Simple question that I have yet been unable to find any answer to: how long after the Dark Brotherhood's emergence from the shattered Morag Tong (or Thieves Guild in some odd accounts) in the second era did the phenomenon of Shadowscales crop up?

I understand how the Shadowscale system works, but what I want to know is if there is any information on when it came to be and why and who was the mastermind behind it? I imagine it wasn't right away, or perhaps it was?

Thanks to any loremasters who can help me with this conundrum.