Theory of endosymbiosis

The theory of Endosymbiosis explains the origin of
chloroplasts and mitochondria and their double membranes.

This concept postulates that chloroplasts and mitochondria
are the result of years of evolution initiated by the
endocytosis of bacteria and blue-green algae.
According to this theory, blue green algae and bacteria
were not digested; they became symbiotic instead.