D-types were a dead end in my opinion. I don't think there'd many offshoots from that at all. It's a very limited technique.

Anyway, photography almost certainly started with leaf prints, long before Scheele and all the rest. Starting with leaf printing, on the other hand, I can well imagine an incremental process of isolating light sensitive pigments and making colour images and so forth.

But it's really very remarkable that we are where we are with photography. There were a great number of shot-in-the-dark experiments done by people who didn't always understand what they were doing but whose curiosity and patience drove photography forward. It's really one of the most amazing histories in all of technology.