What is Endosymbiosis?
Endosymbiosis, meaning “inside symbiosis”, is the theory that the cellular unit is actually a product of the ancient merging of multiple separate organisms. It is thought that early Eukaryotes (organisms with a cell nucleus) were formed from the encapsulation of Proteobacteria by Archaebacteria. These early cells did not use oxygen, instead making ATP by lactic acid fermentation of sugars in the cytoplasm. Later on, the proto-mitochondria were encapsulated, which gave the cell the advantage of being able to use oxygen to produce some 20 times more ATP. In a similar way, there is also strong evidence that plants developed their plastids (photosynthesis organelle containing chlorophyll) from encapsulation of blue-green algae. Follow this offsite link for a more detailed explanation.
