Simplified answer:
The theory of endosymbiosis proposes that eukaryotes descended from a combination of both archaea and bacteria. Evidence that supports this theory includes the similarities between mitochondria and chloroplasts to free-living bacteria, the presence of double membranes around mitochondria and chloroplasts, and the fact that mitochondria and chloroplasts have their own DNA.
Step-by-step explanation:
The theory of endosymbiosis proposes that eukaryotes descended from a combination of both archaea and bacteria. Evidence that supports this theory includes the similarities between mitochondria and chloroplasts to free-living bacteria, the presence of double membranes around mitochondria and chloroplasts, and the fact that mitochondria and chloroplasts have their own DNA.
Mitochondria and chloroplasts contain some of the same enzymes and transport systems as bacteria, and are similar in size and shape to bacteria. In addition, they both reproduce by binary fission, similar to bacteria. The presence of double membranes around mitochondria and chloroplasts also supports the theory of endosymbiosis, as this suggests that the organelles were engulfed by the host cell and retained their own membranes.
Finally, mitochondria and chloroplasts have their own DNA, which is circular like bacterial DNA. The DNA of mitochondria and chloroplasts also lacks histones, which are present in eukaryotic DNA, and is more similar to bacterial DNA in terms of gene expression and replication.
Together, these lines of evidence support the theory of endosymbiosis and suggest that mitochondria and chloroplasts were once free-living bacteria that were engulfed by a host cell and evolved into the organelles found in eukaryotic cells today.