Ultimately, probably Sabbath, Pol. But there's little doubt they in turn were influenced by the heavier elements of the Amimals, Hendrix, Cream, Steppenwolf.......plus a host of others.
I have an interview with Chas Chandler here somewhere where he says the first time he heard the term "heavy metal" used was in reference to Jimi's guitar playing. Whilst it's not "metal" as we know it now, obviously there were sounds there that were later advanced, developed and built upon. Zepp had early metal elements there, as did Purple....but I'd probably hand it to Sabbath as the band that created something more identifiable to what we know today.
The more difficult question to pinpoint is probably not where has metal come from.....but where the hell is it headed? It's fracturing itself into so many bloody sub genres, soon there'll be nothing left. ;-)
They were the counter to the supposed niceties of the Beatles. In that way they certainly started an attitude that has been copied by rock (including punk and metal) bands for decades. You've only gotta have a look around to see..........bands are still trying to copy that swagger of the Stones to this very day.....they may not know it, but that's what they're doing.
It's official. You have NO taste!
I don't know anyone who doesn't like the Stones...........I don't
wanna know anyone who doesn't like the Stones.