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Symbiosis And Evolution

Ideas about symbiosis have made some important contributions to theories that help explain the evolution of complex life forms on Earth. The first organisms on Earth were prokaryotic viruses and blue-green bacteria, which do not have an organized nucleus. Eukaryotic cells are more complex, having their nuclear material bounded within a nucleus, as well as other cellular organelles such as ribosomes, chloroplasts, cilia, flagellae, and other important structures. An exciting theory of the origin of eukaryotic cellular organization postulates the occurrence of a series of symbioses, in which prokaryotic cells became intimately associated with each other, with certain cellular functions being mutualistically divided amongst the symbionts. For example, certain tiny symbionts might have become responsible for the most of the respiratory function of the mutualism, and could then have evolved into mitochondria. Other symbionts, such as blue-green bacteria, could have been responsible for photosynthesis by the cell, and may have evolved into chloroplasts. To a degree these ideas are supported by the observation that both ribosomes and chloroplasts contain small amounts of genetic material (DNA, or deoxyribonucleic acid), which may be relict from an earlier, independent existence.

Another recent and highly controversial theory, called the Gaia hypothesis, suggests that Earth may represent an enormous, quasi-organismic entity, in which all species comprise a global, symbiotic, physiological system that maintains environmental conditions within a range that life can tolerate. Supporting evidence for this hypothesis includes the suggestion that the oxygen in Earth's atmosphere is ultimately of biological origin, having been emitted by photosynthetic organisms. Without oxygen, of course, most species could not survive. In addition, some ecologists suggest that the concentration of carbon dioxide in Earth's atmosphere is to a large degree regulated by a complex of integrated biological and physical processes by which CO2 is emitted and absorbed. This gas is well known to be important in the planet's greenhouse effect, which is critical in maintaining the average surface temperature within a range that organisms can tolerate.

See also Parasites.



Begon, M., J.L. Harper, and C.R. Townsend. Ecology: Individuals, Populations and Communities. 2nd ed. London: Blackwell Sci. Pub., 1990.

Brewer, R. The Science of Ecology. 2nd ed. Fort Worth: Saunders, 1994.

Margulis, L., and L. Olendzenski, eds. Environmental Evolution: Effects of the Origin and Evolution of Life on Planet Earth. Cambridge: MIT Press, 1992.

Bill Freedman


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—A mutually beneficial relationship between species.


—A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.

Additional topics

Science EncyclopediaScience & Philosophy: Swim bladder (air bladder) to ThalliumSymbiosis - Various Types Of Symbiosis, Examples Of Natural Symbioses, Symbioses Between Humans And Other Species, Symbiosis And Evolution