Symbiosis – derived from a Greek word meaning 'living together' – is the ecological relationship between organisms of different species that are in direct contact. If one of the symbiotic organisms is much larger than the other, the largest is called a host and the smallest is called a symbiont.
There are several types of symbiotic relationships depending on the benefit that organisms get. In mutualism, the organisms that participate in the symbiosis benefit, while in commensalism one of the 'participants' in the relationship benefits while the other does not benefit, but is not harmed either. Finally, in the parasitic relationship an organism lives at the expense of its host.
Many of the eukaryotic organisms – made up of cells with nuclei – depend on their symbiotic relationships with other organisms. In fact, the same mitochondria, essential organelles of our cells, come from a very primitive symbiosis , as postulated by the theory of endosymbiosis popularized by Lynn Margulis.
The more we know about the symbiotic relationships between different organisms, the more we realize that these are not isolated associations, but that they are part of the day-to-day life of a good part of Earth's organisms – including humans – and that, in many cases, life as we know it would not be possible without symbiosis.
