ENDOSYMBIOSIS

What is Endosymbiosis?

Endosymbiosis refers to the phenomenon of one organism (often a small prokaryote) living inside another organism, and the two effectively live in a symbiotic relationship. The larger cell usually engulfs the smaller one, and it becomes an 'organelle' for the host/larger cell. 

When we break apart the word, it makes sense! 'Endo' means within, and 'symbiosis' refers to any effective two-way relationship between organisms, often a mutually beneficial one. 

⇧ Evidence for this theory is supported through this mnemonic, MAD DR. (mad doctor!). Mneumonic courtesy of BioNinja!

The Theory of Mitochondria and Chloroplasts

All eukaryotes that have been studied thus far have mitochondria in their cells (or versions of mitochondria). Mitochondria are organelles, as they are membrane-bound bodies in cells with specific functions, but unlike other organelles, mitochondria have their own DNA. This supports the theory that mitochondria evolved from an independent single-celled organism, that was engulfed by a larger single-celled organism in the earlier stages of life on earth. That organism underwent massive evolutionary processes to eventually become the common ancestor for the eukaryotic domain of life. A very similar theory has been developed about chloroplasts, which also have their own DNA.