The perspective from population genetics
Ancient human DNA is revolutionising our ability to detect human migration that occurred hundreds of years ago. Joscha Gretzinger and Stephan Schiffels explain how these advances have transformed our understanding of the past.
Archaeogenetics – the study of ancient human DNA – has become an important contributor to cross-disciplinary endeavours to understand our past. It can provide illuminating insights into long-vanished human groups – particularly in time periods with no or only few written sources – complementing the information that we can gain from archaeological material and through other research methods like osteological assessments and isotope analysis.
Ancient DNA (aDNA) plays an especially important role, though, in questions of human mobility and migration. When people move, they bring with them their individual genetic profile, which gets preserved in aDNA extracted from their remains. Thereby, population movements undertaken hundreds or even thousands of years ago can be detected as changes in genetic profiles, seen in aDNA sampled through time. In the last decade, archaeogenetic research has shed new light on the spread of farming across Europe during the early Neolithic (c.6,000-4,000 BC), revealing that it was driven by multi- generational waves of people from the Near East, and not (only) by the transmission of ideas and cultural diffusion from that region. Similarly, and more recently, such analyses have demonstrated that the emergence of the Bell Beaker cultural package in Britain around 2500 BC was dominated by such large-scale movements of people from Central Europe that some 90% of the British gene pool was replaced within a few hundred years (see CA 338).
Besides the Beaker phenomenon, Britain in particular has undergone several other well-documented transitions – perhaps most prominently the shift from Common Brittonic and Latin to the Old English language, changes in burial tradition, and the rise of new political formations during the early and middle Anglo-Saxon periods in England. While genetics have been helping to make sense of these events for almost three decades, it was not until 2016 that the first genome-wide aDNA evidence from this period demonstrated clear genetic differences between early and middle Anglo-Saxon individuals on the one hand, and Roman-period and Iron Age individuals on the other. This was a breakthrough in recognising population demographics, but unfortunately the small sample size used in these two studies – comprising only eight early medieval genomes – did not allow for a detailed understanding of the dynamics of mixing and immigration. Nor did their spatial coverage permit a more finely resolved comparison to the archaeological record.
Thanks to the recent research (initially published in Nature) that we are exploring in this special issue, though, that picture has changed dramatically. We have now increased the genomic coverage from the original eight samples to 278, and have increased the number of cemetery sites represented by these to 38, covering most of the south and east coasts of England. We have also filled some critical gaps in the archaeogenetic record for this period, in what today is Germany, the Netherlands, and Denmark, with 142 newly generated ancient genomes from these areas.
MAPPING PAST POPULATIONS
How does this analysis work? We can visualise the genetic profiles of groups, whether ancient or modern, through a technique called Principal Components Analysis. This is a statistical method to lay out genetic data on to a two- dimensional plane, much like a map, where distances match genetic distances between samples. On this genetic map, the genetic profiles of individuals from Britain’s past, spanning from the Bronze Age through to the Roman period, were found to overlap largely with those of many people in present-day Wales, Northern Ireland, and Scotland, at least those with local European descent. In other words, over that period of nearly 3,000 years, from 2500 BC up until the Roman period, the genetic profiles in Britain appear not to have changed that much. This view can be misleading, though – in fact, a recent study by Patterson et al. indicated that, during the late Bronze and Iron Age, there was actually a substantial influx of newcomers from south-western Europe, probably from what today is France, which generated genetic changes in Britain over the course of many centuries – albeit very subtle ones, due to the relative genetic similarity of these incoming groups.
In stark contrast to this relative genetic stability, though, when we examine individuals from the early medieval period we find a surprisingly strong signal of population shift, and a large spread of genetic diversity encompassing the previous Iron Age groups all the way to contemporary groups from the continental North Sea zone. By differences in ‘genetic profiles’, we mean subtle differences in how common genetic variants segregate in different populations. Indeed, these differences are so subtle that they require hundreds of thousands of genetic markers (called ‘single nucleotide polymorphisms’, or SNPs), and thousands of present-day individuals, to become visible.
At this resolution, classification into clusters becomes statistically possible, and Principal Components Analysis – as well as the more quantitative modelling that we will describe below – are able to zoom in on the only 0.9% of genetic variation that indeed correlates with historical and geographic patterns. This striking shift of genetic profiles in early medieval Britain can only be explained by an unprecedented increase in mobility of groups migrating across the North Sea, at a larger scale than seen in the entire preceding 3,000 years (since the Bell Beaker transition).
