They didn't. At least not genetically.
You must not trust any contemporary account that says the Angles went around slaughtering the Britons. The archeology presents no
evidence of widespread violence against the Romano-Britons in the
century that the Angles were supposed to be invading. No slash marks on
bones, no burned houses, nada. It appears there was no Demographic Replacement
through massive violence. Had there been such a thing, it would have
been far, far less likely that Cheddar Man’s DNA could closely match
that of a man in a nearby village. The DNA that prevails in Britain is about what it has ever been.
I imagine that the
Angles and Saxons, coming over as a
relatively small number of people, were able to take advantage of
the fact that a massive plague hit the island in 664. The newcomers
were not yet crowded into towns and cities while the Romano-Britons had not very much dispersed from the urban centers, so more of the Romano-Britons were lost, and I imagine that the
Anglo-Saxons had the chance to move into a lot of key leadership
positions that had been left vacant, thus to exert a greater influence
on language and culture than they had before the plague.
Then
again, the Danes didn’t need a big old plague to get the people of the
Orkneys to start speaking Old Norse. They just had to be the people who
dominated the trade routes. The Orkneys started to speak Norse
because that became the language of trade. And most of the world knows a little
English now because Americans dominated international trade in the 1950s
and the Internet in the 1990s. If the Anglo-Saxons were in a key
position to control trade then their language would have become
necessary for basic commerce.
Yet it was not simply their
language that predominated, but their culture, for by the 700s the
island was definitely not following a Roman model of governance. It may
be the great plague of 664 that tipped the scales towards the Anglo-Saxons. Or maybe it was a violent invasion that somehow involved no violence of the sort that would leave an archaeological record, as if the Angles slaughtered all the Romano-Britons by smothering them with pillows. We know, at the very least, that these people were willing to raid the Gaulish coast, for they are mentioned in that role within 3rd-century Roman records. That makes it easier to assume violence, if we know they replaced both language and custom on the isle.
And yet, proven violence on the part of the British and French towards West Africa did NOT have the same effect that the Angles had upon the Brits. You go to Nigeria and they all speak English, sure, but they're all black. You go to Cote d'ivoire and they're all black and they all speak French. Somehow the colonial administrations of Britain and France did not result in a measureable amount of replacing the natives.
I think we underestimate the level of effort it takes to completely replace the natives of an area, much as we underestimate the sheer amount of force it takes to blow apart a planet. There is such a thing as inertia, in both cases, and the efforts of the Chinese to replace Tibetians have been going slowly for years, and their efforts to supress the Uighyrs winds up replying largely on local Uighyrs who are loyal. If the Chinese were to ease up on the pressure in either case, the larger number of natives would likely cause the newcomers to go native quickly. Demographic replacement doesn't work very well or very fast unless the natives are all killed, and that only comes with war or plagues. The United States is as White as it is because of both, but mostly because of the latter, over the course of hundreds of years.
The Anglo-Saxons had the plague on their side for about 20 years in the 7th century, but that was only 20 years, and if they didn't go around slaughtering the Brits then -- how did they come to turn Britania into England?
But that raises the question of how the Normans failed to have the same results as the Angles. We do not call the island "Normandonia" or anything of the sort. It's still Britain, the biggest part of it is stilled called England, who still speak English, a language heavily influenced by Norman french but not replaced, unlike what Old English did to the native Brythonic. Why did the Normans have less of a cultural impact than the Angles? Why is it that half our words are still direct from the Old English, but nearly none of them come from the old Brythonic?
The Norman administration, especially in the years of King William The Conqueror, may have been more violent towards the natives than the Angles had done, for there was his Harrying of the North, whereby he burnt many Northumbrian settlements to quell their rebellion, and there were the fights against the Kentish people, who held out against him for a while. And yet this violence did not get rid of all the English, the way the Romano-Britons seem to have disappeared under less duress of violence.
Maybe it was the great plague of 664 that made the difference, or maybe it was a difference in who controlled the daily
affairs of the trade routes. Maybe the Angles were more resilient in
general for being relatively free, compared to the Britons whose warrior
culture had been erased by the Romans, and whose standing army up and
left once the empire started to fall apart.
Or maybe it was a matter of the Angles being, oh, just a little bit racist
about letting people have power and influence. In the initial period of
their dominance anyone in their lands had to claim Germanic descent in
order to receive freedom at law, access to patronage, and privilege to
bear arms. They were all about claiming descent. You could make yourself
sound much cooler by claiming descent from Wotan, and in the early
years a lot of their warrior leaders did. So any Briton in Angle-land
would have had significant incentives to make everyone else forget that
they were Britons. Yeah, I’m a Sa – I mean I’m an Angle, my
great-grandfather was uh…Hengist. Your majesty.
I do not recall
the Norman French invaders imposing the same ethnic requirements for
patronage, despite what Walter Scott would have you believe. While they
required French to be spoken at court, and this alone put significant
pressure on the language to change it from old to Middle English, they
did not coerce the native population to claim ancient French descent in
order to gain influence. As far as I can tell the Normans didn’t set up a
situation where you had to pretend you were a Norman in order to access
patronage and hold a weapon.
