Biological and social kinship reflected in genomes from medieval Swedish multiple burials.
Multiple burials have been of much interest to archaeologists and social-scientists. During the Middle Ages in Scandinavia such burials were rare but not unheard of. They can contain adults of the same or opposite sexes; however burials including or consisting of children are more common, provoking discussions concerning childhood, the age of transition to adulthood and the social status of children in the past. Hence, sex but also kinship analyses of sites where apparently affiliated individuals were interred together can yield valuable information about mortuary practices, local traditions, beliefs, social and legal status of the deceased, etc. Until recently the lack of reliable sexing (especially in children) and kinship identification tools rendered analyses of similar demographics impossible. However, the latest developments in molecular genetics and bioinformatics provided tools which can now with confidence be applied in testing concrete hypotheses rooted in such features. The proposed project aims at applying those techniques to the analyses of over 100 individuals from Viking Age and medieval multiple burials from Scandinavia in order to characterise mutual relations of persons buried together. Analysis of kinship patters for boys and girls in multiple burials offers the possibility to understand socio-cultural contexts and to avoid unsubstantiated assumptions about biological relationship, making it relevant and of great importance for future research.
Final report
PURPOSE OF THE PROJECT
The original purpose of the study was to investigate biological and social kinship in Swedish multiple burials from Medieval period. Primarily, we were aiming to use genomic analyses to explore biological kinship in relation to burial traditions governing deposition of multiple individuals in same burial – an unusual practice in Christian contexts. Secondly, as many of such burials contain children and subadults for whom sex identification cannot be reliably obtained from osteological material we were hoping to use molecular sexing to identify boys and girls. This information would shed more light on the treatment of children and add to the developing field of childhood archaeology by focusing on finding whether we may witness differential treatment or socialisation processes of boys and girls in the Middle Ages.
A SHORT DESCRIPTION OF HOW IT WAS IMPLEMENTED
We have sampled and analysed a total of 120 individuals from 50 multiple burials in four localities in Sweden (Sigtuna, Fjälkinge, Västerhuas and Lund). The investigated individuals underwent DNA extraction and multiple rounds of whole genome sequencing allowing for accumulation of enough shotgun sequencing data for sex and kinship identification. After validation the obtained data was merged with previously generated Swedish ancient human genomes from the sites of interest to be used for comparative analyses. The dataset has then been analysed using a number of statistical tests. In total we analysed 150 individuals ranging from 0.01X to 21X genome coverage) to identify correlations between different individual arrangements in multiple burials and combine the information with kinship identification to answer two fundamental questions within the study: can we use genetic data from sexing of subadults to learn more about medieval burial traditions and can such data be used to identify differential treatment of boys and girls? And whether kinship patterns can be used to explain or exclude reasons for two or more individuals to occupy the shared burial space after death.
THE PROJECT’S THREE MOST IMPORTANT RESULTS AND CONTRIBUTIONS TO THE INTERNATIONAL RESEARCH FRONT
Most important findings in the study include observation that: a) the majority of co-burials consist of adults and children of the same sex, i.e. the individuals’ sex seems to determine where people were buried in terms of the localization relative to the church. This is a well-known tradition for adult burials which is hereby for the first time confirmed in subadults as well. Thus, it seems that there was no ‘infancy’ period prior to puberty in which boys and girls were treated as ‘gender-less’. This observation largely holds for both sexes but there seem to be a degree of freedom judging from slightly more widespread distribution of girls; b) furthermore this observation is not accompanied by detection of close biological kinship between the co-buried people. In fact, we find (i) very few individuals closely related by blood within burials while the majority of detected biological kin associations is distributed between burials. This suggests that social factors, other than being buried with close kin, played an important part in the practice. We also cannot exclude that there were other family ties than a biological connection between the individuals (e.g. foster children). Alternatively, (ii) this finding could also reflect differences between urban and rural burial traditions as close biological kinship co-burials were observed at a higher frequency in medieval (urban) Sigtuna; c) we note that at least in Västerhus the cemetery is organized around dominant pedigrees, with individuals representing the most extensive kin buried closest to the church building itself. d) Finally, we also confirm that based on research questions answered by this project we corroborate the need to analyse whole sites rather than selecting specific individuals as this creates a sample-specific bias. In other words, analysing whole sites would enable far better pedigree reconstructions with minimal missing data, which could in turn reveal further kin associations between seemingly unrelated individuals interred together or in individual burials.
