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An organism is 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. Several criteria, few of which are widely accepted, have been proposed to define what constitutes an organism. Among the most common is that an organism has autonomous reproduction, growth, and metabolism. This would exclude viruses, even though 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.

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

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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

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One criterion proposes that an organism cannot be divided without losing functionality.[6] 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 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

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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.[21]

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 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]
Level Example Composition Metabolism,
growth,
reproduction 		Co-operation
Virus Tobacco mosaic virus Nucleic acid, protein No No 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 functions Yes Inter-cellular (inter-organismal) signalling[22]
Swarming protistan Dictyostelium (cellular slime mould) Unicellular amoebae Yes Free-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 fungus Cells, grouped into organs for specific functions (e.g. reproduction) Yes Cell specialisation, communication[22]
Permanent sexual partnership Anglerfish Male and female permanently fastened together Yes Male 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 together Yes Organism specialisation; inter-organism signalling[8]
Superorganism Ant colony Individuals living together Yes Organism 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

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Viruses

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Main article: Virus
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 Yes No, rely entirely on host cell
Growth Yes No, just self-assembly
Reproduction Yes No, 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

Evolutionary emergence of organisms

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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.[25] 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.[26][27] 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[26][27] 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

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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]
Function Colonial siphonophore Jellyfish
Buoyancy Top of colony is gas-filled Jelly
Propulsion Nectophores co-ordinate to pump water Body pulsates to pump water
Feeding Palpons and gastrozooids ingest prey, feed other zooids Tentacles trap prey, pass it to mouth
Functional structure Single functional individual Single functional individual
Composition Many zooids, possibly individuals Many cells

Synthetic organisms

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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.[28]

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.[28]

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.[28]

References

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Organism

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An organism is a living entity, such as an , , bacterium, or , that constitutes a basic unit of distinct in time with a defined beginning and end. Organisms are characterized by several fundamental properties that distinguish them from non-living , including cellular , (the use of and materials), growth and development, , response to environmental stimuli, (maintenance of internal stability), adaptation through , and regulation of internal processes. These traits enable organisms to interact with their surroundings, sustain themselves, and propagate their genetic material, often encoded in or , across generations. Organisms exhibit vast diversity in form, size, and , ranging from microscopic single-celled microbes in extreme environments to complex multicellular life forms spanning ecosystems worldwide. They are classified into three primary domains based on cellular structure and genetic differences: (prokaryotic cells without a nucleus, including most common microbes), Archaea (prokaryotes, many of which are adapted to extreme conditions, such as high temperatures or salinity), and Eukarya (organisms with complex cells containing a nucleus, encompassing protists, fungi, , and animals). This classification reflects evolutionary relationships and underpins the study of , revealing how life has diversified over billions of years from a common .

Definition and Characteristics

Core Definition of an Organism

In biology, an organism is defined as a contiguous living system composed of one or more cells that maintains its integrity through , exhibits growth and development, reproduces to propagate its lineage, responds to environmental stimuli, and adapts over generations through evolutionary processes. This definition emphasizes the organism's capacity for self-maintenance and continuity as a discrete , distinguishing it from inanimate or mere chemical reactions. A prominent contemporary , developed for , describes —and by extension, —as a self-sustaining chemical system capable of Darwinian evolution. This working definition, proposed by , highlights the role of evolutionary mechanisms in enabling to undergo heritable changes that enhance survival and reproduction in varying conditions. It addresses the challenge of delineating from non-life by focusing on systemic properties rather than specific molecular compositions, allowing for potential extraterrestrial forms while grounding the concept in observable biological principles. Key criteria for conferring organism status include individuality, , and . Individuality refers to the organism's bounded, cohesive structure that persists as a unit despite internal complexity or external perturbations. Heredity ensures the transmission of genetic information across generations, enabling continuity and variation. involves the coordinated physiological processes that sustain , such as energy processing and regulatory feedback, unifying the system's components into a singular . These criteria collectively demarcate organisms from aggregates like crystals or populations, though debates persist regarding edge cases like symbiotic consortia. Bacteria exemplify minimal free-living organisms, possessing the smallest known genomes sufficient for independent replication and metabolism, as seen in species like Pelagibacter ubique with volumes around 0.01 cubic micrometers. In contrast, complex ecosystems, such as coral reefs or forests, represent supraindividual assemblages of multiple organisms interacting without forming a single integrated entity, underscoring the scalar boundaries of organismality.

