What theory explains chloroplast origin from cyanobacteria?

The endosymbiotic theory explains chloroplast origin from cyanobacteria. It proposes that an ancestral eukaryotic cell engulfed a photosynthetic cyanobacterium, which survived inside the host and gradually evolved into a chloroplast.

The relationship became permanent as the internal bacterium supplied photosynthetic benefits while receiving protection and nutrients from its host. Over evolutionary time, many of the bacterium’s genes moved into the host cell’s nucleus, leaving the modern chloroplast with a much smaller genome and dependence on proteins made elsewhere in the cell.

Several observations support this history. Chloroplasts retain their own usually circular DNA, divide by a process resembling bacterial fission, contain bacteria-like ribosomes, and have double membranes. Genetic comparisons also place plastids close to cyanobacteria. These features are evidence of descent from a once-independent organism, not merely a superficial resemblance.

The idea has a long history. Andreas Schimper noticed chloroplast-like division patterns in the 19th century; Konstantin Mereschkowski proposed a symbiotic origin in 1905; and Lynn Margulis helped revive and substantiate the theory in the 1960s. “Endosymbiotic theory” is more precise here than the general theory of evolution, which describes the broader process rather than this specific origin.