Suppose again that those aliens hung around to observe and guide the development of the civilizations they helped create- and then stepped up their involvement when their creatures split the atom and began to venture out into space.
It's not what they teach us in school, but it's as plausible as the narrative that has everything happen by chance- and it makes sense of a lot of odd and anomalous stuff that the standard histories can't find room for.
You know what, I think I buy it.