Mitochondria are said to arise from proteobacteria through the process known as endosymbiosis, which was first postulated in the 1960s by the biologist Lynn Margulis. Endosymbiosis is the condition wherein one organism resides within another organism for the mutual benefit of both organisms.
Cells are generally classified into two types: prokaryotic and eukaryotic. Eukaryotic cells are distinguished from prokaryotic cells by the presence of specialized structures called organelles in their cytoplasm. Plants and animals contain eukaryotic cells, while bacteria are prokaryotic microorganisms.
Mitochondria are specialized compartments that are referred to as the "powerhouses" of eukaryotic cells where the high-energy molecule adenosine triphosphate, or ATP, is produced. These structures differ from other organelles because they replicate by pinching in half and possess their own DNA and cellular membranes. Due to these characteristics, mitochondria are said to bear a striking resemblance to bacterial cells.
Margulis based her hypothesis on the strong similarities between mitochondria and bacteria. The extracellular origin of mitochondria claims that an autotrophic bacterium, possibly rickettsialis, was enveloped by a primordial, heterotrophic eukaryotic cell. The two cells then developed a symbiotic relationship where the bacterium supplied added energy to its host, which in turn gave shelter and nourishment to the prokaryotic cell. The prokaryotic bacterium gradually lost its ability for self-sufficiency and came to rely on its host for survival. Through time, the bacterium evolved into the mitochondria of eukaryotic cells.