In the context of the evolution of the eukaryotic cell, the best-supported explanation for how mitochondria and chloroplasts arose is which of the following?

• A. Nuclear budding: mitochondria and chloroplasts originated as vesicles that budded from the nucleus and later acquired metabolic enzymes
• B. Endosymbiosis: an ancestral eukaryotic cell engulfed aerobic bacteria (becoming mitochondria) and later engulfed photosynthetic cyanobacteria (becoming chloroplasts), with the symbionts retained as organelles
• C. Autogenous origin: mitochondria and chloroplasts formed de novo by invagination and specialization of the eukaryotic plasma membrane
• D. Spontaneous generation: mitochondria and chloroplasts repeatedly assembled from nonliving cellular components whenever energy demands increased

Answer: (B)