If you're talking about the origins of the genre you don't begin with DoTA.
before dota came out there was aeon of strife, sure not as many players as dota, but it was still a popular map to play on wc3,
many more map variations and mods later... getting slightly more popular
one guy thought it'd be cool to do diagonal progression instead of left to right
calls it defense of the ancients, everyone loves the diagnonal setup, versions later and further development with custom skins ect ect ect, there you go.
aos gets left in the dark no hardcore following or further development , majority of dota community don't realize aos existed ever