According to the endosymbiotic theory, the larger prokaryotic cell may have been predating on the smaller prokaryotic cell, before a mutual relationship developed later on. Another hypothesis is that it was probably a parasitic relationship. Over time, the phagocytosed cell survived inside the larger cell and a symbiotic relationship developed over time because it provided an evolutionary advantage.
It is useful to use light years when you have to measure a super long distance as a light is approximately 6 trillion miles. And astronomical units are known as measurements within our solar system.
