endosymbiotic theory

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A theory stating that the eukaryotes evolved through a process whereby different types of free-living prokaryotes became incorporated inside larger prokaryotic cells and eventually developed into mitochondria, chloroplasts, and possibly other organelles.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun ecology The theory that concerns the origins of mitochondria and chloroplasts, which are organelles of eukaryotic cells.