Endosymbiosis refers to the co-existence of living matter where one organism lives within the body or within the cells of another organism. This is facilitated by the fact that in most cases the relationship benefits both the organisms unlike a parasitic relationship where one feeds off the other.
In eukaryotic cells, mitochondria and chloroplasts are an example of endosymbiosis. Studies show that mitochondria and chloroplasts have a structure that would have allowed their independent existence in the past. The same can be said about cilia, flagella and centrioles. These are thought to have come by symbiosis between early eukaryotic cells and other bacteria.
Endosymbiosis is also the case with bacteria that live in the digestive system of animals and the nitrogen fixing bacteria that live in plants.
The endosymbiosis theory attempts to explain the origins of organelles such as mitochondria and chloroplasts in eukaryotic cells. The theory proposes that chloroplasts and mitochondria evolved from certain types of bacteria that eukaryotic cells engulfed through endophagocytosis. These cells and the bacteria trapped inside them entered a symbiotic relationship, a close association between different types of organisms over an extended time. However, more specifically, the relationship was endosymbiotic, meaning that one of the organisms (the bacteria) lived within the other (the eukaryotic cells).
According to endosymbiosis theory, an anaerobic cell probably ingested an aerobic bacterium but failed to digest it. The aerobic bacterium flourished within the cell because the cell's cytoplasm was abundant in half-digested food molecules. The bacterium digested these molecules with oxygen and gained great amounts of energy. Because the bacterium had so much energy, it probably leaked some of it as Adenosine triphosphate into the cell's cytoplasm. This benefited the anaerobic cell because it enabled it to digest food aerobically. Eventually, the aerobic bacterium could no longer live independently from the cell, and it therefore became a mitochondrion.
The origin of the chloroplast is very similar to that of the mitochondrion. A cell must have captured a photosynthetic cyanobacterium and failed to digest it. The cyanobacterium thrived in the cell and eventually evolved into the first chloroplast. Other eukaryotic organelles may have also evolved through endosymbiosis; it has been proposed that cilia, flagella, centrioles, and microtubules may have originated from a symbiosis between a Spirochaete bacterium and an early eukaryotic cell, but this is not widely accepted among biologists.
There are several examples of evidence that support endosymbiosis theory. Mitochondria and chloroplasts contain their own small supply of DNA, which may be remnants of the genome the organelles had when they were independent aerobic bacteria.