Why did endosymbiosis occur?
2 Answers
Any symbiosis is a mutual benefit.
Explanation:
The best example of endosymbiosis is mitochondria in eukaryotic cells. Oxybacteria having similar genome as reketsia (bacteria of trench fever) were free living.
In prokaryotic cells glucose metabolism takes place up to pyruvic acid. This pyruvic acid is the starting material of mitochondria.
The end product of pyruvic acid is carbon dioxide and water.
This is oxydative phosphorylation. This process releases more energy in terms of ATP.
During course of time the mitochondria were engulfed in cell bodies with a process of endomytosis.
This was so successful that major multicellular organism have mitochondria in cell bodies.
Endosymbiosis appears by a chance event but it may become naturally selected when both the endosymbiont and the host derive some benefit from the association.
Explanation:
In endosymbiosis, one organism lives within another living organism. There are many examples, like presence of nitrogen fixing bacterium Rhizobium in roots of leguminous plants, or presence of algae in corals.
Such symbiotic associations may evolve to become obligatory, as has happened in case of mitochondria and plastid within eukaryotic cells.
In the beginning, there were only prokaryotic life which diversified into aerobic/anaerobic and autotrophic/heterotrophic organisms. If by chance an aerobic bacteria established itself inside an anaerobic one, the association proved to be beneficial for both: first received a protected environment to multiply while the second received some supply of energy. Thus endosymbiosis could have been the story of evolution of mitochondria.
Plastids, like mitochondria, are with own circular DNA and evolved in a similar manner. Plastids evolved from photosynthetic bacteria which opted for an endosymbiotic life within another prokaryotic organism which lacked the ability to produce own food.
