So the changes we see in the Romance languages, things like final consonants dropping off (e.g., Lat. est [is] --> Sp. es/ Lat. clamat --> It. chiama) and the vowel 'u' becoming 'o' (e.g., Lat. amicus --> It. amico; Sp. amigo) must have been happening long before they were ever written down.
In fact, people probably continued to write their language in Latin spellings even though they were no longer pronouncing it as they wrote it (much as we keep in English the writing of 'gh' in words like 'rough' and 'though').
And this leads me to assert that a certain Latin graffito preserves the oldest written example of Proto-Romance language. It's been known about for over a hundred years, but it has been mistranslated and, therefore, its significance not realized.
The inscription in question is understood to be a sort of vandalism on the early imperial period tombstone of a man named Tiberius Julius Vitalis.
It is not impossible that the inscription was made at the same time as the tombstone itself. That possibility will not affect my interpretation.
There have been two approaches to translating the inscription. One is to assume that the name Marcio is feminine to match the feminine adjective ebria:
"Marcio the constantly intoxicated" ["in mockery of a tippling lady"]
(P. 13, Guide to the Public Collections of Classical Antiquities in Ancient Rome, Wolfgang Helbig  )
The other approach is to assume that the name Marcio is masculine and apparently ignore the fact that ebria seems to be a feminine adjective:
"to Marcius always drunk [dem Marcius immer vollauf]"
(P. 474, Beschreibung der Stadt Rom, Ernst Zacharias Platner )
There are a lot of mistakes an illiterate vandal might make. But gender discord was probably not among them. And there is no attested name Marcio in the nominative.
A Proto-Romance Inscription?
I assert that the changes from Latin to Romance I described above had already happened in common speech during the Classical Period. If that is so, then the better understanding of this inscription is that it was carved by a somewhat illiterate person, who carved, not formal Latin, but the language he or she actually spoke.
The name Marcius in common speech had already become Marcio. It was probably still written Marcius well into the post-Classical period, much as people with the name Stephen may pronounce it with a 'v' but write it with the 'ph' pronounced as an 'f' in Greek.
Semper was still intact, only later to evolve into, for example siempre in Spanish and sempre in Italian.
Ebria is not an adjective at all, but rather the verb ebriat (ebrio, ebriare 'to make drunk' in the 3rd person singular) that has lost its final 't'.
The verb ebriare is only attested twice, both times in the works of 5th century writer Macrobius. One time it is literal:
Mulieres, inquit, raro ebriantur, crebro senes.
Women, he says, rarely are made drunk, old men often (are made drunk).
(Macrobius, Saturnalia 7.6.16)
The other instance is figurative:
ea recognoscimus quae naturaliter noveramus prius quam materialis influxio in corpus venientes animas ebriaret.
We relearn what we knew naturally before the influx of matter intoxicated souls coming into the body.
(Macrobius, Commentary on the Dream of Scipio 1.12.10)
The graffito is a figurative use:
Marcio Semper Ebria
[Marcius Semper Ebriat]
Marcius always inebriates.
I speculate that whoever Marcius was, he made women (and men?) drunk with love for him. The graffito is somewhat analogous to the Pompeian graffito calling the gladiator Celadus the "suspirum et decus puellarum" (heart-throb of the girls).
My reading of the inscription solves the gender discord problems, as well as addressing the absence of the verb est (is), which is just assumed in all other translations.
If my interpretation is correct, this inscription is the oldest extant evidence of precisely the linguistic evolution we actually observe when the Romance language are represented in writing centuries later.
It's a bargain at 0.99 cents on Kindle (or affordably priced at $11.90 on paperback).
You'll travel back to ancient Rome on a harrowing mission to save the modern world. It's the adventure of four lifetimes.