According to the endosymbiosis theory of the origin of eukaryotic cells, how did mitochondria originate?

Mitochondria originated when an ameoba like organism engulfed an aerobic proteobacterium capable of cellular respiration.

Explanation:

Endosymbiotic Theory:

The endosymbiotic theory explains the origin of eukaryotic cells and eukaryotic organelles, particularly the chloroplast and mitochondria. According to this theory, a prokaryotic ameoba like organism increased in size and gradually developed infoldings of its plasma membrane. These infoldings gradually developed into an endomembrane that enclosed the genetic material and nucleus. This cell then engulfed a proteobacterium capable of cellular respiration. Over time, this cell thrived as oxygen increased in the atmosphere of the Earth. The proteobacterium survived as an endosymbiont and later evolved into mitochondria.