The theory of endosymbiosis proposes that certain organelles in eukaryotic cells, specifically mitochondria and chloroplasts, originated from free-living prokaryotic organisms that were engulfed by a primitive eukaryotic host cell. This symbiotic relationship began as a mutually beneficial association, where the engulfed prokaryotes provided metabolic advantages to the host, such as efficient energy production, while gaining protection and access to nutrients. Over time, these prokaryotes became permanent residents within the host cell, evolving into organelles that lost much of their original independence. Primary endosymbiosis refers to the process by which a eukaryotic cell directly engulfs a prokaryotic organism. In the case of mitochondria, an ancestral aerobic α-proteobacterium was engulfed by a non-photosynthetic eukaryotic host. This engulfed bacterium possessed the machinery for oxidative phosphorylation, which allowed the host to generate energy more efficiently through aerobic respiration. Similarly, chloroplasts originated when a eukaryotic cell engulfed a photosynthetic cyanobacterium. The cyanobacterium contained the machinery for photosynthesis, providing the host cell with the ability to produce organic molecules from sunlight,