Endosymbiosis played key roles in the development of eukaryotes and plants. Roughly 2.2 billion years ago an archaeon absorbed a bacterium through phagocytosis, that eventually became the mitochondria that provide energy to almost all living eukaryotic cells. Approximately 1 billion years ago, some of those cells absorbed cyanobacteria that eventually became chloroplasts, organelles that produce energy from sunlight.[4] Some 100 million years ago, UCYN-A, a nitrogen-fixing bacterium became an endosymbiont of marine algae Braarudosphaera bigelowii, eventually evolving into a nitroplast.[5] Similarly, Diatoms in the family Rhopalodiaceae have evolved a diazoplast, a nitrogen-fixing organelle.[6]
Symbionts are either obligate (require their host to survive) or facultative (can survive independently).[7] The most common examples of obligate endosymbiosis are mitochondria and chloroplasts, which reproduce via mitosis in tandem with their host cells. Some human parasites, e.g. Wuchereria bancrofti and Mansonella perstans, thrive in their intermediate insect hosts because of an obligate endosymbiosis with Wolbachia spp.[8] They can both be eliminated by treatments that target their bacterial host.[9]