The discovery that significantly supported the endosymbiotic theory was the observation of mitochondria and chloroplasts containing their own DNA, which is distinct from the nuclear DNA found in eukaryotic cells. This DNA is circular, similar to the DNA of prokaryotes, and it suggests that these organelles may have originated from free-living bacteria that were engulfed by ancestral eukaryotic cells.
Additionally, studies have shown that the ribosomes within mitochondria and chloroplasts resemble those of bacteria rather than those of eukaryotic cells, further supporting the idea that these organelles evolved from a symbiotic relationship. The endosymbiotic theory posits that larger host cells and smaller prokaryotic cells formed a mutualistic relationship, which eventually led to the incorporation of these prokaryotic cells as permanent organelles.
Overall, the genetic, structural, and functional similarities between these organelles and bacteria provide compelling evidence for the endosymbiotic theory, illustrating how complex cells may have evolved through the merger of simpler life forms.