The Endosymbiote Hypothesis, developed by Dr. Lynn Margulis, states that structures such as the mitochondrion, chloroplast, cilia and flagella of eukaryotic cells are all derived from symbiotic relationships between early eukaryote cells and bacteria.  The evidence for this theory arises from the similarity of these structures to bacterial cells and in the presence of 70S ribosomes and circular DNA in mitochondria and chloroplasts.