Organism

An organism, or simply a “living creature” as some might colloquially put it, is essentially any entity that functions as an individual within the grand, chaotic theatre of life . However, defining “individual” is a philosophical quagmire that makes defining an organism feel like trying to nail jelly to a wall. [1] We toss around criteria like autonomous reproduction , growth , and metabolism as if they’re definitive, but even these foundational pillars crumble under scrutiny. Take viruses , for instance. They replicate, they evolve, they certainly make their presence known, yet many balk at calling them organisms. Perhaps it’s their utter dependence on hijacking other cells, their lack of internal machinery to churn out their own sustenance. They exist in a liminal space, more akin to complex biological machines than self-sufficient entities.

Then we have the truly perplexing cases, like colonial organisms . Think of a bustling eusocial insect colony – a perfectly orchestrated society where some members are dedicated solely to the continuation of the species, while others perform all the grunt work. It’s a division of labor so profound, it mirrors the specialized cells within a single animal. Or consider the siphonophore , that ethereal, gelatinous marine creature. Its body is a collection of distinct, organism-like units called zooids , each with its own specialized function, yet they coalesce into a single, functioning entity that moves and feeds like a giant jellyfish . It’s a collaborative effort on a grand scale, blurring the lines between individual and collective.

Evolutionary biologists David Queller and Joan Strassmann offer a refreshingly pragmatic, if slightly unnerving, perspective. They posit that “organismality” – the very essence of being an organism – isn’t an inherent quality but an evolved one, born from social cooperation. Groups of simpler units, from cells to entire species, learn to work together, suppressing internal conflicts to achieve a common goal. Under this definition, even the intimate partnership of a fungus and alga in a lichen , or the lifelong, symbiotic bond of a male and female anglerfish , could be considered organisms. It’s a perspective that redefines individuality, suggesting it’s less about what something is and more about how it behaves with others.

Etymology

The word “organism” itself is a rather elegant piece of linguistic engineering, stemming from the Ancient Greek word “órganon.” This term, in its original context, was a versatile tool, capable of meaning “instrument,” “implement,” “tool,” or even an “organ of sense” or “apprehension.” It hints at complexity, at function, at something designed to do something. The term first sauntered into the English language in the 1660s, carrying a somewhat archaic meaning of an organic structure or organization. [3] It’s intrinsically linked to the verb “organize,” a concept that implies arrangement, structure, and purpose. Long before that, in 1790, the philosopher Immanuel Kant , in his seminal Critique of Judgment , offered a more profound definition, describing an organism as a being that is both “organized and a self-organizing” entity. [4] [5] This dual nature, the inherent structure and the capacity for self-direction, is what truly sets organisms apart.

Whether Criteria Exist, or Are Needed

The quest for a definitive checklist to identify an organism is a notoriously thorny one. One proposed criterion suggests that an organism cannot be divided without losing its functional integrity. [6] Yet, even a simple basil plant cutting, a mere fragment, can embark on a new life, sprouting roots from a bit of stem and becoming a whole new plant. This demonstrates that the boundaries of an “individual” can be remarkably fluid.

The commonly cited criteria for organismhood include:

• Autonomous reproduction , growth , and metabolism : The ability to independently create more of oneself, to increase in size or complexity, and to process energy and matter. [7]
• Noncompartmentability: The idea that an organism’s structure is so integrated that dividing it renders it non-functional. [6] As Richard Dawkins eloquently put it, it’s “the quality of being sufficiently heterogeneous in form to be rendered non-functional if cut in half.” [8] But, as we’ve seen, many organisms defy this rigid definition, readily regenerating from fragments.
• Individuality : This encompasses genetic uniqueness, genetic homogeneity within the entity, and a degree of autonomy . [9]
• An immune response : The capacity to distinguish “self” from “foreign” invaders, a fundamental aspect of self-preservation. [10]
• “Anti-entropy ”: Coined by Erwin Schrödinger , this refers to the ability to maintain internal order against the universal tendency towards disorder. [11] Alternatively, from the perspective of Claude Shannon ’s information theory , an organism is something that can maintain its own information content. [12]

However, a significant faction of scientists argues that the very concept of the “organism” is becoming an outdated relic in modern biology. [13] The notion of strict individuality, they contend, is problematic. [14] From a philosophical standpoint, the necessity of such a rigid definition is even called into question. [15] [16] [8]

The complexities multiply when we consider colonial organisms . A colony of eusocial insects , for example, exhibits a sophisticated, adaptive organization and germ-soma specialization. [17] If we apply the same logic, or perhaps a criterion emphasizing high cooperation and low conflict, we might find ourselves including certain mutualistic partnerships, like lichens , or even those lifelong sexual unions, such as that of the anglerfish , as organisms. [18] The concept of a superorganism , where a group functions as a single adaptive unit, further complicates the picture, particularly if group selection plays a significant role in adaptation . [19]

A more nuanced view suggests that attributes like autonomy, genetic homogeneity, and genetic uniqueness aren’t all-or-nothing propositions. Instead, they exist on multiple dimensions of biological individuality, leading to various classifications of organisms. [20]

Organisms at Differing Levels of Biological Organisation

The way we perceive an organism can shift dramatically depending on the level of biological organization we’re examining. Take a lichen . It’s a composite entity, a partnership primarily between fungi and unicellular algae or cyanobacteria , with a whole bacterial microbiome thrown in for good measure. [21] These species are so intertwined, so mutually dependent, that they function much like the cells within a single, complex multicellular organism.

