Of course I didn't quite realize it's the songs not played on the radio, mostly concentrated in the Terry Knight-era (1969-71) that should be concern of people digging into the roots of metal, as many of them are quite heavy, especially 'Sin's a Good Man's Brother' from Closer to Home (which is hard to believe is on the same album that has 'I'm Your Captain'). 'We're An American Band' seems more like mainstream hard-ish rock, and 'The Locomotion' and 'Some Kind of Wonderful' were full-on R&B covers. Not to mention the nearest radio station (KZEL 96.1 FM, major Eugene rock station, still around) would play that song at that time. It even resonated with my hippie spirit living as a small child in rural Oregon in the late '70s in a semi-hippie fashion (helped that there was a hippie commune at the time within walking distance, and my father owned an old, beat-up VW split window Microbus). It's certainly a great song, and even the band's detractors tend to think fondly of it.

From hearing 'I'm Your Captain', I was thinking that song has as much metal going as your typical Crosby, Stills & Nash song.

When I was in my teens (circa 1989) I kept hearing how Grand Funk Railroad was the forefathers of heavy metal.

Grand funk railroad poster