Three clues support bacterial ancestry: their own bacterial-like DNA, bacterial-like ribosomes and reproduction by binary fission. Oxygen-using bacteria connect to mitochondria; photosynthetic bacteria connect to chloroplasts. A stable relationship inside a larger cell explains how distinct cells became organelles over evolutionary time.

The model has a domain of validity. Explain its assumptions before using it in a changed context, then compare your prediction with evidence.

• Mitochondria and chloroplasts descend from bacteria that became long-term partners inside ancestral cells.
• The theory concerns specific organelles, not a claim that every cell structure was an engulfed bacterium.