Wednesday, February 28, 2018

A Short History of Demographic Change In Britain

Britain has seen five or more rounds of near total population replacement, in addition to other more modest tweaks to its gene pool (and new cultural eras that had surprisingly little demographic impact).

The most notable less than complete population replacements have been the Anglo-Saxon migration, the Viking migrations, and modern immigration, each of which has been more heavily concentrated in some geographic regions than others. The Normans had little genetic impact outside the British aristocracy.

There is overwhelming evidence of a great Celtic cultural impact, but the demic impact of the Celts was not obviously great. But, there are methodological problems with determining what their demographic impact was on Britain because the Celts would have been genetically and physically quite similar to native Britons. Romans and Punic people meanwhile, definitely had little demographic impact.

1. Pre-Neanderthals Hominin occupation of Britain was intermittent in pre-history.

The first members of the genus Homo in Britain were pre-Neanderthal archaic hominins who had arrived by 814,000 years ago, and were forced out by an ice age about 200,000 years ago, leaving Britain without any members of the genus Homo for the next 100,000 years.
Early pre-Neanderthals inhabited Britain before the last ice age, but were forced south by a previous glaciation about 200,000 year ago. When the climate warmed up again between 130,000 and 110,000 years ago, they couldn't get back because, similar to today, the Channel sea-level was raised, blocking their path.
Homo heidelbergensis arrived in Britain around 500,000 years ago and used Acheulean flint tools, but then left during a severe ice age from 478,000 years ago to 424,000 years ago.  Pre-Neanderthal hominins were then present intermittently for the next 200,000 years or so.

2. Neanderthals Starting around 100,000 years ago, Neanderthals arrived in Britain. Early Neanderthals or "pre-Neanderthals" were also present from 230,000 years ago to around 200,000 years ago.

The Neanderthals were probably weakened by climate factors partially related to a string of extreme volcanic eruptions in Europe and possibly also by improving anatomically modern human capabilities both cultural and individual. Still, Neanderthals in Europe kept anatomically modern humans at bay for about 32,000 years after modern humans expanded to West Asia and South Asia, and at least 82,000 years after modern humans first left Africa.

Neanderthals persisted in Jersey (and probably also Doggerland) until about 42,000 years ago.

3. Cro-Magnon Neanderthals were completely replaced in Britain by the first wave on anatomically modern humans in Europe, the Cro-Magnon, who first appeared in Britain around 43,000 years ago. There was probably some admixture at that time, but most Neanderthal admixture in the Cro-Magnon probably pre-dated their arrival in Britain rather than occurring in situ. Neanderthals and Cro-Magnon typically overlapped in any one place for about 1,000 years at most before Neanderthals were replaced.

Britain was not a refugium during the last big ice age, however. Its entire Cro-Magnon population was eliminated in the run up to the Last Glacial Maximum as glaciers covered Britain. The Last Glacial Maximum was 20,000 years ago.

Very few relic populations in refugia during the ice age that including the Last Glacial Maximum had British Cro-Magnon migrants among them. These refugia had an effective male population as small as 30 men. Thus, any admixture between Neanderthals and Cro-Magnon that took place in Britain was eliminated in the last great ice age.

4. Mesolithic Western Hunter-Gatherers Then, Britain was repopulated in the Mesolithic era by Western Hunter-Gathers like Cheddar Man (from about 9500 years ago) over a period somewhere in the range of about 14,500 to 6,000 years ago. 

Despite coming from a much more restricted gene pool, Western Hunter-Gathers were actually pretty similar in terms of Y-DNA and mtDNA, and even, to a lesser extent, autosomal genetics, to Cro-Magnon population, whose relic populations in refugia like the Franco-Cantrabrian refuge and Italy repopulated Europe from a very restricted founding population after the Last Glacial Maximum, even though direct continuity was absent in Britain.

Also notably, around 6200 BCE, a megatsunami driven by runoff from melting glaciers suddenly flooded an inhabited land bridge between Britain and continental Europe called Doggerland (which had been shrinking with rising sea levels since 9000 BCE). The Dogger Bank, however, an upland area of Doggerland, remained an island until at least 5000 BCE.

5. Neolithic Farmers Around 6000 years ago (about 4000 BCE), in the Neolithic Revolution in Britain, early European Farmers, probably more Mediterranean Cardial Pottery folk than LBK farmers with more direct links to Anatolia, largely (90%+) replaced Western Hunter-Gatherers. Farming supports more than an order of magnitude greater population density than a hunter-gatherer lifestyle does, and as an isolated island with relatively little megafauna after the Last Glacial Maximum, Britain was probably not the most abundant place for Western Hunter-Gatherers to try to survive in, so the hunter-gatherer to farmer population surge may have been particularly great.

