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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.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.
endosymbiotic theorythe proposition that symbiosis between different prokaryotes gave rise to eukaryotic cells.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005