What does the theory of endosymbiosis explain?

The theory of endosymbiosis explains how chloroplasts and mitochondria evolved from free-living prokaryotic organisms that entered into a symbiotic relationship with ancestral eukaryotic cells. This theory is supported by several pieces of evidence, such as the similarities in the structure and function of these organelles with certain prokaryotes, including their own circular DNA, double membranes, and ribosomes that resemble those of bacteria.

According to this theory, an ancestral eukaryotic cell engulfed these prokaryotes, which then became integral to the cell's metabolism and energy production processes. The engulfed prokaryotes provided advantages, such as the ability to perform photosynthesis in the case of chloroplasts and aerobic respiration for mitochondria, leading to the evolution of complex cells. This hypothesis thus provides a vital explanation for the origin of key organelles within eukaryotic cells, illustrating how symbiotic relationships were fundamental to the development of complex life.