One key subtlety genetically is that early European Farmers were themselves a mix of European hunter-gatherers and Fertile Crescent farmers, and the hunter-gatherers that the original farmers admixed with were only modestly genetically drifted from Western hunter-gatherers. So, crude ancestry estimates overestimate the extent to which British or Western European hunter-gatherers admixed with local hunter-gatherer populations.

This said, on the European continent, there was significant enrichment of local hunter-gather admixture following first wave Neolithic collapse before Bell Beaker and Corded Ware people emerged onto the European scene.

But, the first wave Neolithic farming civilization collapsed, in my view, most likely as a result of crop failures from some combination of soil exhaustion due to poor farming practices of first wave Neolithic farmers (repeated everywhere first wave Neolithic farmers went) and climate, returning Britain to a predominantly hunter-gatherer-herder society with a much lower population density. There is some evidence a wave of plague that swept Europe at this time as well, but disease is often an effect rather than a cause of famine.

6. Bell Beaker People The semi-hunter-gatherer/herder ancestors of the first wave Neolithic Britons were then almost entirely replaced or overwhelmed demographically (93%+), in perhaps a few centuries or less (in a period starting around 2400 BCE and ending before 2000 BCE, see also here suggesting 2500 BCE to 2100 BCE), by the Bell Beaker people who brought a more sophisticated Copper/Bronze age farming package with them that endured.

The Bell Beaker people who colonized Britain were genetically very similar to the Bell Beaker people of Continental Europe, with significant steppe ancestry and regionally specific Y-DNA R1b clades and mtDNA H clades, rather than like the Iberian Bell Beaker people who were genetically more similar to the Neolithic people of that region with only a sprinkling of the steppe genetic ancestry that is predominant in other European continental Bell Beaker people. This was a quite surprising discovery, because in terms of ceramics and other physical relics, the Bell Beaker culture appears to have originating in Iberia, and in particular in Portugal, which is the least like European continental Bell Beaker people genetically.

Also, despite the heavy rate of population change associated with the appearance of the Bell Beaker people, both in Britain and in Europe, the change must not have been complete, because there was, for example, a continuity of architectural styles and religious practices to some extent, between the descendants of the first wave Neolithic people and the Bell Beaker people. For example, Stone Henge was built by the Neolithic people, but their Bell Beaker successors continued to use it.

The Bell Beaker colonization of Britain was its last nearly complete population replacement through the present. Today's British people are on average perhaps 80% identical to the Bell Beaker people genetically (in terms of ancestry percentages, which ignore the large portion of the genome in which all humans and all Europeans which are basically fixed; in a raw, model independent genetic overlap the percentage similarity is much, much higher) with Germanic admixture making up the balance. The successive Neolithic and Bell Beaker waves of replacement left only about 1% or less of the British gene pool attributable to the Mesolithic Western Hunter-Gatherers of Britain. Previous estimates from the early 2000s that concluded that most British ancestry was traceable to the Mesolithic era, or to the Neolithic revolution in Britain, have been revealed by ancient DNA evidence to be incorrect.

There was significant population exchange and trade between Bell Beaker Britain and Bell Beaker areas in continental Western Europe for pretty much the entire Bell Beaker area until Bronze Age collapse (ca. 1200 BCE).

In terms of physical traces of culture ,and probably language as well, based upon the time depth of the Celtic languages, the Bell Beaker derived cultures collapse, and are replaced by recognizably Celtic cultures, within a few centuries of Bronze Age collapse, which was a climate driven collapse of many cultures over a geographic range from Britain to Egypt to the Indus River Valley (at least).

The language(s) spoken by the Bell Beaker people of Britain remains an open issue that may never be definitively resolved. The oldest historically attested linguistic layer in Britain is Celtic with discernible impacts from later populations that were historically present in Britain that are discussed below.

7. Celts, Romans and Punic People Subsequently, the Celts (coinciding with the British Iron Age ca. 800 BCE), Romans (43 CE to 410 CE plus an earlier invasion in 55-54 BCE), and heavy maritime trade handled by Punic people from Northwest Africa (Iron Age ca. 1100 BCE to early Middle Ages ca. 800 CE) arrived in Britain. Each of these peoples had a powerful toponym impact and a significant cultural impact on Britain, but none of them had much of a long term population genetic impact. The demographic impact of the Celts was fairly minor (although difficult to determine by genetic means because continental and British Bell Beakers people were genetically very similar and exchanged people for the entire Bell Beaker era) and the Romans and Punic traders had only a negligible long term demographic impact on Britain. 

