If you don't restrict the list to living things, then salt and water are surely the oldest answers. :)

Hydrogen ions are edible, although usually dissolved in water. Hydrogen ions existed before oxygen or water

Yep, I ensure I ingest a healthy amount of plain old-fashioned protons. Surely among nature's most ancient foods.

I don't like them, they taste sour: https://youtu.be/FSYE1T5d9jc?si=jQP90-q7QAHnrcJt

Don't forget dirt - https://en.wikipedia.org/wiki/Geophagia#Humans

I remember when I was a very young child - more than half a century ago - getting into trouble for eating dirt. I was amused more recently to hear about my nephew doing the same thing. It seems, from my point of view, to be instinctive. I wonder why?

Bored? Low on minerals? Too many parasites?

Dogs do this instinctively too when they might need something from the dirt.

don't little kids sometimes eat play-doh, bugs, crayons, etc? new experiences/curiosity i'd imagine