I'm finding it hard to get a clear defination of what Heavy Metal is, but most seem to agree on the following: loudness, distorted guitar, reliance on chorded riffs, a loud consistent beat and macho lyrics.
If I were a music archeologist, digging up the bones of songs, probably the best thing to do would be to look for songs that had the most of these characteristics.
Plenty of songs are loud. And due to technology a song recorded in 1964 will never sound as loud as a song recorded in say 1968, so we have to judge them by the standrads of their day. In 1963 Twist and Shout was loud, but no one would call that proto metal. Rain is loud and has distorted guitars, but no riff or macho lyric. All of The Day is loud and has chorded riffs and macho lyric.
Dunno.