Only about 4% of the population of Roman Britain was Roman, and some of those would have been foreign soldiers on a short term tour of duty, rather than permanent settlers.
Roman Britain had an estimated population between 2.8 million and 3 million people at the end of the second century. At the end of the fourth century, it had an estimated population of 3.6 million people, of whom 125,000 consisted of the Roman army and their families and dependents. 
The urban population of Roman Britain was about 240,000 people at the end of the fourth century. The capital city of Londinium is estimated to have had a population of about 60,000 people. Londinium was an ethnically diverse city with inhabitants from across the Roman Empire, including natives of Britannia, continental Europe, the Middle East, and North Africa. There was also cultural diversity in other Roman-British towns, which were sustained by considerable migration, both within Britannia and from other Roman territories, including North Africa, Roman Syria, the Eastern Mediterranean, and continental Europe.
The limited Roman demic impact coincides with the shallowness of its cultural impact. For example, while Christianity arrived in Britain first from the Romans, as did Roman law, both nearly completely died out, with Christianity re-emerging from Irish and continental European missionaries, and Roman legal concepts returning only with the Norman elites.

The permanent Punic population appears to have been confined to expatriot neighborhoods in some select port cities.

The certainty with which we can say that the demographic impact of the Celts was small, however, is fairly weak. As noted above, the significant population exchange between Britain and Western Europe in the Bell Beaker era means that the population genetic makeup of the Celts may have been very similar to that of the British, to the point where limited ancient DNA samples, and population genetic studies of modern samples three thousand years removed, may not be able to discern the differences between the populations as distinct ancestral groups. If invading Celts were genetically very similar to Bell Beaker era Britons who were invaded, even a major population shift might have been almost invisible.

In the past, I have estimated Celtic demographic impact by assuming that the Celtic elite was initially mostly Y-DNA R1a and was a male dominated migration, and then assuming that the percentage of men with Y-DNA R1a (of the Northern European clade) in Western Europe is roughly half the percentage change in population due to Celtic migration. This was supported by ancient Urnfield Y-DNA R1a. Y-DNA R1a rates in men range from 0% to 8% in the British Isles, suggesting an average on the order of 4% which in turn suggests a 2% population turnover with the Celts.

But, the trouble with this hypothesis is that the R1a distribution in the British Isles appears to be a better fit geographically for Angles, Saxon and Viking demographic impacts than it does for Celtic demographic impact. Ireland, Scotland and Wales, which should be higher than average if Celts are the source of Y-DNA R1a against a Bell Beaker R1b source, are actually lower in Y-DNA R1a than England (Ireland is about 1% and Wales is 1%-2%) except on islands where maritime invaders would have had an edge, suggesting that Y-DNA R1a in Britain has a source that is more likely mostly Germanic than Celtic. Likewise, the Y-DNA R1a frequency in France which was historically Celtic before Romance languages replaced Celtic languages (excluding French Basque for which the percentage is 0%) is only about 2%, again disfavoring a hypothesis that even Celt elites had Y-DNA R1a.

Ancient dental remains also support a primarily cultural diffusion model of Celtic culture, rather than a mass migration, although if the populations are genetically and physically similar, that degeneracy may also be hard to resolve,

This doesn't detract from, however, and indeed reinforces, the possibility that degeneracy in population genetic makeup between the Bell Beaker people and the Celts could cause us to underestimate to the extent to which the Celtic cultural transition in Britain involved a mass migration of people from Europe to Britain.

It is also notable that: Celtic parts of the U.K. (presumably Scotland, Wales, Northern Ireland), have more steppe ancestry than Southern and Eastern England proper, presumably because Norman invaders ca. 1066 CE had less steppe ancestry than the pre-existing residents of the U.K. The modern residents of England proper also have less steppe ancestry than Anglo-Saxon ancient DNA. Keep in mind, however, that this is a subtle difference that is discernible only because of a huge sample size (N=113,851) in a generally very homogeneous population.

The hypothesis that the Norman invaders had less steppe ancestry is consistent with the evidence that the Bell Beaker people, who had significant steppe ancestry, almost fully replaced the population in Britain but less fully replaced populations in Continental Europe, the balance of whom would have been predominantly early European farmers with little or no steppe ancestry, who were most similar to modern Sardinians and Basque people.

8. Anglo-Saxons, Vikings, Normans and Jews. There were subsequent waves of Angles and Saxons (early Middle Ages after the fall of Rome ca. 400 CE) who are the source of the Germanic Old English language, Vikings (late first millennium in the middle Middle Ages ca. 865 CE or 800 CE-950 CE) with a lasting genetic impact mostly limited to the Orkney Islands and a few other coastal and island localities in Northern Scotland and between Britain and Ireland, and Normans (in the late Middle Ages, conventionally 1066 CE) whose Norman French influences caused the transition from Old English to Middle English. 

