Some historians consider the hypothesis that the language spoken in Portugal in the Middle Ages and in the early Modern Age was close to Brazilian Portuguese. The modern European Portuguese was created in the late Modern Age, when Portuguese people started to speak in a fancier way. Of course, there is no recording of people speaking centuries ago. But there are some clues. The poems of Luís de Camões (1524-1580) have a much better melody when spoken in Brazilian Portuguese than in modern European Portuguese.
There is also this hypothesis concerning English. The language spoken in England in the Middle Ages and in the early Modern Age might be close to the American English.
I have seen it claimed that the Southern US accent is a more direct descendant of the way 17th century English people talked than the RP accent or most other British accents.