Currently there are two theories that try to explain cell evolution:

• Autogenous theory (F. Taylor and E. Dodson). He states that the eukaryotic cell arose when the prokaryotic cell was increasing in size and complexity. The development of the endomembrane system would give rise to the current organelles.
• Endosymbiont theory (Lynn Margulis and D. Sagan). According to this theory, the most accepted at present, the eukaryotic cell has been formed by the permanent symbiosis between different types of prokaryotes. The chloroplasts and mitochondria, originated when a prokaryote was engulfed by other, which would have established an endosymbiotic relationship.

Some characteristics of mitochondria and chloroplasts that support the endosymbiont theory are:

• Double membrane presence.
• Presence of ribosomes the size of the ribosomes of prokaryotic cells (70S). 
• Circular DNA.
• Mitochondria and chloroplasts divide by bipartition.

Animation: Endosymbiotic Theory .

Video: Endosymbiotic theory (Amoeba sisters).