Final answer:
The Endosymbiotic Theory states that mitochondria evolved from alpha-proteobacteria and chloroplasts from cyanobacteria. This is now widely accepted, supported by evidence like DNA, ribosomal similarities, and binary fission.
Step-by-step explanation:
The hypothesis regarding the Endosymbiotic Origin of Eukaryotic Organelles states that organelles like mitochondria and chloroplasts originated from prokaryotic cells establishing a symbiotic relationship within a eukaryotic host. Mitochondria are believed to have evolved from an aerobic bacterium, more specifically an alpha-proteobacterium, while chloroplasts are believed to have originated from a photosynthetic prokaryote, like cyanobacteria. Therefore, the correct answer to the question is that alpha-proteobacteria are the ancestors of mitochondria and cyanobacteria are the ancestors of chloroplasts.
Lynn Margulis was a key figure in proposing this theory in 1967. Moreover, the discovery of mitochondrial and chloroplast DNA, which resemble their bacterial counterparts in both sequence and chromosome structure, along with other evidences such as the similarity of their ribosomes to bacterial ribosomes and their replication through binary fission, support this hypothesis. Thus, the Endosymbiotic Theory is now widely accepted in the scientific community.