It was Chaos War (SC) -> Hero Arena (WC3) -> Attack on the Light v0.1a (WC3) -> Defense of the Ancients v1.3 (WC3).
Source: I made Chaos War (SC1 Map).
A multi-billion dollar industry hinges on the fact that an eight year old figured out how to make a map trigger which spawned units with an attack-move command in a custom editor in an RTS.
P.S: (I’ve seen exactly $0 of DOTA “money”. It was free and clear, for any and all, a source of happiness with no entry fee except Use Map Settings.)