The moon's massive size means its gravity pulls our oceans towards the sky and gives us the tides. The water bulges out to follow the orbit of the moon. If life began in the oceans, which is most people's theory, then the moon's tides would have mixed the water in such a way that it would have eventually dragged those early primitive life forms around. And why does it not exist somewhere else? And how rare is that? How rare is this collision between Thea and an early Earth that leads to this particular moon and this particular planet we live on? It might be really, really uncommon and that might be a unique situation in the entire broader cosmos.