* Mitochondria: These are the powerhouses of the cell, responsible for cellular respiration and ATP production. They are thought to have originated from aerobic bacteria that were engulfed by early eukaryotic cells.

* Chloroplasts: These are found in plant cells and are responsible for photosynthesis, the process of converting sunlight into energy. They are believed to have originated from photosynthetic bacteria that were engulfed by early eukaryotic cells.

The theory that explains the origin of these organelles is called the endosymbiotic theory. It suggests that these organelles were once free-living bacteria that formed a symbiotic relationship with a host cell. Over time, these bacteria became integrated into the host cell, eventually evolving into the organelles we see today.