Fossil evidence indicates that the first organisms on Earth were single-called prokaryotes. The first eukaryotes are believed to have formed from these prokaryotes. How does the endosymbiotic theory explain the development of the first eukaryotic organisms?.

The endosymbiotic theory explains how eukaryotic cells evolved. The large and small cells formed a symbiotic relationship in which both cells benefited. Some of the small cells were able to break down the large cell's wastes for energy. They supplied energy not only to themselves but also to the large cell.