Final answer:
Scientists believe that bacteria, mitochondria, and chloroplasts share a common evolutionary history due to their structural and functional similarities. These similarities support the theory that mitochondria and chloroplasts were once bacteria that were engulfed by host cells in an endosymbiotic relationship.
Step-by-step explanation:
Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size and have other similar features. We also know that bacteria have DNA and ribosomes, just like mitochondria and chloroplasts. Scientists believe that host cells and bacteria formed an endosymbiotic relationship when the host cells ingested aerobic bacteria and cyanobacteria but did not destroy them. Through evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the photosynthetic bacteria becoming chloroplasts.