Final answer:
Mitochondria were once free-living aerobic prokaryotes that used fermentation and aerobic respiration to acquire energy. They entered into a symbiotic relationship with larger cells, providing them with energy, and over time became dependent and evolved into organelles.
Step-by-step explanation:
Your question relates to how mitochondria, which are now organelles within eukaryotic cells, once lived on their own and acquired energy. The endosymbiotic theory suggests that mitochondria were once free-living prokaryotes that had the ability to create their own energy.
Initially, these prokaryotes may have used processes like fermentation to extract energy from organic molecules in an oxygen-poor environment. As autotrophic organisms evolved, which could use light or inorganic molecules to create energy, the dependency on organic molecules lessened. The bacteria that would become mitochondria probably used aerobic respiration to produce energy more efficiently, especially as oxygen levels in the atmosphere increased.
These early aerobic prokaryotes were engulfed by larger cells, leading to a mutualistic relationship where the smaller prokaryotes provided an extra source of energy to the host cell. Over time, these prokaryotes became the mitochondria we are familiar with today, losing their ability to live independently and instead becoming integral components of the eukaryotic cells.