Fairly sure it would predate even that, and go all the way to lambda calculus, and predicate logic before that, and that's where my knowledge stops and somebody else can tell us where the current conventions around variables in logic and mathematics come from.

In ocaml (and I assume SML) it helps that the generic types have a `'` before them, so

    val map : ('a Box) -> ('a -> 'b) -> 'b Box