I recently came across this question and the following answer on Quora. The responder, an assistant professor of linguistics, does a marvellous job explaining what could possibly have happened. Further, he explains the various aspects that sometimes bring about linguistic change.
Answer by Thomas Wier:
Old English is the descendant of the Germanic dialects brought over from what is now the northsea coasts of the Netherlands, Germany and Denmark sometime in the 5th century AD. This so-called Migration Period (sometimes referred to by the German word Völkerwanderung, ‘migration of peoples’) in Britain is one of the least well-understood and the most controversial in the history of Britain because of the general lack of contemporary written historical sources, the complexity and ambiguity of the archaeological record, and the way such arguments are used to espouse specific modern political and national narratives and ideologies.
Let’s break it down into its component questions:
(I) Why do languages die and become replaced by other languages?
(II) What were the specific sociolinguistic conditions at the time of settlement that resulted in language replacement?
I. Language Replacement
To answer this first most basic question I have already discussed elsewhere:Just to rehearse those basic points, language should be seen as a kind of communicative currency the value of which rises and falls depending on seven or so main factors:
(1) Demographics: how many people are already using the language?
(2) Cost-benefit analysis: so-called rational choice factors, including what kind of life-benefits use of the language gets you;
(3) Presence/absence of competing languages;
(4) Geospatial distribution of language variaties;
(5) Domains of language: people use different language, or different varieties of the same language, for particular purposes, and if a variety loses a domain it has to that extent effectively died;
(6) Prestige or symbolic value of language: to what extent do speakers feel a language reflects on them as speakers (positive prestige), and to what extent do speakers feel others’ use of language reflects on other people (negative prestige);
(7) [Marginally, and relevant only for adults:] how closely related is one language to another?
II. Sociolinguistic conditions of Britain in the mid first millennium
When the (possibly mythical) Hengest and Horsa arrived on the shores of Britain in the mid 5th century, they arrived into an already sociolinguistically complex environment. Britain had been a Roman province since the middle of the first century, around four centuries prior, and Roman culture had become part of the cultural furniture that Britons were used to. However, the evidence available to us suggests that Britain was never quite as fully Romanized as some other provinces, such as Gaul. By the late 4th century, even this influence was under threat: Roman military installations were being progressively pulled back: from the Antonine Wall to Hadrian’s Wall in 383, from Hadrian’s Wall to the Southeast in 401, from the southeast in 407, and Roman magistrates were finally expelled from British cities in 409.
Germanic raiding along the coast (and as this map shows, also Celtic raiding from Ireland) had been a problem for more than a century before the final removal of Roman legions. Before the mid 5th century, these raids had been just that: not acts of colonization, but mostly just brigandage. When the Angles, Saxons and Jutes finally arrived, then, somewhere between 441 and 446, they arrived into a partially Romanized conglomeration of Celtic tribes and chieftanships, speaking at least Latin and probably multiple Celtic tongues.
Demographics and Linguistic Ecology
We don’t know exactly how many people were living in Britain in the 5th century — estimates are generally based on projecting back from the more reliable figures available for the 10th-11th centuries — but reasonable speculation has often given a figure of about a 800k to a million people. This represented a serious decline from an earlier estimated 2 to 4 million people in Britain. What caused this decline? One factor was disease: we know that major waves of plagues swept through the Roman Empire during the reign of Marcus Aurelius and in the Plague of Cyprian, suspected to be two different diseases, either small-pox or measles. War and conflict also took their toll.
Whatever the reason, we know that some of the people from this diminished number spoke Latin, especially among the elite. It is nearly certain that wide-segments of the population also still spoke one of several different Celtic languages, including:
- Cumbric, a sparsely attested language of the far north in Lancaster;
- possibly also Pictish
To what extent these varieties were mutually intelligible at this time is very hard to estimate. Celtic scholars believe that these had become distinct languages by about 600, so in the mid 5th century they were well on their way to disunion. In addition to this, there was probably also another more distantly related Celtic language called Pritenic in the far north of Britain, which had become distinct from common Brittonic (the ancestor of Welsh, Cornish, etc) around the first century. Late Roman Britain, in other words, was a linguistically complex place with at least four and possibly more distinct Celtic languages spoken.
Into this mixture of language varieties came various Germanic settlers. Traditionally said to derive from the Angles, Saxons and Jutes, these were probably not distinct, coherent ‘peoples’, but they probably did speak mutually intelligible varieties of West Germanic. (Experts on Germanic historical linguistics agree that the later attested Anglo-Saxon dialects did not result from distinct dialects across the sea, but arose locally.) Like the Celtic peoples in Britain, they had also been under the influence of the Romans on the continent, but far less directly: they still maintained much of their pre-Roman tribal and clan structures and traditions.
We do not have any exact numbers of how many such Germanic settlers arrived in Britain starting around 441-6, but we have reason to believe they were fairly considerable: based on the distribution of 5th century Anglo-Saxon cemeteries, 100 thousand is not an extreme estimate, and they came not all at once, but rather progressively over a period of over a century and a half or more. The implication of this is that the earliest Germanic settlers were in continual contact with their linguistic relatives back home.
Warfare and Ethnic Conflict
According to Gildas, as reflected through Bede, the migration was not a peaceful one: an early agreement that Saxon mercenaries would provide protection in return for food/money broke down, and the Saxons turned on their British hosts. Whatever the truth of this tale, this conflict resulted in a possible depopulation of some parts of eastern Britain and the replacement of that population with West-Germanic speaking settlers. This depopulation could have had several causes:
(i) The Angles, Saxons, etc. actively killed the Britons in what amounted to a genocide;
(ii) The Angles, Saxons, etc. ethnically cleansed the Britons from the parts of eastern Britain that they settled, forcing them westward into Wales, Cornwall and across the sea into Britanny;
(iii) The indigenous population suffered from some other crisis event, such as plague or famine. We know that the Plague of Justinian occurred (ca. 540s) during the later part of the time of migration (ca. 450-600) and reached almost every corner of the former empire. If this plague killed as many people in Britain, percentage-wise, as it did in the Mediterranean Basin (around 30-50%), it would have left the Britons very vulnerable to the further encroachments from Anglo-Saxon settlers who continued to arrive throughout this period.
Taken together, all this results in the following scenario: Anglo-Saxons with a common tongue move into new territory, and wipe out or push out its earlier, already vulnerable inhabitants who spoke no common language (Latin probably being still a foreign tongue for most).
No part of the scenario is without its critics. Archaeologists, especially, are wont to demand evidence of changing burial patterns and destruction layers; these are missing in many parts of Britain, or only ambiguous. Historians have texts which were generally written only centuries afterward by the victors with axes to grind, e.g. the Venerable Bede was an Anglo-Saxon who treated the Britons’ defeats as a just punishment from God.