What is the serial endosymbiotic theory? How does it explain the origin of mitochondria and chloroplasts?

Scientists have discovered that the genomes of ricketsia, the bacteria that cause trench fever, and mitochondria are similar. This indicates that anaerobic prokaryotic cells engulfed aerobic bacteria, increasing the production of ATP in the cells. This process is known as endosymbiosis. Originally, cells were anaerobic, but as oxygen in the earth's environment increased due to photosynthesis, giving rise to aerobic bacteria. Ancestors of mitochondria were aerobic bacteria.

The serial endosymbiotic theory proposes that mitochondria and chloroplasts in eukaryotic cells originated from free-living prokaryotic organisms that were engulfed by a primitive eukaryotic cell. This theory suggests that mitochondria originated from an endosymbiotic alpha-proteobacterium, while chloroplasts originated from an endosymbiotic cyanobacterium. Over time, these endosymbiotic organisms evolved a symbiotic relationship with the host cell, eventually becoming integrated into the cell and losing their ability to live independently. This theory is supported by several lines of evidence, including the similar size and structure of mitochondria and chloroplasts to bacteria, their own DNA separate from the nuclear DNA, and their ability to replicate independently within the cell.