Essential Properties of Life

Organisms are distinguished from non-living matter by seven essential properties: cellular organization, , , growth and development, , response to the environment (also known as ), and adaptation through . These properties are observable across all forms of life, from to complex multicellular organisms, and collectively enable the maintenance and propagation of life. Cellular organization refers to the fundamental structure of , where all organisms are composed of one or more cells, the basic units that carry out vital functions. In unicellular organisms like , a single cell performs all necessary processes, while multicellular organisms consist of specialized cells organized into tissues and organs. This compartmentalization allows for efficient coordination of biological activities. Metabolism encompasses the sum of all chemical reactions within an organism that sustain life, divided into —the breakdown of complex molecules to release —and —the synthesis of complex molecules from simpler ones using that . These processes are powered by (ATP), the primary energy currency of cells, which stores and transfers for cellular work. A key catabolic pathway is , where glucose is oxidized to produce ATP, , and water, summarized by the equation: C6H12O6+6O2→6CO2+6H2O+energy\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + energy} This reaction exemplifies how organisms convert from nutrients into usable forms. is the ability to maintain a stable internal environment despite external changes, achieved through regulatory mechanisms. In mammals, body temperature regulation exemplifies this via loops: when temperature rises above the set point (around 37°C), sensors in the trigger responses like sweating and to dissipate heat; conversely, if temperature drops, and generate and conserve heat to restore balance. Growth and development involve an increase in size and complexity, driven by cell division, differentiation, and morphogenesis. Unicellular organisms grow by expanding and dividing, while multicellular ones undergo patterned development from a single fertilized cell into a functional adult, influenced by genetic instructions. Reproduction ensures the continuation of species through the production of offspring, occurring via asexual or sexual means. Asexual reproduction, common in prokaryotes, involves binary fission, where a parent cell duplicates its DNA and splits into two genetically identical daughter cells. In contrast, sexual reproduction in eukaryotes relies on meiosis, a specialized cell division that reduces the chromosome number by half to produce gametes (sperm and eggs), which fuse during fertilization to restore the diploid state and introduce genetic variation. Response to the environment, or , allows organisms to detect and react to stimuli, enhancing survival. grow toward light (), while animals exhibit reflexes like withdrawing from , enabling adaptive behaviors to changing conditions. Adaptation and occur over generations as populations respond to selective pressures, leading to heritable changes that improve fitness. Through mechanisms like , organisms accumulate advantageous traits, such as antibiotic resistance in , ensuring long-term persistence.

Etymology and Historical Context

Origin and Evolution of the Term

The term "organism" originates from the ὄργανον (organon), denoting a "tool," "instrument," or "organ of ," which evolved through into organismus by the . This linguistic root emphasized structured functionality, reflecting early views of living bodies as composed of purposeful parts. The word's conceptual foundation thus bridged mechanical and vital processes, setting the stage for its biological adoption. The Latin organismus first emerged prominently in 1684 within the medico-physiological works of German physician and chemist , particularly in his Theoria medica vera, where it described living bodies as organized machines governed by an animating soul or of order. Stahl's usage marked a shift from purely mechanistic depictions of life—prevalent in Cartesian thought—to a holistic notion of internal regulation and tonic movement in organic entities, distinguishing them from inanimate mechanisms. By the early 1700s, the term spread to vernacular languages: organisme in French, organism in English (noted as early as 1701 by botanist ), and organismo in Italian, increasingly applied to and animals in taxonomic contexts. During this period, Carl Linnaeus's systematic classification in works like (1735 onward) incorporated "organism" to denote discrete living units within hierarchical taxa, embedding the term in the emerging science of . In the early 19th century, the term underwent a conceptual evolution amid debates between mechanism and . , in (1809), employed "organism" to signify dynamic, individual living forms shaped by environmental influences and internal drives, moving beyond Stahl's toward a vitalist framework where organisms actively adapted through use and disuse of parts. This usage contrasted with lingering mechanistic interpretations, positioning organisms as self-organizing entities rather than mere machines. further integrated the term in (1859), applying it over 200 times to describe diverse life forms as products of descent with modification, thus aligning "organism" with evolutionary processes and in a unified biological . Around 1830, "organism" supplanted vaguer phrases like "organized body" to become a standardized technical term across emerging disciplines such as and . The 20th century refined the term through , which reconceived the organism as a complex, integrated network of interacting components rather than a static entity. Pioneered by figures like Lawrence J. Henderson in the 1910s–1920s, this approach—detailed in works such as The Fitness of the Environment (1913)—viewed the organism holistically, emphasizing emergent properties from physiological and biochemical interdependencies. By mid-century, ecological and cybernetic influences, as explored in Ludwig von Bertalanffy's (1968), extended the concept to include feedback loops and environmental interactions, solidifying "organism" as a dynamic system in .

