Behind the paper: mobility and migration in Bronze and Iron Age Britain

WARNING: contains a photo of archaeological human remains.
This article relates to Maddy Bleasdale and Claire-Elise Fischer’s joint first-author poster.

Recently the COMMIOS team, along with many other institutions and authors, published the largest aDNA study to date! We generated genome-wide data for a staggering 793 archaeological individuals (Patterson et al., 2022). But what did we find? And why is it important?

Early European Farmer (EEF) Ancestry

Genetic data has shown that present-day people from England and Wales have more Early European Farmer (EEF) ancestry than people from the Bronze Age – but when did this happen? By analysing hundreds of individuals from across a large time transect we tried to find out! The  skeletons were analysed at the Reich Lab at Harvard Medical School, USA.

The genetic results revealed that the increase in EEF ancestry occurred between the Middle c.1600 – 1200 BC) and Late Bronze Age (c.1200-700 BC), but did not affect all regions to the same extent, as the increase in EEF ancestry was not detected in Scotland. Once this was established, we quantified this component in all samples and were able to determine an average for each period – this is shown in Fig. 1 (graph) on our poster. With these values, we were able to identify individuals who are ‘outliers’, i.e. their genetic signature differs from what would be expected for a given region/period. 

Interestingly, we noticed that many of the so-called genetic ‘outliers’ were found in Kent. Previously published mobility isotope results also showed non-local individuals at several of the sites sampled in the aDNA study (Millard, 2014; Millard & Nowell, 2015). Due to its geographical position and the archaeological data, this area has already been identified as an important contact zone between Britain and the continent. But now we want to look even closer at this region and try to answer: what was the nature of these migrations? And can we see any differences related to age, sex (and possibly gender)?

The gateway to the continent: ‘the eastern route’

Archaeological evidence for Kent as a contact zone dates back to at least the Neolithic period, called ‘the eastern route’ by T. Allen (Allen, 2012). A significant archaeological find that encapsulates this cross-channel trade and exchange, is the Dover boat, which is a magnificent Bronze Age shipwreck which is around 3500 years old. Kent occupied a major position in the bronze trade, linking tin from Cornwall with copper from across the UK and areas of France (e.g.  the Haut-Rhin), and later lead from Wales. Thus, it is argued that an extensive and complex system of intercontinental trade and logistics existed at the end of the Bronze Age, with coastal Kent sitting at the heart of the North European intersection of major trade routes (Allen, 2012)

At a broad-level, archaeological and genetic data has revealed that Kent was an important contact point.

The Dover Boat, https://100objectskent.co.uk/object/bronze-age-boat/

How can genetics contribute to identifying the origin of individuals?

This is what we call phylogeography. In Europe, current populations are made up of 3 principal components: the hunter-gatherer component, the Anatolian component brought by the first farmers during the Neolithic period, and the Steppe component, also called Yamnaya, brought by pastoralists from the Pontic and Caspian Steppes at the end of the Neolithic/beginning of the Bronze Age (Allentoft et al. 2015; Haak et al. 2015; Olalde et al. 2018). What changes, however, is the proportion of each component within the populations. What is true for modern populations is also true for older populations. At least after the Neolithic-Bronze Age transition.

If we look at a PCA (Principal Component Analysis) with modern-day populations and the Bronze Age and/or Iron Age populations, we notice that the ancient individuals fit into the variability of the modern populations. For example, Iron Age samples from France fall into the diversity of modern French populations, and there is a genetic structure linked to geography with, for instance, French samples occupying an intermediate position between Great Britain and Spain (Fischer et al., 2022). On a basic level, this data can allow us to identify ‘outliers’: the Margetts Pit individuals from Kent (Fig. 2 in poster), for example, plot closer to the Bronze Age populations from Germany or France…so that’s the first clue! 

We can then carry out more detailed analyses, such as f3-statistics, which allows us to measure the affinity between two given populations. We can assess whether the Margetts Pit samples have more similarities with populations in England, France, Germany, etc. We can also use qpWave which can tell us if, in relation to a model with X components (e.g. hunter-gatherers, Anatolian farmers and Yamnaya from the Steppes), two populations form a clade*, i.e. if they can be explained by the same model. We can also apply qpAdm, which works on the same principle but, in addition, tells us the percentage of each component! We can also test, for instance, whether the Bronze Age populations of a given region can be used to explain the Iron Age populations of the same region. It was through this kind of analysis that we were able to determine that there was an influx of new populations into England during the Middle to Late Bronze Age. 

We carried out these analyses for individuals from Kent, particularly from Margetts Pit. It turns out that these individuals do not form a clade with the other Bronze Age individuals found in Kent. However, they do form a clade with the Bronze Age individuals from the south of France. And that’s also reflected in the PCA. Therefore, we are beginning to generate several lines of evidence to discuss the geographical origin of these individuals. 

What’s pretty cool is that for some of the outlier individuals, the isotopic data obtained in previous studies have also shown evidence of mobility…

What can the isotope results tell us about mobility?

Previous isotope work using strontium and oxygen has identified a number of ‘non-local’ individuals at archaeological sites in Kent. Most notably, the site of Cliffs End Farm shows a high degree of variation in human oxygen values with many individuals having values that are not consistent with having grown up in Britain. 

