"For most organisms on Earth, rocks are objects, not food."
I can't find a video I had about it, but the first organisms digested minerals and created a shitload of the other minerals we know and rely on today. It's just a hell of a lot more efficient to consume life that already did the hard work. So, ancient lifeforms digested anything they could, starting with 'Rock'. Sorry for lack of source.
However, cool discovery of a 'directional' rock digestion..