Historical Concepts of Organisms

In pre-modern thought, the concept of organisms was profoundly shaped by Aristotle's scala naturae, or "ladder of nature," a system that arranged living beings along a continuous scale from inanimate matter to plants, animals, and humans based on increasing complexity and soul possession. This framework, often extended into the "," posited organisms as fixed links in a divinely ordered , with each level exhibiting greater perfection and purpose-driven organization. Aristotle's influence persisted through medieval , where scholars like integrated it with , viewing organisms as manifestations of purposeful creation. Medieval and early modern views further emphasized vitalism, the doctrine that organisms are animated by an immaterial "life force" or vital spirit distinct from mechanical processes, enabling growth, , and beyond physical laws. Drawing from Galenic , this perspective portrayed organisms as holistic entities governed by internal teleological principles, such as the balance of humors, rather than reducible to inert matter. contrasted with emerging mechanistic ideas but dominated until the , reinforcing the notion of organisms as purposeful, ensouled systems. During the Enlightenment, , advanced a more dynamic understanding by highlighting interactions between organisms and their environments, suggesting that climatic and geographical factors could induce "degenerations" or variations in species over time. In his (1749–1788), Buffon argued that environmental conditions molded organismal traits, challenging static hierarchies and implying a historical dimension to life forms without invoking supernatural forces. This relational view laid groundwork for later evolutionary theories, portraying organisms as responsive to external influences rather than isolated entities. The marked a pivotal shift with Charles Darwin's theory of , which explained organismal diversity through "descent with modification," where heritable variations arise and are preserved if advantageous in specific environments. In (1859), Darwin rejected teleological design, proposing instead a mechanistic process driven by competition and environmental pressures, transforming organisms from fixed creations into products of gradual, branching . This paradigm emphasized contingency over purpose, fundamentally altering biological inquiry. In the 20th century, molecular biology revolutionized concepts of organisms by identifying DNA as the hereditary material, with James Watson and Francis Crick's 1953 elucidation of its double-helix structure revealing a universal biochemical blueprint for life. This discovery shifted focus from vital forces to mechanistic replication and information transfer, enabling organisms to be understood as gene-directed systems governed by physicochemical laws. Concurrently, the Gaia hypothesis, proposed by James Lovelock in the 1970s, reintroduced holistic thinking by positing Earth as a self-regulating superorganism, where biotic and abiotic components interact to maintain conditions suitable for life. Lovelock's model, initially outlined in 1972, portrayed global ecosystems as cybernetic entities with emergent properties, bridging individual organisms to planetary-scale regulation. These developments reflect a broader historical transition from teleological models—where organisms were seen as goal-oriented by inherent purposes or divine intent—to mechanistic frameworks emphasizing causal, material explanations. Aristotle's immanent final causes gave way to 17th-century mechanists like , who viewed organisms as complex machines, though vitalist hybrids persisted. Darwin's and molecular insights further demystified , reducing to functional descriptions without invoking foresight. Post-2000 advancements in have further blurred traditional boundaries, enabling the design and construction of novel organisms through genetic engineering and , thus expanding the definition beyond naturally evolved . Pioneered by works like the 2000 synthetic genetic circuits, this field treats organisms as programmable systems, challenging vitalist remnants and prompting debates on what constitutes "life." Such innovations underscore an ongoing shift toward viewing organisms as malleable, engineered entities integrated with human intent.

