Organism
An organism is any living thing that functions as an individual. That sounds simple, and it is one of biology's oldest assumptions, but the definition raises more problems than it solves. The trouble starts with the word individual, which is itself difficult to pin down. Several criteria have been proposed to decide what counts as an organism, yet few of them are widely accepted. A jelly-like marine animal, a partnership of two species locked together for life, a colony of insects with some members that never reproduce: each one strains the boundary. What does it take to be a single living thing? Why do viruses sit outside the definition while still managing to evolve? And could cooperation itself, rather than autonomy, be the trait that makes an organism an organism? The answers reach from a single cell to an entire ant colony.
The term organism first appeared in English in the 1660s, carrying a meaning now obsolete: an organic structure or organization. It is related to the verb organize, and from the start it pointed at arrangement rather than mere aliveness. The Ancient Greek root behind it reinforces that sense of an ordered, working whole. In his 1790 Critique of Judgment, Immanuel Kant offered a definition that biologists still wrestle with. He called an organism both an organized and a self-organizing being. That phrase folds two ideas together: a thing with parts arranged for a purpose, and a thing that produces and maintains that arrangement itself. Centuries later, the question of what self-organization really requires would split scientists into opposing camps.
Autonomous reproduction, growth, and metabolism form the most common test for being an organism. By that standard viruses are excluded, even though they evolve. Other criteria push at the problem from different directions. Noncompartmentability holds that an organism's structure cannot be divided without losing functionality. Richard Dawkins phrased it as the quality of being sufficiently heterogeneous in form to be rendered non-functional if cut in half. The catch is that many organisms can be cut into pieces, and each piece grows into a whole organism. Individuality offers another route: the entity holds genetic uniqueness, genetic homogeneity, and autonomy at the same time. Some scientists point instead to an immune response, the body's way of separating self from foreign. Erwin Schrodinger proposed anti-entropy, the ability to maintain order. A related version borrows Claude Shannon's information theory, identifying organisms as things capable of self-maintaining their information content. No single criterion has settled the matter, and some scientists conclude that the concept of the organism is simply inadequate for biology.
David Queller and Joan Strassmann, both evolutionary biologists, propose a different anchor for the whole question. They argue that organismality, the qualities that define an entity as an organism, has evolved socially. Groups of simpler units, from cells upwards, came to cooperate without conflicts, and that cooperation is what they would use as the defining trait. The consequences are sweeping. A lichen, which joins a fungus and an alga of different species, would count as an organism on this view. So would the permanent sexual partnership of an anglerfish, where male and female are fastened together for life. Samuel Diaz-Munoz and colleagues, writing in 2016, accept that organismality can be measured wholly by degrees of cooperation and conflict. They add a striking implication: this situates organisms in evolutionary time, so organismality becomes context-dependent. They suggest that highly integrated life forms may evolve through context-dependent stages toward complete unification.
A colony of eusocial insects is organised adaptively and shows germ-soma specialisation. Some insects reproduce while others never do, much like the cells in an animal's body, where only some pass on genes. If group selection occurs, such a group could be viewed as a superorganism, optimized by group adaptation. The siphonophore makes the puzzle physical. This jelly-like marine animal is built from organism-like zooids, yet the whole structure looks and functions much like a jellyfish, its parts collaborating as one. The philosopher Jack A. Wilson lined up the two side by side to test the boundary. In a siphonophore the top of the colony is gas-filled for buoyancy, while a jellyfish uses jelly. Nectophores coordinate to pump water for propulsion, where a jellyfish simply pulsates its body. Palpons and gastrozooids ingest prey and feed other zooids, where a jellyfish traps prey with tentacles. Both end up as a single functional individual, one made of many zooids, the other of many cells. Wilson placed sponges, lichens, slime moulds, and eusocial colonies of ants or naked molerats all in the same boundary zone, between definite colonies and definite organisms.
