Who started the generic rank structure amongst all nations?

After a very little research, it looks like the grades associated with military ranks for NATO nations were probably developed, or at least approved, by the NATO Military Committee. There are many similarities among NATO nation's rank structures, but also significant differences. Countries greatly influenced by the UK use a rank structure similar to the UK with Army, Navy, and Air Force officer ranks having many different names, for example, "commander" instead of "Lieutenant Colonel."

The introduction of the NATO rank codes or grades allows military personnel from different nations serving on NATO staffs or NATO Task Forces to understand their relationships in terms of rank. This is very helpful when the rank titles don't actually describe the job the Service Member holds. For example, not all RAF Squadron Leaders actually lead squadrons.

The Romans had a rank structure, but today's ranks really took shape in medieval Europe. Officers were members of the aristocracy, commissioned by the King to lead, and enlisted men were commoners. Each lord would raise a company of men, commanded by a Captain (who was assisted by a Lieutenant), and make that company available to his King. There were three main enlisted ranks: Sergeant (roughly equivalent to US SNCOs), Corporals (in charge of a squad, comes from the Italian world for "head"), and Privates (called such because they signed a private enlistment contract with the Captain).
The Navy got its ranks from the British Empire, who developed the modern naval rank structure with their massive fleet. I'm not as well versed on that though, so I can't explain the individual ranks.

As time went on, ranks got sub-divided down, and here we are today!

Have no real idea- but probably the brits several hundred yrs ago.