Answer
endosymbiosis
Work Step by Step
When one organism lives inside another organism, the result is an endosymbiotic relationship. Researchers speculate that organelles such as mitochondria and chloroplasts arose from such endosymbiotic relationships. When the host cell reproduced, it passed on some of its endosymbionts to its offspring. Over time, these two organisms could no longer live without one another, and functions that were not needed were lost.
