Answer:
Secondary endosymbiosis is one of the two types of endosymbiotic interaction which played a major role in the formation of complex organism on Earth.
Secondary endosymbiosis refers to the endosymbiosis of the primary endosymbiotic organism by another eukaryote that is a eukaryote is engulfed by another eukaryote.
The secondary endosymbiosis explains how the eukaryotic organism was formed which can be supported by the double membrane of the chloroplast and the mitochondria.