Endosymbiotic Theory in a nutshell

Endosymbiotic theory explains the evolution of eukaryotic cells (euks) from prokaryotic cells (proks).

Just to remind, euks have membrane-bound organelles and a nucleus whereas proks do not have those.

Once upon a time, chloroplast and mitochondria were free-living proks and then one day they either ended up being eaten by euk hosts or became parasites in these hosts. Fun thing, they were not digested or killed the host. The inner cells (proks) and the host (euks) developed a symbiotic relationship. Endosymbiont means one organism living in another. It’s a win-win situation: the inner cells provide energy to the host while the host provides a safe, nutritious environment for the organelles.

Now they cannot live without each other.