Being an old geezer [born in 1960] I grew up with the original heavy metal bands. Steppenwolf, Iron Butterfly,The Who, Deep Purple, Led Zeppelin, Black Sabbath, Uriah Heep, Golden Earring, Blue Oyster Cult, Alice Cooper, Judas Priest, UFO, Scorpions, VanHalen.
The term " heavy metal " can be traced to the mid to late sixties - I think it appears in the book 'Naked Lunch' and in the same way that terms like 'soft machine', 'the doors', 'psychedelic' etc came from books that the emerging rock royalty were into and found their way into lyrics, bandnames, album titles etc, so it happened with genres too {rock and roll was Black American slang for sex, coz their beds were rocking and rolling during the action, reggae came from a word that simply meant 'the regular people'}. I think musically it was first applied to the Byrds.
Heavy metal however, was, in my opinion,
an evolution, not an invention { check out some of the guitar sounds on the Stones' "I wanna be your man", the Kinks' "You really got me", the Nashville Teens' Tobacco Road" - all from '64 }. Things that guitarists were experimenting with began to show themselves on those kind of records and moved in leaps thru the likes of the Hendrix Experience, Cream and the Yardbirds (and others).
For about 12 years {say, 68-80} the terms 'hard rock', 'heavy rock', 'heavy metal rock', 'heavy metal' and 'heavy music', were
interchangeable terms that quite often meant the same thing. But because of the earlier usage of 'heavy metal' sometimes it had a different meaning. So it's not surprizing that Jon Lord and Ritchie Blackmore referred to Purple as hard rock while John Lennon in 1980 called 'Ticket to ride' one of the earliest heavy metal tunes written. It's also worth making the point that in the late 60s and early 70s, heavy/ hard rock was actually marketed as 'progressive' rock
along with some of what we now refer to as 'prog', hence you get Ian Gillan referring to "Fireball" as 'a good progressive rock album' and Rush's Geddy Lee referring to Rush as a kind of progressive metal outfit. To slightly confuse the issue further, Lee also drew distinctions between hard rock and metal but historically, such a distinction is hard to find. I was using the term heavy metal in '79 and if you look at the New musical express encyclopedia of rock (published jan '77 but written in '76) the number of times phrases like 'heavy metal', 'heavier rock', 'harder rock' and 'hard rock' turn up is amazing - as is also the bands to whom the phrases are applied {Iggy and the Stooges, Kiss, Humble Pie, Three Dog Night, Grand Funk Railroad, Queen ~ for example}. Led Zeppelin are called "the definitive dizz-buster heavy metal rock combo, spawning a host of imitators" {many forget that Hendrix, Cream and Led Zep were the original inspiration for many groups to go heavy ~ Sabbaths influence was probably more far reaching in the long run, but not for close to a decade} and of their first album "from the opening few bars this was absolutely dynamite heavy metal rock" and later on "their understanding of heavy rock textures certainly had no precedent". 'Whole lotta love' is called "the definitive heavy metal anthem", of Zeppelin 3 we are told, interestingly, that it "appeared in 1970 with similar hard rock foundation" (note how the terms are already being used interchangeably) and a quote from Page is given where he describes part of their sound as "heavy blues". The 1980 'Rock On heavy metal A-Z' {published in '79} calls Led Zeppelin the "kings of heavy metal", refers to Motorhead as "one of the loudest hard'n'heavy bands in the UK", Blue Oyster Cult as "initially considered the foremost exponents of heavy metal rock in the US" {interestingly, it was Sabbath's early impact that pushed BOC's record company to look for a heavy act, settling on the Blue Oysters}, Boston as "the softer side of heavy metal", Rush as "heavy metal with classical undertones", Thin Lizzy as "inimitable hard rock though not strictly heavy metal", Staus Quo as the "granddads of heavy metal.....playing good, hard upfront rock".
One of the things that is notable in reviews written in the 70s about heavy bands is that quite a number of them had other strings to their bows {like Queen or The Stooges}. A broad range of bands fell into the heavy category, some of which moreorless exclusively stuck to the genre, others dipped in and out and went elsewhere. Some of the heavy metal arguments that have come up in this thread could have been and indeed were being made
before the 80s even began.
Ozzy is cartoonish but with Black Sabbath he was occassionally funny - but that's the point: they had a sense of humour as in "Fairies Wear Boots".
Their music wasn't humorous. It was uniformly HEAVY METAL until Technical Ecstatcy anyway. That's a 1/2 dozen albums in a row that set, made &/or met the mark in the genre.
There are hundreds of heavy bands that existed prior to the 80s, far more than ever made it big. But many many of those bands, with each release contributed something new to the evolving genre. Alot of them knew each other, drank and smoked together, toured together or played the same festivals, recorded either for the same labels or in the same studios and there was much cross pollination and the genre grew and expanded in a number of different directions and paved the way, in my opinion, for the expansion that emereged in the 80s and beyond. Five of the bands Ocnor mentions were blasting peoples' eardrums long before Black Sabbath existed as a band. Their debut came after Zeppelin had done 2 albums that contained heaviness. They without a doubt were, across their first six LPs, one of heavy metal's foremost contributors. Inventors, I think not. I think it's safe to say the genre would not have been the same without them.
It's almost impossible to write a definitive history because of the many shifting currents and changes. Such is the nature of an evolution.