While the main paper discussing all the upper mentioned findings is to be submitted for publication later this year we note that the questions addressed in the study are of much interest to the general public. This observation is based on the fact that the first published case-study from the project (Krzewinska et al. 2021) was recently highlighted as being in top ten papers downloaded from Journal of Archaeological Science: Reports in 2021.
NEW RESEARCH QUESTIONS GENERATED THROUGH THE PROJECT
Through developed collaborations with other researchers and additional analyses performed on data produced within a scope of this study we found that very few individuals tested were carriers of pathogenic microbes which could be identified as plausible cause of death. This is quite confounding as many of the analysed individuals were children and their premature death should have a detectable cause. It thus may be that we are faced with a form of an “osteological paradox” on a molecular level, or that the newly developed tools for microbial detection are not sensitive enough. Yet another explanation could be too little data or inappropriate input materials, i.e. skeletal elements, for a reliable detection. Sampling bias should be contemplated when molecular pathology is considered. The tests raise more questions not only regarding the actual individuals studied but also present a challenge driving development of novel analytical tools.
Through another collaborative project it has been noted that some of the individuals interred in Västerhus burial ground were carriers of some evidence of North-South population cline structuring which is characteristic of present-day Scandinavia. This is highly important as the observation was made on individual level within a single-family unit which could thus be a direct example of how the North-South genetic variation cline was formed in Sweden. Those findings are preliminary but could possibly serve as starting point for further research focusing on genetic structuring in Scandinavian Peninsula.
THE PROJECT’S INTERNATIONAL DIMENSIONS, SUCH AS CONTACTS AND MATERIAL
We have developed new collaborations both within the presented project by inviting participants but also by participation in ancient genomic papers utilizing kinship as means of understanding past burial traditions (Yaka et al. 2021, Linderholm et al. 2020, Krzewinska et al. 2018a; Krzewinska et al. 2018b). Those studies were critical as they focused on adopting novel kinship identification techniques which were later used in realisation of current project (Yaka et al. 2021). At the same time some collaborations directly resulted in a number of follow up studies. One such study is currently being realised as a master project at Centre for Paleogenetics (CPG) dealing with newly generated genomic data from a previously described Bronze Age Russian site in southern Urals (Krzewinska et al. 2018). The master project is focusing on identification of kinship patterns and pedigree reconstruction as means of explaining variable traditions and practices at a single Bronze Age Russian burial site. Another currently realised project is a direct result of previous collaboration which among others based on kinship analyses allowed for identification of novel burial traditions among Polish Corded Ware Culture individuals (Linderholm et al. 2020). The new study focuses on the role of women in Early Bronze Age societies in Southern Poland (Narodowe Centrum Nauki, grant no 2020/39/B/HS3/00454) and will do so by systematic analyses of whole burial site, an optimal approach as confirmed by this project. Finally, invested interests in kindship analyses and pedigree reconstruction led to participation in the NEOMATRIX program of the HORIZON2020 (Grant agreement ID: 952317).
HOW THE PROJECT TEAM HAS DISSEMINATED THE RESULTS TO OTHER RESEARCHERS AND GROUPS OUTSIDE THE SCIENTIFIC COMMUNITY
Dissemination of results outside the scientific community was somewhat restricted due to Covid-19 and fewer opportunities to participate in public presentation. However, the thus far published results from the study were highlighted through interviews and publications in such journals, sites and news outlets as Atlas Obscura, Archaeology - A publication of the Archaeological Institute of America, Science Direct, Science Daily, Wikipedia and others. The results and proceedings from the project were used in teaching at "Paleoecology, genomics and human prehistory" course at Stockholm University meeting with much interest from the students and were also used by all project participants during various public lectures and presentations.
The original purpose of the study was to investigate biological and social kinship in Swedish multiple burials from Medieval period. Primarily, we were aiming to use genomic analyses to explore biological kinship in relation to burial traditions governing deposition of multiple individuals in same burial – an unusual practice in Christian contexts. Secondly, as many of such burials contain children and subadults for whom sex identification cannot be reliably obtained from osteological material we were hoping to use molecular sexing to identify boys and girls. This information would shed more light on the treatment of children and add to the developing field of childhood archaeology by focusing on finding whether we may witness differential treatment or socialisation processes of boys and girls in the Middle Ages.