Biological Organization

Hierarchical Levels in Organisms

Biological organization exhibits a hierarchical structure, progressing from the simplest components to complex systems. At the most fundamental level, atoms combine to form molecules, which serve as the basic units of matter in . These molecules assemble into organelles, specialized s within cells that perform specific functions, such as mitochondria for production or ribosomes for protein synthesis. The hierarchy continues with cells as the fundamental building blocks of life, where organelles integrate to enable cellular processes. Prokaryotic cells, characteristic of and , lack a membrane-bound nucleus and are generally simpler, with their genetic material free in the . In contrast, eukaryotic cells, found in protists, fungi, , and animals, possess a nucleus enclosing DNA and various membrane-bound organelles, allowing for greater compartmentalization and complexity. From cells, the structure advances to tissues, groups of similar cells working together; organs, structures composed of multiple tissue types; and organ systems, coordinated groups of organs performing major functions. These levels culminate in the organism itself, a self-sustaining capable of , growth, and reproduction. Beyond the organism, populations of interacting individuals form communities, which in turn interact with their environment to create ecosystems. Each level in this hierarchy demonstrates emergent properties—novel characteristics that arise from interactions at lower levels and cannot be fully predicted from them alone. For instance, while individual cells maintain homeostasis independently, their aggregation into tissues via cell adhesion molecules, such as cadherins and integrins, enables collective behaviors like coordinated contraction in muscle tissue. In human examples, the circulatory system, comprising the heart, blood vessels, and blood, transports oxygen and nutrients throughout the body, exhibiting emergent efficiency in distribution that surpasses isolated vascular components. Similarly, the nervous system, including the brain, spinal cord, and nerves, integrates sensory inputs to produce coordinated responses, a property emerging from networked neuronal interactions rather than single cells. This hierarchical integration underscores how organisms achieve complexity through layered organization.

From Unicellular to Multicellular Forms

Unicellular organisms, such as and protists, represent the simplest form of life, consisting of a single cell that performs all necessary functions for survival, growth, and reproduction. , like Escherichia coli, exemplify the advantages of this organization through their rapid reproduction rates; under optimal laboratory conditions, E. coli populations can double every 20 minutes via binary fission, enabling quick adaptation to environmental changes and colonization of new niches. Protists, unicellular eukaryotes such as amoebas and paramecia, similarly benefit from this streamlined structure, allowing efficient nutrient uptake and mobility without the energy costs of intercellular coordination. This simplicity confers evolutionary flexibility, as unicellular forms can persist in diverse, often harsh environments where multicellularity might prove disadvantageous. The evolutionary transition to multicellularity occurred at least 1.7 billion years ago, when unicellular eukaryotes began forming stable aggregates that evolved into cooperative groups, marking a major innovation in life's complexity. Recent studies (as of 2024) suggest multicellularity in eukaryotes emerged around 1.63 billion years ago. Key developments included cell specialization, where cells within the group adopt distinct roles—such as reproductive versus somatic functions—and intercellular communication mechanisms like , which allows to coordinate behaviors such as formation in response to . These innovations promoted division of labor and enhanced survival, as seen in early fossils suggesting multicellular traces around 2.1 billion years ago, though definitive evidence points to a gradual emergence driven by selective pressures for larger size and predation resistance. A notable example of this transition is found in the volvocine green algae, particularly Volvox, which serves as a model for the stepwise evolution from unicellular to multicellular forms; closely related species range from solitary Chlamydomonas cells to Volvox colonies of up to 50,000 specialized cells forming spherical structures with flagella for motility. In animals, multicellular development advances through gastrulation, a process where the hollow blastula reorganizes into a multilayered gastrula with distinct germ layers—ectoderm, mesoderm, and endoderm—enabling tissue differentiation and organ formation from a single fertilized egg. This embryonic stage underscores how cell adhesion and signaling pathways facilitate the shift to complex body plans. However, the evolution of multicellularity faced significant challenges, particularly the tension between cellular and cheating, where selfish cells exploit group resources without contributing, potentially destabilizing the aggregate. models this conflict, showing that mechanisms like and spatial structuring—where related cells cluster together—favor cooperation by punishing cheaters and stabilizing multicellular groups over time. In experimental systems, such as evolving or , cheaters initially proliferate but are outcompeted when cooperation yields synergistic benefits, like improved resource sharing, highlighting how resolving these intracellular conflicts was crucial for the persistence of multicellular lineages.

