Endosymbiotic theory is supported by a wealth of evidence, including:
1) Mitochondria have their own DNA, which is circular and similar to bacterial DNA. This DNA is distinct from the DNA in the nucleus of the eukaryotic cell.
2) Mitochondria have their own ribosomes, which are smaller and structurally different from cytosolic ribosomes. These ribosomes are responsible for translating mitochondrial DNA into proteins.
3) Mitochondria have their own mechanism for protein import, which is different from the protein import machinery of the eukaryotic cell. This suggests that mitochondria were once independent organisms.
4) Mitochondria share many similarities with alpha-proteobacteria, a group of bacteria. This includes the structure of their inner membranes, the arrangement of their genes, and the way they generate energy.
5) Mitochondria reproduce by binary fission, similar to bacteria. This suggests that mitochondria once replicated independently of the eukaryotic cell.
6) The endosymbiotic theory is also supported by the fact that mitochondria are found in all eukaryotic cells, with only a few rare exceptions. This suggests that mitochondria were acquired by an ancestral eukaryotic cell and have since been passed down to all of its descendants.
7) Mitochondria (and chloroplasts) have double membranes, exactly like they would if they were smaller cells engulfed by the host cell.
8) There are multiple examples of ongoing endosymbiosis where the engulfed cell remains a true symbiont, not yet an organelle. Paramecium bursaria is my favorite - a ciliated protozoan with blue-green algae symbionts.
Bonus: there is evidence for secondary and tertiary endosymbiosis too.
1) Mitochondria have their own DNA, which is circular and similar to bacterial DNA. This DNA is distinct from the DNA in the nucleus of the eukaryotic cell.
2) Mitochondria have their own ribosomes, which are smaller and structurally different from cytosolic ribosomes. These ribosomes are responsible for translating mitochondrial DNA into proteins.
3) Mitochondria have their own mechanism for protein import, which is different from the protein import machinery of the eukaryotic cell. This suggests that mitochondria were once independent organisms.
4) Mitochondria share many similarities with alpha-proteobacteria, a group of bacteria. This includes the structure of their inner membranes, the arrangement of their genes, and the way they generate energy.
5) Mitochondria reproduce by binary fission, similar to bacteria. This suggests that mitochondria once replicated independently of the eukaryotic cell.
6) The endosymbiotic theory is also supported by the fact that mitochondria are found in all eukaryotic cells, with only a few rare exceptions. This suggests that mitochondria were acquired by an ancestral eukaryotic cell and have since been passed down to all of its descendants.