The endosymbiotic theory proposes that several organelles within eukaryotic cells, such as mitochondria and chloroplasts, were once free-living prokaryotic organisms that were engulfed by a larger host cell. Instead of being digested, these prokaryotes formed a symbiotic relationship with the host cell, eventually becoming integrated as permanent organelles within the cell. This theory suggests that the origins of complex eukaryotic cells can be traced back to symbiotic relationships between simpler prokaryotic cells.
