Which discovery supported the endosymbiotic theory?

The discovery that supported the endosymbiotic theory is the presence of mitochondria and chloroplasts in eukaryotic cells, which have their own DNA that is distinct from the nuclear DNA of the cell. This DNA is similar to the DNA found in certain prokaryotes, suggesting that these organelles were once independent prokaryotic organisms that were engulfed by a host cell. Additionally, both mitochondria and chloroplasts have double membranes, which is consistent with the engulfing mechanism proposed by the endosymbiotic theory. This evidence supports the idea that these organelles originated from a symbiotic relationship between early eukaryotic cells and prokaryotic organisms.