Viruses are not typically considered organisms because they cannot reproduce, grow, metabolise, or maintain homeostasis on their own. They carry a few enzymes and molecules resembling those in living things, but they have no metabolism. They cannot synthesize the organic compounds from which they are made, which makes them, in this sense, similar to inanimate matter. Viruses still have their own genes, and they evolve, and that fact keeps the debate alive. The argument for counting them as living rests on their ability to undergo evolution and replicate through self-assembly. The counterargument is sharper than it first appears. Some scientists hold that viruses neither evolve nor self-reproduce on their own terms. Instead they are evolved by their host cells, the product of co-evolution between virus and host. Without host cells, viral evolution would be impossible, and for reproduction viruses rely entirely on a host's machinery. The discovery of viruses with genes for energy metabolism and protein synthesis seemed to tilt the scales toward life. But those genes have a cellular origin, most likely acquired through horizontal gene transfer from viral hosts. Where a cellular organism stores its genetic information in DNA, a virus may use DNA or RNA.
The RNA world is a hypothetical stage in the history of life on Earth, a time when self-replicating RNA molecules reproduced before DNA and proteins existed. On this hypothesis, organisms emerged when RNA chains began to self-replicate. That act set in motion the three mechanisms of Darwinian selection: heritability, variation of type, and differential reproductive output. The fitness of an early RNA replicator, its per capita rate of increase, would have depended on its intrinsic adaptive capacities and the availability of external resources. Three primary capacities mattered most. The first was replication with moderate fidelity, faithful enough to give heritability while still allowing variation. The second was resistance to decay. The third was the acquisition and processing of resources. All of these would have worked through the folded configurations that an RNA replicator takes on, shapes determined by its own nucleotide sequence.
Chimaeras built from the cells of two or more species are no longer thought experiments; scientists and bio-engineers are making them now. The roster of synthetic organisms also includes cyborgs with electromechanical limbs and hybrots, which contain both electronic and biological elements. An evolved organism takes its form through evolutionary developmental biology, where the genome directs a series of interactions that build successively more elaborate structures. The existence of chimaeras and hybrids shows that these mechanisms are intelligently robust, holding up under radically altered circumstances from the molecular level to the whole organism. What every synthetic organism shares is a teleonomic, or goal-seeking, behaviour. It lets them correct many kinds of error to reach whatever result they were designed for, a behaviour reminiscent of intelligent action. On this view intelligence is an embodied form of cognition, and the diversity of these designed life forms is only expected to grow.
Common questions
What is the definition of an organism in biology?
An organism is any living thing that functions as an individual. The most common criteria for being an organism are autonomous reproduction, growth, and metabolism, though few proposed criteria are widely accepted.
Why are viruses not considered organisms?
Viruses are not typically considered organisms because they cannot reproduce, grow, metabolise, or maintain homeostasis on their own. They have no metabolism and rely entirely on a host's machinery to replicate, though they do have their own genes and evolve.
What did David Queller and Joan Strassmann propose about organisms?
David Queller and Joan Strassmann proposed that cooperation should be the defining trait of an organism. They argue that organismality evolved socially, as groups of simpler units from cells upwards came to cooperate without conflicts, which would treat lichens and anglerfish partnerships as organisms.
How did Immanuel Kant define an organism?
In his 1790 Critique of Judgment, Immanuel Kant defined an organism as both an organized and a self-organizing being. The term organism first appeared in English in the 1660s, meaning an organic structure or organization.
What is a siphonophore and why is it a boundary case for organisms?
A siphonophore is a jelly-like marine animal composed of organism-like zooids that together look and function much like a jellyfish. The philosopher Jack A. Wilson placed it in the boundary zone between being a definite colony and a definite organism.
What is the RNA world hypothesis in relation to organisms?
The RNA world is a hypothetical stage in the history of life on Earth when self-replicating RNA molecules reproduced before DNA and proteins evolved. According to the hypothesis, organisms emerged when RNA chains began to self-replicate, initiating heritability, variation of type, and differential reproductive output.