Galician-Portuguese was first spoken in the area bounded in the north and west by the Atlantic Ocean, and by the Douro River in the south, comprising Galicia and northern Portugal, but it was later extended south of the Douro by the Reconquista.
It is the common ancestor of modern Portuguese, Galician, Eonavian and Fala varieties, all of which maintain a very high level of mutual intelligibility. The term "Galician-Portuguese" also designates the subdivision of the modern West Iberian group of Romance languages.
Galician-Portuguese developed in the region of the former Roman province of Gallaecia, from the Vulgar Latin (common Latin) that had been introduced by Roman soldiers, colonists and magistrates during the time of the Roman Empire. Although the process may have been slower than in other regions, the centuries of contact with Vulgar Latin, after a period of bilingualism, completely extinguished the native languages, leading to the evolution of a new variety of Latin with a few Gallaecian features.
Gallaecian and Lusitanian influences were absorbed into the local Vulgar Latin dialect, which can be detected in some Galician-Portuguese words as well as in placenames of Celtic and Iberian origin like Bolso. In general, the more cultivated variety of Latin spoken by the Hispano-Roman elites in Roman Hispania had a peculiar regional accent, referred to as Hispano ore and agrestius pronuntians. The more cultivated variety of Latin coexisted with the popular variety. It is assumed that the Pre-Roman languages spoken by the native people, each used in a different region of Roman Hispania, contributed to the development of several different dialects of Vulgar Latin and that these diverged increasingly over time, eventually evolving into the early Romance Languages of Iberia.
It is believed that by 600, Vulgar Latin was no longer spoken in the Iberian Peninsula. An early form of Galician-Portuguese was already spoken in the Kingdom of the Suebi and by the year 800 Galician-Portuguese had already become the vernacular of northwestern Iberia. The first known phonetic changes in Vulgar Latin, which began the evolution to Galician-Portuguese, took place during the rule of the Germanic groups, the Suebi (411–585) and Visigoths (585–711). And the Galician-Portuguese "inflected infinitive" (or "personal infinitive") and the nasal vowels may have evolved under the influence of local Celtic languages (as in Old French). The nasal vowels would thus be a phonologic characteristic of the Vulgar Latin spoken in Roman Gallaecia, but they are not attested in writing until after the 6th and 7th centuries.