The Endosymbiotic Theory

 A theory on the Origins of Eukaryotic Cells: Mitochondria and Chloroplasts

http://www.biology.iupui.edu/biocourses/N100/2k2endosymb.html

The Endosymbiotic Theory was first proposed by former Boston University Biologist Lynn Margulis in the 1960's and officially in her 1981 book "Symbiosis in Cell Evolution".  Although now accepted as a well-supported theory, both she and the theory were ridiculed by mainstream biologists for a number of years.  Thanks to her persistance, and the large volumes of data that support this hypothesis gathered by her and many other scientists over the last 30 years, biology can now offer a plausible explanation for the evolution of eukaryotes. [Quote]
Dr. Margulis was doing reserarch on the origin of eukaryotic cells. She looked at all the data about prokaryotes, eukaryotes, and organelles. She proposed that the similarities between prokaryotes and organelles, together with their appearance in the fossil record, could best be explained by "endo-symbiosis". 



Her hypothesis originally proposed that:

• mitochondria are the result of endocytosis of aerobic bacteria
• chloroplasts are the result of endocytosis of photosynthetic bacteria
• in both cases by large anaerobic bacteria who would not otherwise be able to exist in an aerobic environment.
• this arrangement became a mutually beneficial relationship for both cells (symbiotic).

Margulis' original hypothesis proposed that aerobic bacteria (that require oxygen) were ingested by anaerobic bacteria (poisoned by oxygen), and may each have had a survival advantage as long as they continued their partnership.

Each would have performed mutually benefiting functions from their symbiotic relationship.  The aerobic bacteria would have handled the toxic oxygen for the anaerobic bacteria, and the anaerobic bacteria would ingested food and protected the aerobic "symbiote"..
The result = a cell with a double-membrane bound organelle. The inner lipid bilayer would have been the bacterial cell's plasma membrane, and the ouler lipid bilayer came from the cell that engulfed it.