According to the theory of mitochondria in cells today are the descendants of aerobic prokaryotes that used oxygen to generate energy.

According to the theory of endosymbiosis , mitochondria in cells today are the descendants of aerobic prokaryotes that used oxygen to generate energy.

Explanation:

Endosymbiosis is an evolutionary theory which explains how eukaryotic cells evolved from prokaryotic cells. The theory of endosymbiosis states that mitochondria were once prokaryotes. They could carry out aerobic respiration and used oxygen to produce energy. With the passage of time, the bigger prokaryotes engulfed the smaller prokaryotes like the mitochondria. Mitochondria have their own DNA which proofs the hypothesis that they were once individual prokaryotic cells. Organelles like mitochondria, chloroplast and plastids support the theory of endosymbiosis.

the fossil record provides evidence of a common ancestor to many species.