What observation led researchers to propose that chloroplasts evolved from cyanobacteria?

The theory that claims that, mitochondria and chloroplast's origin is bacterias, is called the endosymbiotic theory. It suggest that, plastids (one of them is chloroplast) and mitochondria were once, bacterias, and about 1.5 billions year ago they started to live with (inside) the pre-eukaryotic orgamisms. This is called endosymbiosis. The chloroplast originates from the cyanobacterias and the mitochondria from the proteobacterias. The some evidences that support this theory are these : - Mitochondria and chloroplast are formed with binary fission (the way bacterias reproduce) - When you remove a cell's mitochondria or it's chloroplast it can not produce them by itself. - Mitochondria and chloroplast have their own ribosomes (which produces protein) and they are smaller than the cell's own ribosomes, their ribosomes are similar to bacterias'.