Dear Gurudev, the soul is eternal, pure and ancient, then how did it become deluded, that it does not know itself in its true nature?
This is called Atiprashna, i.e., the question which does not really concern you, and it is too much to ask for.
So why did the first ignorance happen, these questions were never answered. In the scriptures also, it is said that this is Atiprashna.
What are you going to do knowing that? Even if someone will say, 'Okay, this happened two billion years ago', what is the use? What are you going to achieve, or solve by it?
There are many other things to know. First know who you are, and then you can say, how the first ignorance came.