Symbiosis is an association between two or more different species of organisms. The as sociation may be permanent, the organisms never being separated, or it may be long lasting. These bacteria, together with microorganisms, provide diversity in nature. Any relationship between two populations of species that live together is symbiotic, and these species have no benefit, harm, or any effect on each other. No organism is independent; each has a relationship, directly or indirectly, with other organisms. Even people are an ancient example of symbiosis. Human cells house mitochondria, organelles that were once symbiotic bacteria. Similarly, chloroplasts in plant cells are organelles that have evolved from ancient symbiotic photosynthetic bacteria.
Communities of mutualism, commensalism and parasitism form the foundations of the symbiosis form. Mutualism is the relationship between organisms of two different species which each benefit. Mutualistic arrangements are most likely to develop between organisms with widely different living requirements. The partnership between nitrogen-fixing bacteria and leguminous plants is one example. In addition, cows possess rumen bacteria that live in the digestive tract and help digest the plants the cow consumes. Associations between tree roots and certain fungi are mutualistic as well. Commensalism, on the other hand, is called the relationship between individuals of two species in which one species obtains food or other benefits from the other without harming or benefiting the other. The commensal relationship is usually between a larger living being and a smaller commensal living being. Parasitism is the relationship between two species of plants or animals, in which one benefits the other, sometimes without killing the host organism. Intracellular parasites – such as bacteria or viruses – usually rely on a third organism, known as a carrier or vector, to transmit them to the host.
In the late 1960s, scientist Lynn Margulis studied the structure of cells and introduced the hypothesis of ‘endosymbiosis’ to the world with the argument of Symbiosis in Cell Evolution in 1981. The theory of endosymbiosis deals with the origin of mitochondria and plastids from eukaryotic cell organelles. The theory suggests that the organelles originated from prokaryotic creatures that began to live together with eukaryotic cells. Margulis claims that most of the DNA originated as bacterial genes that became organelles, rather than mutations. According to Margulis, mitochondria are convoluted bodies that produce the energy needed for metabolism, and their structure looks just like that of bacteria. With the help of this similarity, some studies have even suggested that mitochondria arose from bacteria that live in a permanent symbiosis in animal and plant cells. Algae and plant cells, known as chloroplasts, have a second group of bodies that they use to carry out photosynthesis. The chloroplast captures the incoming sunlight energy, and this energy activates biochemical reactions involving the combination of water and carbon dioxide to make organic matter. Chloroplasts, like mitochondria, are like bacteria. Scientists argue that chloroplasts evolved from symbiotic bacteria. Margulis spent much of the rest of the 1960s developing the argument that symbiosis was an unnoticed but important factor in the evolution of cells.