In biology, an organism is any living thing (such as animal, plant, fungus, or micro-organism). In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole. An organism may either be unicellular (single-celled) or be composed of, as in humans, many billions of cells grouped into specialized tissues and organs. The term multicellular (many-celled) describes any organism made up of more than one cell.
The term "organism" (Greek á½ÏγανιÏμÏÏ - organismos, from Ancient Greek á½Ïγανον - organon "organ, instrument, tool") first appeared in the English language in 1701 and took on its current definition by 1834 (Oxford English Dictionary).
Organisms may be divided into the prokaryotic and eukaryotic groups. The prokaryotes represent two separate domains, the Bacteria and Archaea.[1] All fungi, animals and plants are eukaryotes. The word "organism" may broadly be defined as an assembly of molecules that function as a more or less stable whole and has the properties of life. However, many sources propose definitions that exclude viruses and theoretically-possible man-made non-organic life forms.[2] Viruses are dependent on the biochemical machinery of a host cell for reproduction.
Chambers Online Reference provides a broad definition: "any living structure, such as a plant, animal, fungus or bacterium, capable of growth and reproduction"[3].
In multicellular life the word "organism" usually describes the whole hierarchical assemblage of systems (for example circulatory, digestive, or reproductive) themselves collections of organs; these are, in turn, collections of tissues, which are themselves made of cells. In some plants and the nematode Caenorhabditis elegans, individual cells are totipotent.
Superorganism
Main article: Superorganism
A superorganism is an organism consisting of many individuals working together as a single functional or social unit. Ants, and other eusocial invertebrates such as termites, are examples of superorganisms that form large colonies of individuals with highly specialised roles and division of labor. Individuals may be unable to reproduce or to survive for extended periods of time when separated from the colony. By working cooperatively, superorganisms can achieve feats that comparable solitary organisms cannot. Thermoregulation, for example, does not occur in individuals or small groups of honeybees or other invertebrates, but colonies of bees with more than about 5000 individuals can thermoregulate.[4] James Lovelock, with his "Gaia Theory" has paralleled the work of Vladimir Vernadsky, who suggested the whole of the biosphere in some respects can be considered as a superorganism.
A sea sponge is a very simple type of multicellular organismThe concept of superorganism is under dispute, as many biologists maintain that in order for a social unit to be considered an organism by itself, the individuals should be in permanent physical connection to each other, and its evolution should be governed by selection to the whole society instead of individuals. While it's generally accepted that the society of eusocial animals is a unit of natural selection to at least some extent, most evolutionists claim that the individuals are still the primary units of selection.
The question remains "What is to be considered the individual?". Advocates of evolution like Richard Dawkins suggest that the individual selected is the "Selfish Gene". Others believe it is the whole genome of an organism. E.O. Wilson has shown that with ant-colonies and other social insects it is the breeding entity of the colony that is selected, and not its individual members. This could apply to the bacterial members of a stromatolite, which, because of genetic sharing, in some way comprise a single gene pool. Gaian theorists like Lynn Margulis would argue this applies equally to the symbiogenesis of the bacterial underpinnings of the whole of the Earth.
It would appear, from computer simulations like Daisyworld, that biological selection occurs at multiple levels simultaneously.
It is also argued that humans are actually a superorganism that includes microorganisms such as bacteria. It is estimated that "the human intestinal microbiota is composed of 1013 to 1014 microorganisms whose collective genome ("microbiome") contains at least 100 times as many genes as our own. . . . Our microbiome has significantly enriched metabolism of glycans, amino acids, and xenobiotics; methanogenesis; and 2-methyl-D-erythritol 4-phosphate pathway–mediated biosynthesis of vitamins and isoprenoids. Thus, humans are superorganisms whose metabolism represents an amalgamation of microbial and human attributes." [5]. An NIH-coordinated and -funded effort is currently in progress to characterize the human microbiome.
[A superorganism is an organism consisting of many individuals working together as a single functional or