Genetics, family history and population history
This area of research looks at the DNA from skeletons to better understand whether there was population change in or since medieval times, and how people interacted with disease. To do this, we compare the DNA of medieval skeletons against the DNA of other medieval people, people from other time periods, and modern people. How much has the population of Cambridge changed? Can changes be ascribed to particular historic events or processes, such as the Black Death, and if so, how?
To do this, we extract DNA out of teeth and bones from archaeological skeletons, and analyse it bioinformatically. Once we have sequenced the DNA we can compare it to similar data from ancient and modern individuals. We can then find out whether the medieval people were local or if they came from a population with a different genetic composition somewhere else. We also try to see how genetically similar people in medieval Cambridge were to each other and whether they may have been related, and whether the population has higher or lower frequencies of different versions of genes – for instance, genes for eye and hair colour, genes related to metabolism, and genes that protect against disease.
DNA doesn't always preserve very well; it can get fragmented and damaged over time. DNA from bacteria and soil fungi invades bones, and often only a tiny fraction of the DNA extracted from archaeological skeletons is actually human. It is difficult sometimes to accurately work out from which organism and which part of the genome a particular fragment of DNA that we read comes from. Nevertheless, methods in ancient DNA research are rapidly developing and some results are emerging. (We also learn about the DNA of the pathogens that infected humans – see Palaeopathology).
DNA results: human genetics
As other researchers have noted before, the most striking population change in this region happened between the Neolithic and the Iron Age. Our samples show strong change between the Neolithic and the Bronze Age, perhaps reflecting an influx of people or genetic components from the continent. After this, the genetic composition of the area remained relatively stable until the Anglo Saxon period. New genetic components appear, perhaps reflecting new mixture with continental populations, which have been estimated to account for about one third of genetic ancestry in the present-day East Anglian population. In Cambridgeshire, we observe notable genetic differences between the Early Anglo Saxon site and the Middle/Late Anglo Saxon sites, possibly representing groups connected with multiple migrations from across the Channel. From Anglo Saxon times onward, the genetic composition of the area remains stable through medieval and post-medieval times. In comparison with other areas of Britain such as Western England, it always shows close relations with the Low Countries – perhaps reflecting ongoing mobility between East Anglia and areas such as Flanders.
In terms of kinship, cemeteries from the Anglo Saxon, Roman and prehistoric periods periods often reveal close family links among people buried there. Since these are usually quite small sites where the people from small, local communities were buried, it is not surprising to find family relationships. In the medieval period, we detected few or no close relatives within the same burial site. This is not a surprise for the Hospital of St. John, which was intended for the urban poor who had no relatives to turn to; nor for the Augustinian Friary, which mostly served a monastic order. The parish burial sites (All Saints by the Castle, Cherry Hinton, Clopton) reveal some kinship links among burials; for instance, we found several two- or three- generation linkages between people buried at All Saints. However, these kinship links are much fewer proportionally than in earlier sites; this makes sense, since they represent larger and more diverse populations.
Results from principal component analysis, showing the separation in genetic ancestry between individuals from the Neolithic and after the Neolithic period and their relation to modern populations in western Eurasia. Distances in the plot approximate the genetic divergence between the genomes.
Within the medieval period, we found little difference in the genetic composition of the thirteenth, fourteenth and fifteenth century samples, or before and after the Black Death pandemic. Despite its devastating consequences, it is unlikely that the pandemic left long-lasting marks on the genome. To make things more complicated, later plague outbreaks in the fifteenth to seventeenth centuries could have had a different direction of selection when the social environment, demography and pathogen changed.
We also examined the frequency of about 70 single nucleotide polymorphisms (SNPs), which are specific locations in the genome which are linked to specific traits (in appearance, biological processes such as metabolism, risk or protection factors for diseases, etc.). In line with the continuity of the genetic background on a broad time scale, we found that the frequencies of most genetic variants remained relatively stable. But a few variants with significant changes are related to the risk of diseases such as leprosy and the plague. We cannot distinguish whether the changes result from selective pressure or just random fluctuations. They would affect the immune response in the population, but they do not show a strong signature of selective pressure from epidemics. We also found that people’s appearance changed a lot between the Iron Age/Roman period and the Anglo Saxon period: blue eyes became more common than brown eyes, and the hair shades became lighter (the first red-haired person appeared during the Anglo Saxon period!).