What is endosymbiosis?

According to the University of Utah, endosymbiosis occurs when two different species benefit from living and working together and one of the species lives entirely within the other. A common example of endosymbiosis is in the case of mitochondria and chloroplasts, which may have once been separate single celled organisms.

As the University of California - Berkeley explains, the evidence for endosymbiosis lies in the observation that mitochondria have both their own cell membranes and their own circular DNA genome. Mitochondria also reproduce separately from their host cells, and if a cell's mitochondria are removed, the cell cannot build new ones from scratch.