MODELLING MIGRATION
In order to understand this shift more quantitatively, we created a model of mixture between two distinct groups – one named CNE (for Continental North European) and the other WBI (Western Britain and Ireland) – which correspond to the cluster overlapping with the North Sea countries on the continent, and the cluster of pre-medieval samples respectively. We then used this admixture model to tease apart the genetic profiles of every single sample in both the newly and previously published dataset. This process revealed that CNE ancestry was present in Britain before the early medieval period, but only sporadically – 1% during the Bronze/ Iron Age, rising to 15% during the Romano-British period – followed by a dramatic spike up to 76% during the early and middle Anglo-Saxon periods.
in central and northern England show exclusively CNE ancestry, which implies that the ancestors of those individuals originated directly from the above-identified areas and did not admix with other populations on their way across the North Sea. In contrast, in southern England, especially Kent, many individuals exhibit additional French-related ancestry. We cannot rule out that this admixture between CNE and French-related ancestry occurred already on the Continent, potentially in a contact zone between both ancestries, for example in what is now the southern Netherlands, Belgium, western Germany, or northern France.
Beyond this large change in average genetic ancestry, our research has also revealed detailed insights into how this incoming ancestry was distributed across different sites. Many cemeteries that we sampled reveal major amounts of CNE ancestry, sometimes involving many unadmixed direct migrants of CNE ancestry, sometimes with many mixed individuals of both ancestries. Within many sites we find individuals with zero CNE ancestry too, as well as many people with mixed ancestry.
By analysing these patterns, we can see that ancestry played a key and complex role in how people were buried. For example, taking all early Anglo-Saxon cemeteries we surveyed together, women with immigrant ancestry were more likely to be buried with grave goods than women with local ancestry. However, wherever both ancestries were present, people from both groups were buried together, suggesting a high degree of interaction despite any differing funerary customs. In some cases, reconstructed family trees have even allowed us to identify locals and newcomers producing mixed- ancestry children.
Looking at the strong change in genetic ancestry seen here, the obvious question is: where did it come from? Our additional samples – taken from areas in what is now northern Germany, the Netherlands, and extra sites in England – combined with already published samples from these locations, provide the means to study this in detail. To this end, we focused in particular on samples from England that contained exclusively continental European ancestry without admixture from the local British gene pool, and compared these to various groups from the Continent. We found that the greatest similarity is seen in a region spanning Friesland (present-day Netherlands), Niedersachsen and Schleswig-Holstein (present-day Germany), and modern Denmark up to the southern tip of Sweden. Strikingly, all of these areas had a remarkably homogeneous genetic profile during the period that we are studying, making all three probable source regions for the migration process into early medieval England.
wear full-body suits, gloves, and other protective clothing – but in contrast to typical laboratories with security status, this is not about protecting staff from the samples, but vice versa, in order to minimise contamination of samples with modern DNA.
This is an extract of an article that appeared in CA 392. Read on in the magazine or on our new website, The Past (click here to subscribe), which details of all the content of the magazine. At The Past you will be able to read each article in full as well as the content of our other magazines, Current World Archaeology, Minerva, and Military History Matters
What the aDNA evidence indicates is that the -traditional- view of the Anglo-Saxon migrations to post-Roman England are precisely accurate; a mass migration from across the North Sea, replacing or partially replacing the Romano-Britons.
The contortions of recent generations, attempting to minimize the migration, are thereby swept aside. They always looked fishy to me.
This is precisely what the linguistic evidence would indicate, too. There are about 15-20 Brythonic loanwords in Old English, even the Wessex dialect. By way of comparison, modern English has around 900 Old Norse loanwords.
Nor is this likely to have been a happy, peaceful cultural mingling with the Romano-Britons welcoming the newcomers with glad cries of “take my farm, I don’t care if I starve or have to flee to Brittany!”. Just as the written sources say, it was a violent invasion accompanied by a folk-migration.
This is much more complicated that it seems here.
Saxon Foederati in the period 200 – 440 CE? Was the Saxon Shore a settlement of Saxons by the Romans?
Punishment of the Romano-British for secession? Did the the Romans exterminate the Pelagians?