Published data from Cliffs End Farm (Millard 2014, Figure 4.11). Strontium isotope values against drinking water oxygen values. Note that individual 3673 on the left hand side of the plot is not in the ‘local’ range for oxygen.

Our poster highlights one individual in particular – ‘bundle burial’ 3673 from Cliffs End Farm. 

This individual was an adult male (~30 years old) who was transported to the site already in a partly decomposed state (i.e. in a ‘bundle’). They were buried on top of a cow’s foot along with an as-yet-unique object consisting of a copper ring and worked piece of bone, which may have been some sort of pendant. 

LEFT: Plan of Cliffs End Farm Late Bronze Age communal grave 3666 containing individual 3673. Image taken from poster by Wessex Archaeology.  RIGHT: photo of individual 3673 ‘bundle burial taken from article by Wessex Archaeology. 

Tooth enamel analysed from this individual gave a strontium value that fits within the range for the UK but their oxygen value is consistent with somewhere north or east of Britain (Alpine areas of Europe, or Scandinavia) (Fig.3 in poster). 

Isotope methods used to look at mobility such as strontium and oxygen are best viewed as exclusion methods (they can tell us where a person isn’t from) rather than an “x marks the spot” approach. This is why it is common to use different isotopes to try and help form an interpretation of an individual’s likely origin. We can also combine isotope results with genetics too! Now we know this individual has a high EEF ancestry value we have proposed it is more likely they came from Europe (potentially an Alpine area).

So, what next?

The COMMIOS team plan to continue to bring together all the available results (archaeological, funerary, genetic, isotopic, osteological, etc.) for Kent to figure out who these individuals were and where they came from. 

We also plan to do some new work too! In addition to applying these methods to sites not yet analysed for ancient DNA or isotopes, we also want to conduct more isotope work at the site of East Kent Access Road (previously only 4 individuals were analysed – see Millard 2014). And plan to conduct a pilot study using lead (Pb) analysis on some isotopic outliers from Cliffs End Farm.  

We are all very excited to see what new results our work will yield!

References

Allen, T. (2012). Bronze, Boats and the Kentish seaboard in prehistory: the role of coastal Kent in a major trans-continental trade route. Archaeologia Cantiana, 1, 1–21.

Allentoft, M. E., Sikora, M., Sjögren, K.-G., Rasmussen, S., Rasmussen, M., Stenderup, J., Damgaard, P. B., Schroeder, H., Ahlström, T., Vinner, L., Malaspinas, A.-S., Margaryan, A., Higham, T., Chivall, D., Lynnerup, N., Harvig, L., Baron, J., Casa, P. D., Dąbrowski, P., … Willerslev, E. (2015). Population genomics of Bronze Age Eurasia. Nature, 522(7555), 167–172.

Fischer, C.-E., Pemonge, M.-H., Ducoussau, I., Arzelier, A., Rivollat, M., Santos, F., Barrand Emam, H., Bertaud, A., Beylier, A., Ciesielski, E., Dedet, B., Desenne, S., Duday, H., Chenal, F., Gailledrat, E., Goepfert, S., Gorgé, O., Gorgues, A., Kuhnle, G., … Pruvost, M. (2022). Origin and mobility of Iron Age Gaulish groups in present-day France revealed through archaeogenomics. iScience, 25(4), 104094.

Haak, W., Lazaridis, I., Patterson, N., Rohland, N., Mallick, S., Llamas, B., Brandt, G., Nordenfelt, S., Harney, E., Stewardson, K., Fu, Q., Mittnik, A., Bánffy, E., Economou, C., Francken, M., Friederich, S., Pena, R. G., Hallgren, F., Khartanovich, V., … Reich, D. (2015). Massive migration from the steppe was a source for Indo-European languages in Europe. Nature, 522(7555), 207–211.

Millard, A. (2014). Isotopic Investigations of Residential Mobility and Diet. In McKinley, J., Leivers, M., Schuster, J., Marshall, P., Barclay, A.J., Stoodley, N (Ed.), Cliffs End Farm Isle of Thanet, Kent. A mortuary and ritual site of the Bronze Age, Iron Age and Anglo-Saxon Period (pp. 133–144). Wessex Archaeology Ltd.

Millard, A., & Nowell, G. (2015). Chapter 13 -Appendix. Isotopic Investigation of Residential Mobility of Individuals from the Zone 12 Middle Iron Age Cemetery. In P. Andrews, Booth P Fitzpatrick, & K. Welsh (Eds.), Digging at the Gateway: Archaeological landscapes of south Thanet. The Archaeology of the East Kent Access (Phase II) Volume 2: The Finds, Environmental and Dating Reports (Vol. 8, pp. 429–432). Oxford Wessex Archaeology.

Patterson, N., Isakov, M., Booth, T., Büster, L., Fischer, C.-E., Olalde, I., Ringbauer, H., Akbari, A., Cheronet, O., Bleasdale, M., Adamski, N., Altena, E., Bernardos, R., Brace, S., Broomandkhoshbacht, N., Callan, K., Candilio, F., Culleton, B., Curtis, E., … Reich, D. (2022). Large-scale migration into Britain during the Middle to Late Bronze Age. Nature, 601(7894), 588–594.

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