Traces of these migrations are visible in modern British regional population genetics, despite the fact that Britain is actually very homogeneous in terms of population genetics, due to the large sample sizes and precision genetic sampling of individuals whose genomes were sampled in the latest genetic surveys of the British people, and despite the fact that Angles, Saxons, Vikings and Normans are all genetically only subtly different from the pre-existing mostly Bell Beaker and Celtic derived populations of Britain.

Anglo-Saxon demic impact may have been as high as 38% in Eastern England, although it declines with distance from that epicenter (other regional estimates are in the 10%-40% range with considerable regional variation).

As noted here
A study into the Norwegian Viking ancestry of British people found that there is evidence of particular concentrations in several areas; especially in Lowland and Eastern Scotland - and the North Sea islands Shetland and Orkney, Western Scotland and the Western Isles including Skye in Scotland, Anglesey in Wales, the Isle of Man and the Wirral, Mid-Cheshire, West Lancashire and Cumbria in England.
The percentage of modern British ancestry attributable to the Normans is more slippery to determine, and although it is not zero, it is closer in order of magnitude to the Viking and Roman contributions to British population genetics which are small. An article in the New York Times from 2007 references some historical information regarding this issue:
Dr. Oppenheimer . . . cites figures from the archaeologist Heinrich Haerke that the Anglo-Saxon invasions that began in the fourth century A.D. added about 250,000 people to a British population of one to two million, an estimate Dr. Oppenheimer notes is larger than his but considerably less than the substantial replacement of the English population assumed by others. The Norman invasion of 1066 A.D. brought not many more than 10,000 people, according to Dr. Haerke.
This would suggest a Norman genetic impact of about 0.5% or less on the British gene pool, which would make it almost impossible to discern outside the British aristocracy, many of whom hold hereditary titles traceable to those 10,000 or so Normans.

Britain does not have anything approaching the endogamous caste features in its gene pool that India does, but there are subtle enhancements of Norman ancestry in the British upper classes and there are some very subtle but traceable genetic connections between the modern British upper classes/lower classes and their ancestors many generations earlier, with corresponding traces in surnames.

The earliest evidence of Jews in England is from shortly after the Norman invasion in 1070 CE. Also notable from a population genetic perspective is the fact that in the post-Norman era in Britain (as a result of attitudes associated with Norman involvement in the Crusades) there was, in 1290 CE, an expulsion of every Jew from England and Gascony (4,000-16,000 people), except 128 Jewish converts to Christianity in a single communal building in London (following multiple prior massacres), by the Normans, which left the entire region without any Jews for the next 365 years (i.e. until 1655 CE), more than a century after the Church of England broke from the Roman Catholic Church in 1534 CE so the King could get a divorce.

9. Irish Travelers.  "Travelers" in Ireland, who emerged in the modern era (ca. 1650 CE), while culturally similar to European Roma with South Asian ancestral roots, are genetically pretty much indistinguishable from other native of Ireland. They make up about 0.1% of the population of the U.K.

10. Modern Era Migration. Modern era immigrants from the British empire began to arrive starting around the 16th century CE and has had a modest demographic impact on the British gene pool, at least regionally, but has never come remotely close to total replacement. 

Frequently, researchers try to screen out modern immigration when characterizing a country's gene pool, but in Britain this is more problematic than in most places, because Britain's maritime capability has been a global population draw (from Europe to India, Indonesia, Africa and China) for people from its empire for five or six hundred years, which is long enough to make these introgressions part of what is the indigenous Britain gene pool at this time.

Modern global migration have probably contributed more to the British gene pool, for example, than Western hunter-gatherers like Cheddar man have, and also more than the Romans or the Vikings or the Punic people. The impact of modern migration in Britain is closer to the population genetic impact of Anglo-Saxon migration to Britain in its magnitude and timing.

For example, about 14% of the current population of the U.K. is foreign born, and that percentage has never been less than 4% in the post-World War II period. About 13% of the population of the U.K. is non-white. About 7% of the population of the U.K. is Muslim, Hindu, Sikh or Buddhist. The largest share of the foreign born population is South Asian, followed by Chinese. About two-thirds of foreign born citizens of the U.K. have "Asian" ancestry and about a third have African ancestry (often Afro-Caribbean). These are rapidly growing sectors of the population relative to the native born population (mostly due to immigration). Several percent of people in the U.K. are native born and are not of British or Irish ancestry, roughly in accord with the historical 4% of the population was the foreign born in earlier times.

Modern migrants have settled mostly in Greater London (which has about 47% of the population that is white from the U.K. or Ireland, and about 60% white of any kind) and a few other major British cities.

UPDATE March 12, 2018:

A chart showing the population of the regions of the British Isles since the Norman Invasion:

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