An organism (from Greek: ὀργανισμός, organismos) is any individual entity that embodies the properties of life, mostly resulting from a complex of parts (organs), coordinated with each other, to form a single harmonious whole. Peculiarities of each organism are the specificity of form and chemical composition, the ability to perceive stimulations from the environment and to react to them, the ability to reproduce, giving rise to other organisms with similar characteristics.
As the organism is subordinated to the purpose of survival of the being, so the parts are subordinated to the organism: this is the concept of organism in Aristotle and Descartes, even if the latter calls the organism “machine”; on this path Leibniz defines the organism “divine machine” or “natural automaton”. Kant, on the other hand, distinguishes the machine from the organism, because the first has its productive cause outside itself, while in the organism the parts are together cause and effect of each other. After Kant, the metaphysical dispute between finalism and mechanism does not touch the concept of organism; however, the cosmic finalism of the organism (supported by vitalism and the various metaphysical schools) falls, while its internal finalism remains.
Every living organism known today derives from a network, in first approximation similar to an arborescent phylogenetic line, common to all other organisms, regardless of the time of separation between the evolutionary lines. Every extant life form derives from one or a few common ancestors that appeared on Earth billions of years ago, possesses metabolic pathways, reproduces, transmits information to its offspring, and organizes its structures. These characteristics constitute the core of the biological concept of life, an emerging peculiarity that distinguishes it from non-living entities. These peculiarities by convention are represented by some aspects common to all living organisms:
- Evolution: evolves, resulting related to all other living organisms.
- Order: turns out to be structured.
- Encoding: contains within itself the information and instructions that control and define its structure and function.
- Regulation: it is capable of autonomously maintaining homeostasis.
- Growth and development: it is capable of autonomous growth.
- Energy: it represents an open thermodynamic system, able to assimilate energy, store it, transform it, and give it to the environment.
- Irritability, Sensitivity or Motility: it is independently capable of responding to external stimuli.
In a broader sense, organisms may also be able to, and as a whole possess:
- Reproductive capacity: able to give rise to fertile offspring which will give rise to adult-like organisms.
- Evolutionary capacity: can vary its genotype and phenotype, giving rise to new anatomical structures, physiological pathways and genomic combinations, never previously appeared within the phylogenetic line to which it belongs (evolutionary divergence) or already appeared within phylogenetic lines previously separated (evolutionary convergence).
These characteristics are the basis of the sufficiently shared view of a living being as a known organism of the terrestrial biota. Other definitions deviate in one or more points from the previous one, generated by the study of natural sciences.
The synthetic biology that can lead to the genesis of organisms endowed with so-called synthetic life, such as Mycoplasma laboratorium, and the topics of exobiology and hypothetical biochemistry can lead to defining the concept of a living organism more generally.
Known organisms span a dimensional and temporal range from the 80.000 years and more than 6.600 tons of a single (Pando tree), to the tiny mycoplasmas of 200 nanometers in diameter, which “live” (duplication time) a few minutes, with a mass less than one billionth of a gram.