Were "en" and "y" always used in French?

Question

To a beginning learner like me, these two pronouns, en and y, are so quintessentially French that they seem to have been part of the language since its very inception, really.

However, I have no knowledge of the history of these typically French linguistic structures; nor do I know when they first entered the language.

So my question is: As Latin, the language from which French evolved, seems to have nothing similar to either en or y, what is then the linguistic source for these pronouns in French, and when were they first introduced into the language? (So a very concise introduction to the history of these words would be greatly appreciated).

Answer 1

Grevisse, le bon Usage (qui a en tout douze pages sur en et y dans la 12^ième édition)

En vient de l'adverbe latin inde « de là ». Pour y (ordinairement écrit i en ancien français), les étymologistes sont partagés : les uns proposent l'adverbe latin ibi « là » (certainement représenté par la forme iv des Serments de Strasbourg) ; d'autres l'adverbe latin hic « ici ».

As they come from adverbs, and behave both like adverbs and pronouns, they are sometimes called adverbes pronominaux or pronoms adverbiaux.