Question

The endosymbiotic theory states that chloroplasts and mitochondria evolved as a result of

Answer 1

Mitochondria and chloroplasts likely developed from engulfed prokaryotes that lived as independent organisms.

A eukaryotic cell engulfed an aerobic prokaryote which then formed an endosymbiotic relationship with the host eukaryote and thus slowly developed into a mitochondrion.  

Eukaryotic cells containing mitochondria then engulfed photosynthetic prokaryotes which developed to form specific specialized chloroplast organelles.