[ Who did the first speaker of Italian have to conserve with if everyone else on the peninsula spoke Latin? ]
Italian evolved.. as did English.. actually all languages evolve..
Latin is not spoken nor is Elizabethan English.. Norman.. Saxon.. Jute.. or Frisian..
Populations evolve in their DNA much as the language of a human population evolves.
Small changes accumulate in separate populations until there is no longer a free exchange of information.
It is not a case where there would be a “human” born to a “nonhuman” that is suddenly looking for another “human” to mate with - any more that it would be the case that an “Italian” speaker would be born from a “Latin” speaker and would be looking for someone who understands him or her.
Small changes accumulating over time.
Modern Italian is based on the dialect of Tuscany but some dialects are so different they can be regarded as separate languages.
The language of Sardinia is considered a separate Romance language. Some consider Sicilian to be a different language. Up in the extreme NE part of Italy there is an area where Friulian is spoken, which is generally considered a separate Romance language.
Besides that, there are a few places where Albanian or Croatian is spoken (their ancestors were refugees from the Turks), and in the far north there is an area where German is spoken (the South Tyrol). There are some places near Trieste where Slovenian is spoken. I think there are places near the French border where French is spoken.
Supposedly less than 50% of the population of Italy speaks standard Italian at home.