Diversity and Classification

Major Domains and Kingdoms

The of biological classification, proposed by and colleagues in 1990, organizes all cellular life into three primary domains—, , and Eukarya—based on differences in (rRNA) sequences, which reveal deep evolutionary divergences. This system supplanted earlier two-kingdom (prokaryote-eukaryote) or five-kingdom models by recognizing as a distinct lineage separate from , emphasizing over morphological traits alone. Within this framework, the domain Bacteria encompasses prokaryotic organisms characterized by their lack of a nucleus and membrane-bound organelles, forming a single kingdom often referred to as Eubacteria or simply Bacteria. These include diverse forms such as cyanobacteria, which perform oxygenic photosynthesis, and pathogens like Escherichia coli. The domain Archaea, also prokaryotic, comprises organisms adapted to extreme environments—such as methanogens in anaerobic conditions and halophiles in high-salt habitats—but many also thrive in moderate settings like oceans and soils. The domain Eukarya includes all eukaryotes with complex cells featuring nuclei and organelles, subdivided into several kingdoms: Protista for mostly unicellular forms like amoebae and algae; Fungi for heterotrophic, chitin-walled organisms such as yeasts and mushrooms; Plantae for photosynthetic, cellulose-walled multicellular life including mosses and flowering plants; and Animalia for motile, multicellular heterotrophs ranging from sponges to vertebrates. Modern classification adheres to cladistic principles, which define monophyletic groups—clades—as lineages sharing a common and all its , identified through shared derived characteristics known as synapomorphies. Developed by Willi Hennig in the mid-20th century, prioritizes these evolutionary innovations, such as the peptidoglycan cell walls in or the presence of introns in Eukarya, to construct hierarchical phylogenies that reflect true ancestry rather than superficial similarities. Post-2010 refinements to the system have incorporated genomic data to adjust kingdom-level boundaries within Eukarya, such as recognizing as a distinct kingdom encompassing photosynthetic stramenopiles (e.g., diatoms) and other alveolate-related lineages derived from secondary endosymbiosis. Similarly, has been elevated to infrakingdom status in some schemes due to multigene phylogenetic analyses highlighting its unique amoeboid and radiolarian forms within the broader or . These updates maintain the three-domain structure while enhancing resolution of eukaryotic diversity through integrative molecular and morphological evidence.

Patterns of Biodiversity

Biodiversity encompasses the variety of organisms at all levels, from within species to the ecological complexity of communities. On , the total number of eukaryotic species is estimated at approximately 8.7 million, with only about 14%—roughly 1.2 million—formally described, leaving 86% undescribed. Prokaryotic microbes, including and , vastly outnumber these, with projections indicating around 1 trillion microbial species, of which fewer than 0.001% have been identified. These figures highlight the immense, largely untapped scope of organismal diversity, predominantly shaped by microbial life that underpins ecosystems globally. Geographic distribution of biodiversity follows pronounced patterns, most notably the latitudinal diversity gradient, where peaks in tropical regions and declines toward the poles. This gradient, observed across taxa from to vertebrates, is attributed to factors like stable climates and higher energy availability in the . —species unique to specific locales—is particularly concentrated in biodiversity hotspots, such as tropical rainforests, which cover just 7% of Earth's land but harbor over 50% of terrestrial , including high proportions of endemic and animals in areas like the Amazon and . In contrast, terrestrial environments support about 80% of known , compared to 15% in marine habitats and 5% in freshwater systems, reflecting disparities driven by habitat productivity and isolation. Among major organismal groups, exemplify extreme diversity, with over 1 million described, comprising more than half of all known animal and potentially up to 5.5 million total. This dominance underscores arthropods' role in terrestrial ecosystems, far outpacing in described numbers despite oceans covering 71% of the planet. Current threats to these patterns are severe, with rates 100 to 1,000 times the background level, as reported by the Intergovernmental Science-Policy Platform on and Services (IPBES) in 2019, affecting around 1 million . Recent assessments link accelerating losses to , projecting that 7.6% of face risk under ongoing warming, with impacts intensifying in vulnerable hotspots like coral reefs and polar regions.

