Most people see Black Sabbath, Led Zeppelin, Deep Purple as the start of heavy metal. Their albums Black Sabbath, Led Zeppelin I and In Rock had a massive influence that is still felt today.
Heavy metal was essentially the Blues Explosion plus fuzz guitars, a more aggressive sound essentially a rejection of the folk of the Summer of Love, Woodstock,etc.
The earliest protoype of metal may well be Cream (1966); certainly the combination of Jack Bruce and Pete Brown (lyricist) meant that they were much more than a simple blues band.
Iron Butterfly are also contenders (1966), there is alot more to them than In-a-Gadda-Gadda-Da-Vida. Blue Cheer, however, probably take the cake. Vincebus Eruptum (1968) leaves the blues unrecognisable, despite the choice of Summertime Blues, Parchment Farm and Satifaction as cover tracks.
Like an onion, however, you can unpeel more and more layers: check Haggard Bastard's list of sixty albums from this era, many of which i simply have never heard of. The Ripple Effect, a great site on lost albums, also has a monthly Proto-Metal Report. A really great giveaway CD with Mojo magazine last year (Heavy Nuggets) is also a revelation; High Tide, Buffalo, Blackwater Park, Hard Stuff.
Here's ten tracks to get you going:
1 Aeroplane Head Woman: Pete Brown and Piblotko
2 Freelance Fiend Leaf Hound
3 NSU Cream
4 Summertime Blues Blue Cheer
5 Hush Deep Purple
6 The Wizard Black Sabbath
7. Possession Iron Butterfly
8 Peace Lovin Man Blossom Toes
9 Dont Make My Baby Blue The Move
10 Timepiece Bloodrock
Image: Polygram/ Vertigo/ Polydor/ Cactus