It would have been absurd to do
such a thing, anyhow – the Normans knew they were conquerors on the
land, they knew exactly how long they’d been there, the guy who brought
them over was sitting right over there, they knew exactly when Normandy
had become Franco-Danish and who had conquered it for them and when, and
if there had suddenly been a number of people claiming to be Norman far
beyond what could be expected, it would have been very suspicious. No.
There was no Murky Mythic Norman Past that people could use to fabricate
credentials like the Britons had been able to do with the Angles. There
were, instead, records. Administrative records. They even did a survey
of the whole kingdom and put it in a book. Nobody was going to get away
with claiming to be a Norman.
The whole business of Brits claiming
to be Angles had been based on a warrior culture where you could become
a great leader if you were brave and heroic enough. Vikings carried
that notion forward far beyond the point where Angles left it behind,
but by the time the Normans invaded England that ideal was no longer
current among them. Social status was fixed, nobody could rise and
nobody could fall, so you couldn’t sneak your way into power simply by
saying you were a Norman. They didn’t display that sort of
ethnocentrism.
What kind they DID have can be seen in the fact
that nearly every important government and ecclesiastical position in
the land was taken from English people and given to Normans. They didn’t
want any English leaders gathering any remaining resistance.
And
yet, unlike the Angles, they never demanded that their peasants should
Be More Norman, nor tried to replace them with any Norman peasants. The
Harrying of the North was about as genocidal as William ever got, and
that was one campaign, and probably not half as bad as the records hold.
William’s kingdom was based from the start on royal authority over a
conquered people, not upon ethnicity.
In contrast the Angles
were more obviously ethnocentric, if you trust the account of what
happened to the Britons in Exeter. Aethelstan kicked them all out and
banished them to beyond the river Tamar, and that became the border
between a now mostly-Anglo Devon and a Celtic Cornwall. It would not be
surprising if this treatment was similar at a small scale everywhere.
King Aethelred Unraed was willing to openly demand a genocide against
the Danes in England (which effort was a failure), so maybe something
similar happened to the Britons over a longer time period?
On
the other hand, during the period where all these Britons were
supposedly dropping their heritage in the garbage so they could survive,
there was evidence of cultural fusion that belied the idea of
straight-up Apartheid some people propose. For one thing, Angle and
Briton settlements might have initially lived apart, but it only took a
hundred years for them to start intermingling, and thereafter there was
no distinction. For another, many of the names of early Angle kings
betray a distinct British influence, such as Cerdic, the founder of the
royal line of Wessex. It is as if, once the Britons and Angles began to
mingle, people claimed Germanic descent without anyone being especially
picky about the details. If you could speak Anglo-Saxon without any
British inflection, great, you’re in. You’re descended from Hengist? I
bet you’re a great warrior. Have a spear.
And there is another key to the mystery, in terms of the word "Saxon".
There were people called the Saxons, who raided the Gaulish coast along with the Angli. They were the pagans that Charlemagne sought to convert by the sword. These people existed. That's fair enough. But...how much did they exist, in the land of the Angles? There's a place called "Wessex" which probably means 'West Saxons" and there's Sussex and Essex and Middlesex, so it sounds like there were people calling themselves Saxons in the formative years of those kingdoms. But as to that name itself, it might be an exonym, after the famous knife called the Seax...except that these people chose to call themselves that, which is not something one normally does with an Exonym. Supposedly people across the Anglo-Saxon kingdoms would refer to themselves as Saxons or Angles interchangeably, as if it didn't matter much to them after a while.
Alfred the Great may have been the first to actually put the term "Anglo-Saxon" in writing. Or maybe Athelstan, depending on which part of Wikipedia you believe. Either way it wasn't something the common people called themselves. They went by the name of their kingdom, or by Angle, or Saxon -- but the Catholic Church in England preferred to call them Angles, fully intending the angles/angels pun. Saint Bede preferred to call them Angles.
Meanwhile, Gildas over in Wales, whose Welsh name is actually Gweltaz, and who clearly hated the Angles, called them Saxons without ever calling them Angles. He preferred to emphasize their sharp knives, and to link them to the damn pirates that raided Gaul. And so did the rest of his fellow Britons. To this day the Gaelic word for Englishman is "Sassanach" as if things haven't changed at all in 1500 years.
In other words, these Brits were about as ethnocentric as the Angles. So you read in Gildas' work that the dastardly Saxons went around slaughtering the innocent Britons, it's highly likely to be a load of shit. This guy and everyone around him hated the Angles. He wasn't going to write a fair account.
I think what happened on the part of the Romano-Britons, what caused them to be culturally and linguistically erased by the German newcomers, to a greater degree than any demographic on the island since then, is that half the Brits ecided to stay in Angle-land by throwing away their heritage, and the other half booked it to Wales because they wanted to remain pure, and Gildas either thought they had been driven away by violence, or he was deliberately spreading lies because he hated the Angles that much.
How did the Anglo-Saxons replace the Celts? Simple: everyone's a little bit racist!