The traditional view that the Celtic Britons originally migrated from the continent, mostly across the English Channel, with their languages, culture and genes in the Iron Age has been considerably undermined in recent decades by the contention of many scholars that Celtic languages had instead spread north along the Atlantic seaboard during the Bronze Age, and the results of genetic studies, which show a large continuity between Iron Age and older British populations, suggesting trans-cultural diffusion was also very important in the introduction of the Celtic languages.
The earliest evidence for the Britons and their language in historical sources dates to the Iron Age. After the Roman conquest of Britain in the 1st century, a Romano-British culture emerged, and Latin and British Vulgar Latin coexisted with Brittonic. During and after the Roman era, the Britons lived throughout Britain. Their relationship with the Picts, who lived north of the Firth of Forth, has been the subject of much discussion, though most scholars now accept that the Pictish language was related to Common Brittonic, rather than a separate Celtic language.
With the beginning of Anglo-Saxon settlement and Gaelic Scots in the 5th century, the culture and language of the Britons fragmented and much of their territory was taken over by the Anglo-Saxons and Scots Gaels. The extent to which this cultural and linguistic change was accompanied by wholesale changes in the population is still a matter of discussion. During this period some Britons migrated to mainland Europe and established significant settlements in Brittany (now part of France) as well as Britonia in modern Galicia, Spain. By the 11th century, remaining Brittonic Celtic-speaking populations had split into distinct groups: the Welsh in Wales, the Cornish in Cornwall, the Bretons in Brittany, and the people of the Hen Ogledd ("Old North") in southern Scotland and northern England. Common Brittonic developed into the distinct Brittonic languages: Welsh, Cumbric, Cornish and Breton.
The earliest known reference to the inhabitants of Britain seems to come from 4th century BC records of the voyage of Pytheas, a Greek geographer who made a voyage of exploration around the British Isles between 330 and 320 BC. Although none of his own writings remain, writers during the time of the Roman Empire made much reference to them. Pytheas called the islands collectively αἱ Βρεττανίαι (hai Brettaniai), which has been translated as the Brittanic Isles; he also used the term Pretannike. The peoples of these islands were called the Πρεττανοί (Prettanoi), Priteni, Pritani or Pretani. The group included Ireland, which was referred to as Ierne (Insula sacra "sacred island" as the Greeks interpreted it) "inhabited by the race of Hiberni" (gens hibernorum), and Britain as insula Albionum, "island of the Albions". The term Pritani may have reached Pytheas from the Gauls, who possibly used it as their term for the inhabitants of the islands.
The Anglo-Saxon Chronicle, which was originally compiled by the orders of King Alfred the Great in approximately 890, and subsequently maintained and added to by generations of anonymous scribes until the middle of the 12th century, starts with this sentence: "The island Britain is 800 miles long, and 200 miles broad, and there are in the island five nations: English, Welsh (or British, including the Cornish), Scottish, Pictish, and Latin. The first inhabitants were the Britons, who came from Armenia, and first peopled Britain southward." ("Armenia" is possibly a mistaken transcription of Armorica, an area in northwestern Gaul including modern Brittany.)