Coexistence of two organisms from different species (with mutualistic advantages), with one of the two partners (the symbiont) living within the other.

Eukaryotic cells developed as the result from such endosymbioses between members of the archaeae and bacteria. The bacterial partners became more and more dependent during the evolution of these symbioses. The precursors of recent mitochondria and chloroplasts lost a large part of their DNA and today are no longer able to live as independent organisms.