What is the theory of endosymbiosis, and why are we discussing this in a unit about energy generation in microorganisms?

The endosymbiotic theory states that some of the organelles in today's eukaryotic cells were once prokaryotic microbes. ... They eventually lost their cell wall and much of their DNA because they were not of benefit within the host cell. Mitochondria and chloroplasts cannot grow outside their host cell.

i have to report this, it doesn't relate to biology

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".