Boundary Cases and Controversies

Viruses and Subviral Entities

Viruses represent a prominent boundary case in the definition of organisms due to their acellular nature and dependence on host cells for propagation. Unlike cellular life forms, viruses consist of a genome—either or —enclosed within a protective protein coat known as a , with some possessing an additional envelope derived from the host. They lack independent metabolic machinery and cannot replicate or synthesize proteins on their own, instead hijacking the host cell's ribosomes and enzymes to produce viral components. This obligate intracellular positions viruses outside traditional organismal criteria, as they remain inert particles outside a suitable host. Subviral entities further blur these boundaries, comprising even simpler infectious agents devoid of nucleic acids or complete genomes. Prions are misfolded proteins that propagate by inducing conformational changes in normal cellular proteins, leading to diseases such as , commonly known as mad cow disease. These protein-only pathogens lack any genetic material and rely entirely on host protein synthesis for replication. Viroids, in contrast, are naked, circular single-stranded molecules of 246–401 that infect , encoding no proteins and depending on host RNA polymerases for replication. Like viruses, viroids form highly structured RNA without a , causing diseases such as potato spindle tuber disease through interference with host . The primary arguments against classifying viruses as organisms center on their absence of cellular structure and . Lacking ribosomes, organelles, or membranes, viruses fail to meet the cellularity requirement central to most definitions of . They also cannot reproduce independently, requiring host cellular machinery for all processes, which contrasts with the self-sustaining of organisms. The underscores this diversity while highlighting their non-cellular replication strategies, dividing viruses into seven groups based on type and mRNA synthesis method: double-stranded DNA (Group I), single-stranded DNA (Group II), double-stranded RNA (Group III), positive-sense single-stranded RNA (Group IV), negative-sense single-stranded RNA (Group V), single-stranded RNA with (Group VI), and double-stranded DNA with (Group VII). Counterarguments emphasize viruses' dynamic roles in evolution, positioning them as influential agents rather than mere parasites. Viruses facilitate , disseminating genetic material across and driving microbial and eukaryotic evolution through integration and mobilization of host genes. Gene transfer agents (GTAs), virus-derived elements in , exemplify this by packaging and exchanging host DNA, enhancing genetic diversity without lysing cells. Additionally, giant viruses like challenge the notion of viral simplicity, boasting a 1.2 Mb —the largest known for a virus—with over 900 genes, including those for translation machinery and metabolic pathways typically associated with cellular organisms. This complexity suggests viruses may represent an evolutionary bridge, with acquiring eukaryotic-like genes via horizontal transfer, complicating their exclusion from organismal debates.

Emergent and Collective Forms

Emergent and collective forms of life challenge traditional definitions of an organism by exhibiting properties that arise from interactions among multiple entities, often blurring the lines between and group-level organization. These systems demonstrate how , communication, and interdependence can produce functional units that behave as cohesive wholes, despite lacking a single centralized control. In , such forms include colonial aggregations and symbiotic associations where constituent parts specialize and integrate to perform organism-like functions, such as , acquisition, and defense. Colonial organisms represent a key example of , where independent cells or individuals aggregate to form multicellular structures with division of labor. In slime molds like Dictyostelium discoideum, solitary amoebae feed on but, under , release signaling molecules to aggregate into a multicellular slug that migrates toward light and heat before forming a fruiting body; here, about 20% of cells altruistically die to form a supportive stalk, elevating spores for dispersal, thus enabling collective survival. Similarly, eusocial such as form supercolonies, vast networks of interconnected nests spanning thousands of square kilometers with billions of workers and multiple queens, characterized by low genetic relatedness yet high cooperation through shared chemical cues that prevent aggression among nestmates. These supercolonies function as single societal units, with workers foraging, defending, and reproducing collectively, illustrating how genetic and behavioral mechanisms enforce unity in expansive collectives. Symbiotic relationships further exemplify collective forms, where distinct species integrate to create composite organisms. Lichens are stable symbioses between fungi (mycobionts) and photosynthetic or (photobionts), forming self-sustaining thalli that colonize extreme environments; the fungus provides structure and protection, while the photobiont supplies nutrients via , resulting in a morphologically complex greater than the sum of its parts. This partnership has persisted for over 400 million years, adapting to diverse habitats through reciprocal physiological exchanges. Extending this concept, holobionts describe hosts and their microbiomes as integrated ecological units, where the collective genome (hologenome) influences traits like immunity and ; for instance, in humans and corals, microbial communities modulate host , evolving together as a functional despite of both host and microbes. Bacterial biofilms provide another emergent collective through quorum sensing, a density-dependent communication system where cells release and detect autoinducer molecules to synchronize behaviors without a leader. In biofilms, such as those formed by Pseudomonas aeruginosa on medical devices, quorum sensing coordinates adhesion, matrix production, and virulence factor expression, transforming dispersed cells into a resilient, three-dimensional community resistant to antibiotics and immune responses. This decentralized coordination enables collective decision-making, like dispersal under nutrient stress, highlighting how simple molecular signals yield complex group-level adaptations. Debates persist over whether certain colonial forms qualify as single organisms or mere aggregates, particularly in siphonophores like the (Physalia physalis). Composed of specialized s—genetically identical polyps for floating, feeding, stinging, and —this hydrozoan appears as a unified entity with integrated , yet each zooid derives from a single embryo and cannot survive independently, fueling arguments on individuality. Proponents of colonial status emphasize functional modularity and evolutionary origins from clonal , while others view it as an emergent due to coordinated behaviors like synchronized stinging and gas-filled float maintenance, akin to eusocial societies. Such controversies underscore the fluidity of organismal boundaries in .

