Fwiw I did go another step "up the tree" by looking at the references of that paper, and it seems the only real precursor is another one by the same authors, one year previous:

https://core.ac.uk/download/pdf/301667381.pdf

see p.206-207 : these are two protocols doing the same thing (proving DL), but they are a little less elegant than the later ones. That paper itself is "the root" (or at least one such), if you take the authors at face value. Quote: "As far as we know, no other protocol with the same functionality has been presented."

It seems like this line of research came directly out of the Goldwasser, Micali, Rackoff paper in '85 showing that ZKPs are possible; they reference that set of ideas heavily here.