Just like there's no definitive answer as to how Earth formed, there is truly no definitive answer as to which band "invented" heavy metal.

The Beatles and the Rolling Stones may have been the biggest rock 'n' roll bands throughout the early to mid-1960s, but it was The Who, The Kinks, Jimi Hendrix and Cream who really began pushing the limits and innovating with their equipment to create the distorted, deep sounds that defined heavy metal in the coming years.

Some will argue it was Led Zeppelin who swallowed their rock and blues predecessors' sounds and thus regurgitated metal, others will say Black Sabbath. Various sources say it was those two plus Deep Purple. But even the styles of those three bands were a result of the way rock music was evolving throughout the '60s, combining blues, distorted garage rock, acid rock and psychedelia. The birth of heavy metal was a culmination of bands experimenting with heavier sounds, like an intensely flavored musical stew.

Groups began releasing heavy albums left and right starting around 1970, which was a whole five decades ago as of this year. To celebrate 50 iconic years of heavy metal, we compiled a gallery of bands and musicians considered "proto-metal," who are credited for pioneering the genre. Whether it was a single song, instrumental technique or attitude, these artists are the roots of heavy metal.

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