Answer :
Without mitochondria, life on this earth as we know it would not exist. Mitochondria are responsible for producing massive amounts of ATP, without which more complicated multicellular life forms would not have been able to evolve.
Mitochondria are responsible for producing largest amounts of ATP, which become the main source energy for multicellular life forms to operate.
Since the organ system in multi cellular life form is much more advanced, it require larger amount of energy compared to single cellular life form.
Without mitochondria , higher level (multicellular) animals would likely not exist because their cells wouldn't be able to obtain energy from aerobic respiration.