Synthetic and Engineered Organisms

represents a field where scientists design and construct new biological parts, devices, and systems, often blurring the lines between natural and artificial organisms. A landmark achievement occurred in 2010 when researchers at the J. Craig Venter Institute synthesized the 1.08 million of mycoides JCVI-syn1.0 from chemical components, transplanted it into a recipient cell, and created the first self-replicating synthetic bacterial cell, initially termed . This proof-of-principle demonstrated that an organism could be controlled entirely by a chemically synthesized , advancing the concept of minimal life forms. Building on this, efforts to create minimal organisms continued with JCVI-syn3.0 in 2016, a synthetic bacterium with the smallest —approximately 473 genes—capable of autonomous growth and division in conditions. This organism serves as a foundational platform for understanding essential genetic requirements for and bespoke biological systems. Engineered organisms have since expanded through technologies like CRISPR-Cas9, first demonstrated for targeted in 2012, enabling modifications such as bacterial gene drives that bias inheritance to spread engineered traits. More complex examples include , introduced in 2020, which are programmable, multicellular aggregates of frog (Xenopus laevis) skin cells that exhibit collective behaviors like locomotion and self-assembly without genetic alteration. These advancements have sparked significant ethical and legal debates, echoing the precautionary principles established at the 1975 , where scientists recommended guidelines for safe to mitigate biosafety risks. In the 2020s, discussions around projects, such as efforts by to resurrect the through , highlight concerns over ecological impacts, , and regulatory frameworks for releasing synthetic entities into environments. In 2025, announced the successful engineering of mice exhibiting -inspired traits, such as enhanced cold tolerance and woolly coats, marking progress toward their goal of resurrecting mammoths by 2028. These issues underscore the need for updated international oversight to balance innovation with potential unintended consequences.

