The endosymbiotic theory describes a symbiotic relationship between cells in which one cell engulfs another cell, but instead of digesting it, the two cells form a partnership. In the case of eukaryotic cells, the theory suggests that a large cell engulfed a smaller cell that went on to become mitochondria or chloroplasts. This article explores the significance of this theory in understanding how eukaryotic cells evolved and sheds light on the complexity of life on earth.