Fossil records indicate that eukaryotes evolved from prokaryotes somewhere between 1.5 to 2 billion years ago. Two proposed pathways describe the invasion of prokaryote cells by two smaller prokaryote cells. They subsequently became successfully included as part of a now much larger cell with additional structures and capable of additional functions.
Research conducted by Lynn Margulis at the University of Massachusetts supports the hypothesis that two separate mutually beneficial invasions of a prokaryote cell produced the modern-day mitochondria and chloroplast as eukaryotic organelles. In this model, ancestral mitochondria were small heterotrophs capable of using oxygen to perform cellular respiration and thereby create useful energy. They became part of a large cell either by direct invasion as an internal parasite or as an indigestible food source. Later, a second invasion brought ancestral chloroplasts, which are thought to be small, photosynthetic cyanobacteria. Modern-day supporting evidence for endosymbiosis shows that both the mitochondria and chloroplasts have their own genes, circular DNA and RNA, and reproduce by binary fission independent of the host's cell cycle. They therefore appear to be more similar to prokaryotes than eukaryotes.
The invasions of the host prokaryote cell probably were successful because the host cell membrane infolded to surround both invading prokaryote cells and thereby help transport them into the cell. The membrane did not dissolve but remained intact, and thereby created a second membrane around the protomitochondria and protochloroplast. It is also known that in modern-day eukaryotes the inner membrane of both the mitochondria and chloroplast contain structures more similar to prokaryotes than eukaryotes, whereas the outer membrane retains eukaryote characteristics! It is also suggested that continued membrane infolding created the endomembrane system. It can be said that possibly the first eukaryotic cell type was miraculously born from prokaryotic, symbiotic, multicell interactions!
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.