Origins and Evolution

Earliest Emergence of Life

The emergence of life on , known as , is thought to have occurred between approximately 3.8 and 4.1 billion years ago, shortly after the planet's formation and the cessation of the period. This timeline is inferred from geochemical and evidence suggesting that simple self-replicating systems arose from abiotic chemical processes under conditions, including a CO2- and N2-dominated atmosphere, volcanic activity, and liquid oceans. The (LUCA), the hypothetical progenitor of all extant life, is estimated to have existed around 4.2 billion years ago, possessing a encoding about 2,600 proteins and relying on primitive metabolic pathways powered by geochemical energy sources. Key theories of include the hypothesis, which posits that organic molecules formed in shallow pools or oceans energized by lightning, UV radiation, or volcanic heat. The seminal Miller-Urey experiment in 1953 demonstrated this by simulating a with , , , and , producing and other biomolecules after electrical sparking. An alternative is the hypothesis, proposing that life originated at deep-sea alkaline vents where mineral-rich fluids provided energy gradients and concentrated organics, facilitating the synthesis of and peptides. This scenario aligns with the hypothesis, in which self-replicating ribozymes—RNA molecules capable of catalyzing their own replication—served as both genetic material and enzymes, bridging the gap from chemistry to . Recent 2025 research has identified potential prebiotic reactions that could form RNA precursors under conditions, supporting the feasibility of an and suggesting might proceed rapidly on suitable planets. A critical step in these processes was the formation of protocells, primitive compartments assembled from amphiphilic membranes that encapsulated reactive molecules, enabling concentration, protection, and rudimentary division in aqueous environments. The fossil record provides the earliest direct evidence of life through microfossils and dating to about 3.5 billion years ago. In the Apex Chert of Western Australia's Warrawoona Group, filamentous structures interpreted as ancient have been identified, preserved in silica-rich cherts and exhibiting morphological features consistent with prokaryotic cells. from the nearby 3.48 billion-year-old Dresser Formation in the further support this, showing layered microbial mats formed by photosynthetic or chemosynthetic communities in environments. Complementary isotopic evidence includes depleted ratios in from 3.5 billion-year-old rocks, indicating biological during , as well as sulfur isotope anomalies in 3.45 billion-year-old Strelley Pool Formation that suggest microbial reduction. These biosignatures collectively affirm the presence of metabolically active organisms by the mid-Archean eon.

Evolutionary Trajectories of Organisms

The evolutionary history of organisms is marked by a series of transformative milestones that expanded biological complexity and diversity. Prokaryotes, primarily and , dominated Earth's for billions of years following the emergence of life around 3.5-3.8 billion years ago (Ga), with simple unicellular forms thriving in anaerobic environments. A pivotal event was the (GOE), occurring approximately 2.4 Ga, when —photosynthetic prokaryotes capable of oxygenic —began to significantly oxygenate the atmosphere, fundamentally altering global and enabling aerobic respiration while contributing to the rise of more complex metabolisms. This oxygenation, peaking between 2.4 and 2.0 Ga, marked the end of the Eon and set the stage for subsequent evolutionary innovations by shifting ecological niches and selecting for oxygen-tolerant lineages. The transition to eukaryotic organisms, around 2 Ga, represented a major leap in cellular complexity through endosymbiosis, where an archaeal host engulfed an alphaproteobacterium that evolved into the , providing efficient energy production via aerobic respiration. This event, estimated to have occurred between 1.9 and 2.1 Ga, allowed eukaryotes to exploit oxygenated environments more effectively and laid the foundation for larger cell sizes and diverse metabolic strategies. Phylogenetic analyses confirm the alphaproteobacterial origin of mitochondria, with divergence times aligning closely with the post-GOE oxygenation surge. Eukaryotes initially remained unicellular, but this innovation facilitated later developments in organelle acquisition, such as chloroplasts from in photosynthetic lineages. Multicellularity emerged in waves across eukaryotic clades, enhancing specialization and size. Fossil evidence indicates early multicellular forms in around 1.2 Ga, exemplified by Bangiomorpha pubescens, a filamentous alga from the Era that demonstrates primitive tissue differentiation and . This development in archaeplastids predated animal multicellularity by over half a billion years. A dramatic acceleration occurred during the approximately 540 million years ago (Ma), when diverse animal phyla rapidly diversified, driven by ecological pressures like predation and rising oxygen levels, resulting in the appearance of most modern animal body plans within a geologically brief span of 20-25 million years. Mass extinctions punctuated these trajectories, reshaping organismal diversity through selective bottlenecks and subsequent radiations. The Permian-Triassic extinction event, at 252 Ma, was the most severe, eliminating about 96% of marine species and 70% of terrestrial vertebrate genera due to massive volcanic activity, global warming, and ocean anoxia, which disrupted ecosystems worldwide. This catastrophe cleared ecological space, enabling the Triassic radiation of archosaurs, including early dinosaurs and mammals, which dominated Mesozoic faunas and restored biodiversity over tens of millions of years. The current biodiversity crisis, widely regarded by many scientists as the onset of a potential sixth mass extinction and driven primarily by anthropogenic factors such as habitat destruction, climate change, and overexploitation, threatens up to 1 million species with extinction (as of 2019 IPBES assessment), with rates exceeding background levels by 1,000-10,000 times, though the classification as a full mass extinction event remains debated.

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