There is compelling evidence that mitochondria and chloroplasts were once primitive bacterial cells. Both mitochondria and chloroplasts contain a ring of DNA double helix, just like that contained by a prokaryote. They also contain the small ribosomes, like those of prokaryotes. These features have caused some evolutionary biologists to suggest that some organelles are descendants of free- living prokaryotic organisms that came to inhabit larger cells. Present day prokaryotes are similar to fossil prokaryotes, some of which are 3500 million years old. By comparison, the earliest eukaryote cells date back only 1000 million years. Thus eukaryotes must have evolved, surrounded by prokaryotes that were long-established organisms. It is possible that, in the evolution of the eukaryotic cell, prokaryotic cells came to survive as organelles instead. If so, with time they would have become integrated into the biochemistry of their host cell. This concept is known as the endosymbiotic origin of eukaryotes. The endosymbiotic theory describes how a large host cell and ingested bacteria could easily become dependent on one another for survival, resulting in a permanent relationship. Over millions of years of evolution, mitochondria and chloroplasts have become more specialized and today they cannot live outside the cell.
How did this theory get its name?
Symbiosis occurs when two different species benefit from living and working together. When one organism actually lives inside the other it's called endosymbiosis.
Evidence that supports this theory
* Mitochondria and chloroplasts have striking similarities to bacteria cells. * They have their own DNA, which is separate from the DNA found in the nucleus of the cell. And both organelles use their DNA to produce many proteins and enzymes required for their function. * A double membrane surrounds both mitochondria and chloroplasts, further evidence that each was ingested by a primitive...