• Unicellular organism : This is the fundamental unit, a single cell that forms the entirety of a protist , bacterium , or archaean . Even within this single cell, there can be specialized internal structures, organelles , performing specific tasks. [22]
• Multicellular organism : This is what most people picture when they think of an organism – an animal , plant , fungus , or alga composed of a multitude of cells, often highly specialized for different functions. [22]
• Colonial organism : Here, the individual is a collection of communicating units, or zooids , that function together as a single entity. The siphonophore is a prime example. [8]
• Superorganism : This takes collectivism a step further. A superorganism is a colony, like that of ants , where numerous individuals function as a single, cohesive unit, often with a complex social structure. [23] [17]
• Mutualism : This describes a symbiotic relationship where two or more distinct species rely on each other for survival, each contributing essential functions. The lichen , a combination of fungi and algae or cyanobacteria along with its bacterial microbiome , is a classic case. [18] [21] It’s a testament to cooperation, allowing these partners to thrive in environments where neither could survive alone.

Queller and Strassmann’s assertion that “organismality” arises from social evolution, where simpler units unite through cooperation, is a powerful one. They propose cooperation as the defining trait, acknowledging that this can occur at various levels of biological organization. [18]

Here’s a breakdown of how cooperation plays out across different levels:

Samuel Díaz‐Muñoz and his colleagues (2016) build upon this idea, proposing that organismality can be measured by the degree of cooperation and conflict within a system. They argue that this places organisms within an evolutionary context, making organismality a fluid, context-dependent characteristic. They suggest that highly integrated life forms, those that seem less dependent on their specific context, might actually evolve through stages of increasing unification. [24]

Boundary Cases

The edges of the “organism” definition are where things get truly interesting, and decidedly messy.

Viruses

Viruses , like the notorious tobacco mosaic virus , exist in a state of biological ambiguity. They aren’t cells; they are essentially genetic material encased in a protein shell. [7] Their inability to reproduce, grow, or metabolize independently is the primary reason they’re often excluded from the organism club. They lack the internal machinery to synthesize their own components, making them reliant on the host cell’s resources. In this regard, they bear a closer resemblance to inanimate matter than to living beings. [7]

Yet, viruses possess their own genes and demonstrably evolve . This evolutionary capacity fuels the debate: should they be classified as living? Some argue that their ability to replicate through self-assembly and undergo evolution is sufficient. Others contend that viruses don’t truly evolve or reproduce on their own; rather, they are products of their host cells’ evolution, a constant dance of co-evolution. Without host cells, viral evolution would cease. Reproduction, for viruses, is an act of hijacking, of commandeering cellular machinery. [7] The discovery of viruses with genes capable of energy metabolism and protein synthesis further muddied the waters, though these genes are likely acquired through horizontal gene transfer from their hosts. [7]

Here’s a comparison of cellular organisms and viruses:

Organism-like Colonies

The philosopher Jack A. Wilson delves into these murky waters, using boundary cases to illustrate the ill-defined nature of the “organism” concept. [8] He points to entities like sponges , lichens , siphonophores , slime moulds , and highly social colonies such as those of ants or naked molerats . These, he argues, occupy a transitional zone, existing somewhere between clear-cut colonies and definitive organisms or superorganisms. [8]

Wilson draws a fascinating parallel between colonial siphonophores and the more familiar jellyfish , highlighting their shared functional similarities despite vastly different compositions:

Evolutionary Emergence of Organisms

The prevailing hypothesis for the origin of life, the RNA world , posits a stage where self-replicating RNA molecules predated the more complex systems of DNA and proteins. [25] It’s within this primordial soup that “organisms” may have first emerged, when RNA chains began to copy themselves, setting in motion the fundamental mechanisms of Darwinian selection: heritability , variation, and differential reproductive success. The “fitness” of an RNA replicator, its rate of increase, would have been dictated by its inherent adaptive capabilities, determined by its nucleotide sequence , and the availability of resources. [26] [27] The key adaptive capacities of these early proto-organisms likely included: (1) replication with sufficient fidelity to ensure heritability while allowing for variation, (2) resistance to degradation, and (3) the ability to acquire and process resources. [26] [27] These capabilities would have been intrinsically linked to the folded structures of the RNA replicators, a direct consequence of their nucleotide sequences.

Synthetic Organisms

In the realm of bio-engineering and synthetic biology, scientists are pushing the boundaries of what constitutes an organism. They are creating synthetic organisms in various forms: chimaeras built from cells of different species, cyborgs incorporating electromechanical components, hybrids that blend electronic and biological elements, and other novel combinations of evolved and designed systems. [28]

Evolved organisms develop through the intricate, often poorly understood, mechanisms of evolutionary developmental biology , where the genome orchestrates a cascade of interactions to build increasingly complex structures. The existence of chimeras and hybrids demonstrates the remarkable robustness of these developmental mechanisms, capable of adapting to drastic alterations at all biological levels. [28]

Synthetic organisms are already incredibly diverse, and this diversity is only set to expand. What unites them is their teleonomic or goal-seeking behavior, an ability to self-correct errors and achieve their designed objectives. This behavior echoes the intelligent actions of organisms, suggesting that intelligence itself can be viewed as a form of embodied cognition . [28]