Final answer:
The endosymbiotic theory explains the evolution of eukaryotic cells through symbiotic relationships between prokaryotic cells. Mitochondria and chloroplasts formed from ancient prokaryotes that became organelles within host cells.
Step-by-step explanation:
The endosymbiotic theory explains how eukaryotic cells evolved through symbiotic relationships between prokaryotic cells. According to this theory, mitochondria and chloroplasts originated from ancient prokaryotic cells that were engulfed by larger prokaryotic cells. Over time, these engulfed cells became organelles within the host cells, forming a mutually beneficial relationship. Mitochondria evolved from aerobic bacteria that became endosymbionts in primitive eukaryotic cells, while chloroplasts likely originated from photosynthetic bacteria that were engulfed by eukaryotic cells.