The endosymbiont theory explains the origin of certain membrane-bound organelles in eukaryotic cells, particularly mitochondria and chloroplasts, by proposing that they evolved from free-living prokaryotic organisms that entered into a stable symbiotic relationship with a host cell. According to this theory, early eukaryotic cells arose when a larger primitive cell engulfed smaller aerobic or photosynthetic bacteria but did not digest them. Instead, these internalized organisms continued to live inside the host cell, providing metabolic advantages, while the host offered protection and nutrients. Over evolutionary time, this mutually beneficial association became permanent, leading to the integration of these organisms as organelles. The theory was first suggested in the late nineteenth century