Mitochondria and chloroplasts developed entirely within complex cells and have no evolutionary relationship to bacteria.

Mitochondria and chloroplasts are structures inside complex cells. Mitochondria help cells extract usable energy from food, while chloroplasts carry out photosynthesis in plants and algae.

For a long time, it was natural to assume that these structures had simply evolved inside the cells that contain them.

A different idea proposed that they began as independent bacteria.

In this endosymbiotic model, an ancestral cell engulfed a bacterium but did not digest it. The two cells formed a partnership: the bacterium provided useful chemistry, while the host provided a protected environment and access to nutrients. Over many generations the partnership became permanent, and the bacterium lost much of its independence.

The organelles themselves preserve evidence of that history. Mitochondria and chloroplasts contain their own DNA, divide in ways resembling bacteria, and use ribosomes that differ from those in the surrounding cell. Their membranes and many of their genes also have clear bacterial relationships.

Molecular comparisons eventually made the ancestry especially strong. Mitochondria trace back to a bacterial lineage, while chloroplasts trace back to cyanobacteria, the group of bacteria that performs oxygen-producing photosynthesis.

Many genes from the ancestral bacteria have since moved into the host cell’s nucleus. The modern organelles are therefore no longer free-living bacteria, but their bacterial origin is well established.