Introduction
An organism is defined in a medical dictionary as any living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have been proposed to define what an organism is. Among the most common is that an organism has autonomous reproduction, growth, and metabolism. This would exclude viruses, despite the fact that 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.
“Life is the force that unifies us all—be it human, animal, plant, or microbe.”
Etymology
The term "organism" (from the Ancient Greek ὀργανισμός, derived from órganon, meaning instrument, implement, tool, organ of sense or apprehension) first appeared in the English language in the 1660s with the now-obsolete meaning of an organic structure or organization. It is related to the verb "organize". In his 1790 Critique of Judgment, Immanuel Kant defined an organism as "both an organized and a self-organizing being".
“In its essence, life is an instrument through which nature achieves balance and harmony.”
Whether criteria exist, or are needed
One criterion proposes that an organism cannot be divided without losing functionality. 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:
- Autonomous reproduction, growth, and metabolism
- Noncompartmentalibility – structure cannot be divided without losing functionality. Richard Dawkins stated this as "the quality of being sufficiently heterogeneous in form to be rendered non-functional if cut in half". However, many organisms can be cut into pieces which then grow into whole organisms.
- Individuality – the entity has simultaneous holdings of genetic uniqueness, genetic homogeneity and autonomy.
- Immune response – separating self from foreign.
- "Anti-entropy", the ability to maintain order, a concept first proposed by Erwin Schrödinger, or in another form, that Claude Shannon's information theory can be used to identify organisms as capable of self-maintaining their information content.
Other scientists think that the concept of the organism is inadequate in biology, and that the concept of individuality is problematic. They question whether such a definition is necessary.
Organisms at differing levels
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.
A multicellular organism such as an animal, plant, fungus, or alga is composed of many cells, often specialised.
A colonial organism such as a siphonophore is a being which functions as an individual but is composed of communicating individuals.
A superorganism is a colony, such as of ants, consisting of many individuals working together as a single functional or social unit.
A mutualism is a partnership of two or more species which each provide some of the needs of the other.
| 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; but they evolve, their genes collaborating to manipulate the host |
| Unicellular organism | Paramecium | One cell, with organelles e.g. cilia for specific functions | Yes | Inter-cellular (inter-organismal) signalling |
| 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 |
| Multicellular organism | Mushroom-forming fungus | Cells, grouped into organs for specific functions (e.g. reproduction) | Yes | Cell specialisation, communication |
| Permanent sexual partnership | Anglerfish | Male and female permanently fastened together | Yes | Male provides male gametes; female provides all other functions |
| Mutualism | Lichen | Organisms of different species | Yes | Fungus provides structure, absorbs water and minerals; alga photosynthesises |
| Joined colony | Siphonophore | Zooids joined together | Yes | Organism specialisation; inter-organism signalling |
| Superorganism | Ant colony | Individuals living together | Yes | Organism specialisation (many ants do not reproduce); inter-organism signalling |
Boundary cases
Viruses
Viruses are not typically considered to be organisms, because they are incapable of autonomous reproduction, growth, metabolism, or homeostasis. Although viruses have a few 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. 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.
Synthetic organisms
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.
Early evolution of organisms
All organisms that exist today possess a self-replicating informational molecule (genome), and such an informational molecule is likely intrinsic to life. Thus, the earliest organisms also presumably possessed a self-replicating informational molecule (genome), perhaps RNA[28][29] or an informational molecule more primitive than RNA.
References
For more in-depth details on organisms, please visit the Wikipedia page on Organisms.
Quick Facts about Organisms
- Oldest Organism: The oldest living organism is a Great Basin bristlecone pine tree, estimated to be over 5,000 years old!
- Largest Organism: The largest living organism on Earth is a fungus in Oregon that spans over 3.4 square miles.
- Fastest Organism: The peregrine falcon is the fastest bird, capable of diving at speeds over 240 mph.