells compose the body of an organism and divide through the process of binary fission and mitotic division.

<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, in contrast to unicellular organism. 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> How sexually reproducing multicellular organisms could have evolved from a common ancestor species

Sexual reproduction is an adaptive feature which is common to almost all multicellular organisms and various unicellular organisms. Currently, the adaptive advantage of sexual reproduction is widely regarded as a major unsolved problem in biology. As discussed below, one prominent theory is that sex evolved as an efficient mechanism for producing variation, and this had the advantage of enabling organisms to adapt to changing environments. Another prominent theory, also discussed below, is that a primary advantage of outcrossing sex is the masking of the expression of deleterious mutations. Additional theories concerning the adaptive advantage of sex are also discussed below. Sex does, however, come with a cost. In reproducing asexually, no time nor energy needs to be expended in choosing a mate and, if the environment has not changed, then there may be little reason for variation, as the organism may already be well-adapted. However, very few environments have not changed over the millions of years that reproduction has existed. Hence it is easy to imagine that being able to adapt to changing environment imparts a benefit. Sex also halves the amount of offspring a given population is able to produce. Sex, however, has evolved as the most prolific means of species branching into the tree of life. Diversification into the phylogenetic tree happens much more rapidly via sexual reproduction than it does by way of asexual reproduction.

In biology, a strain is a genetic variant, a subtype or a culture within a biological species. Strains are often seen as inherently artificial concepts, characterized by a specific intent for genetic isolation. This is most easily observed in microbiology where strains are derived from a single cell colony and are typically quarantined by the physical constraints of a Petri dish. Strains are also commonly referred to within virology, botany, and with rodents used in experimental studies.

In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA), of a set of organisms is the most recent individual from which all the organisms of the set are descended. The term is also used in reference to the ancestry of groups of genes (haplotypes) rather than organisms.

<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.

<span class="mw-page-title-main">Last universal common ancestor</span> Most recent common ancestor of all current life on Earth

The last universal common ancestor (LUCA) is the hypothesized common ancestral cell from which the three domains of life, the Bacteria, the Archaea, and the Eukarya originated. It is suggested to have been a "cellular organism that had a lipid bilayer and used DNA, RNA, and protein". The LUCA has also been defined as "a hypothetical organism ancestral to all three domains". The LUCA is the point or stage at which the three domains of life diverged from preexisting forms of life. The nature of this point or stage of divergence remains a topic of research.

<span class="mw-page-title-main">Biologist</span> A scientist studying living organisms

A biologist is a scientist who conducts research in biology. Biologists are interested in studying life on Earth, whether it is an individual cell, a multicellular organism, or a community of interacting populations. They usually specialize in a particular branch of biology and have a specific research focus.

<span class="mw-page-title-main">Non-cellular life</span> Life that has no cellular structure

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 this is for some no longer considered necessary, and modern criteria allow for forms of life based on other structural arrangements.

Microbial genetics is a subject area within microbiology and genetic engineering. Microbial genetics studies microorganisms for different purposes. The microorganisms that are observed are bacteria, and archaea. Some fungi and protozoa are also subjects used to study in this field. The studies of microorganisms involve studies of genotype and expression system. Genotypes are the inherited compositions of an organism. Genetic Engineering is a field of work and study within microbial genetics. The usage of recombinant DNA technology is a process of this work. The process involves creating recombinant DNA molecules through manipulating a DNA sequence. That DNA created is then in contact with a host organism. Cloning is also an example of genetic engineering.

<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.

<span class="mw-page-title-main">Prokaryote</span> Unicellular organism lacking a membrane-bound nucleus

A prokaryote is a single-cell organism whose cell lacks a nucleus and other membrane-bound organelles. The word prokaryote comes from the Ancient Greek πρό 'before' and κάρυον 'nut, kernel'. In the two-empire system arising from the work of Édouard Chatton, prokaryotes were classified within the empire Prokaryota. But in the three-domain system, based upon molecular analysis, prokaryotes are divided into two domains: Bacteria and Archaea. Organisms with nuclei are placed in a third domain, Eukaryota.

<span class="mw-page-title-main">Sexual reproduction</span> Biological process

Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete with a single set of chromosomes combines with another gamete to produce a zygote that develops into an organism composed of cells with two sets of chromosomes (diploid). This is typical in animals, though the number of chromosome sets and how that number changes in sexual reproduction varies, especially among plants, fungi, and other eukaryotes.

<span class="mw-page-title-main">Eukaryogenesis</span> Process of forming the first eukaryotic cell

Eukaryogenesis, the process which created the eukaryotic cell and lineage, is a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. The process is widely agreed to have involved symbiogenesis, in which archaea and bacteria came together to create the first eukaryotic common ancestor (FECA). This cell had a new level of complexity and capability, with a nucleus, at least one centriole and cilium, facultatively aerobic mitochondria, sex, a dormant cyst with a cell wall of chitin and/or cellulose and peroxisomes. It evolved into a population of single-celled organisms that included the last eukaryotic common ancestor (LECA), gaining capabilities along the way, though the sequence of the steps involved has been disputed, and may not have started with symbiogenesis. In turn, the LECA gave rise to the eukaryotes' crown group, containing the ancestors of animals, fungi, plants, and a diverse range of single-celled organisms.

This glossary of genetics and evolutionary biology is a list of definitions of terms and concepts used in the study of genetics and evolutionary biology, as well as sub-disciplines and related fields, with an emphasis on classical genetics, quantitative genetics, population biology, phylogenetics, speciation, and systematics. Overlapping and related terms can be found in Glossary of cellular and molecular biology, Glossary of ecology, and Glossary of biology.


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