A SHORT DESCRIPTION OF HOW IT WAS IMPLEMENTED
We have sampled and analysed a total of 120 individuals from 50 multiple burials in four localities in Sweden (Sigtuna, Fjälkinge, Västerhuas and Lund). The investigated individuals underwent DNA extraction and multiple rounds of whole genome sequencing allowing for accumulation of enough shotgun sequencing data for sex and kinship identification. After validation the obtained data was merged with previously generated Swedish ancient human genomes from the sites of interest to be used for comparative analyses. The dataset has then been analysed using a number of statistical tests. In total we analysed 150 individuals ranging from 0.01X to 21X genome coverage) to identify correlations between different individual arrangements in multiple burials and combine the information with kinship identification to answer two fundamental questions within the study: can we use genetic data from sexing of subadults to learn more about medieval burial traditions and can such data be used to identify differential treatment of boys and girls? And whether kinship patterns can be used to explain or exclude reasons for two or more individuals to occupy the shared burial space after death.
THE PROJECT’S THREE MOST IMPORTANT RESULTS AND CONTRIBUTIONS TO THE INTERNATIONAL RESEARCH FRONT
Most important findings in the study include observation that: a) the majority of co-burials consist of adults and children of the same sex, i.e. the individuals’ sex seems to determine where people were buried in terms of the localization relative to the church. This is a well-known tradition for adult burials which is hereby for the first time confirmed in subadults as well. Thus, it seems that there was no ‘infancy’ period prior to puberty in which boys and girls were treated as ‘gender-less’. This observation largely holds for both sexes but there seem to be a degree of freedom judging from slightly more widespread distribution of girls; b) furthermore this observation is not accompanied by detection of close biological kinship between the co-buried people. In fact, we find (i) very few individuals closely related by blood within burials while the majority of detected biological kin associations is distributed between burials. This suggests that social factors, other than being buried with close kin, played an important part in the practice. We also cannot exclude that there were other family ties than a biological connection between the individuals (e.g. foster children). Alternatively, (ii) this finding could also reflect differences between urban and rural burial traditions as close biological kinship co-burials were observed at a higher frequency in medieval (urban) Sigtuna; c) we note that at least in Västerhus the cemetery is organized around dominant pedigrees, with individuals representing the most extensive kin buried closest to the church building itself. d) Finally, we also confirm that based on research questions answered by this project we corroborate the need to analyse whole sites rather than selecting specific individuals as this creates a sample-specific bias. In other words, analysing whole sites would enable far better pedigree reconstructions with minimal missing data, which could in turn reveal further kin associations between seemingly unrelated individuals interred together or in individual burials.
While the main paper discussing all the upper mentioned findings is to be submitted for publication later this year we note that the questions addressed in the study are of much interest to the general public. This observation is based on the fact that the first published case-study from the project (Krzewinska et al. 2021) was recently highlighted as being in top ten papers downloaded from Journal of Archaeological Science: Reports in 2021.
NEW RESEARCH QUESTIONS GENERATED THROUGH THE PROJECT
Through developed collaborations with other researchers and additional analyses performed on data produced within a scope of this study we found that very few individuals tested were carriers of pathogenic microbes which could be identified as plausible cause of death. This is quite confounding as many of the analysed individuals were children and their premature death should have a detectable cause. It thus may be that we are faced with a form of an “osteological paradox” on a molecular level, or that the newly developed tools for microbial detection are not sensitive enough. Yet another explanation could be too little data or inappropriate input materials, i.e. skeletal elements, for a reliable detection. Sampling bias should be contemplated when molecular pathology is considered. The tests raise more questions not only regarding the actual individuals studied but also present a challenge driving development of novel analytical tools.
Through another collaborative project it has been noted that some of the individuals interred in Västerhus burial ground were carriers of some evidence of North-South population cline structuring which is characteristic of present-day Scandinavia. This is highly important as the observation was made on individual level within a single-family unit which could thus be a direct example of how the North-South genetic variation cline was formed in Sweden. Those findings are preliminary but could possibly serve as starting point for further research focusing on genetic structuring in Scandinavian Peninsula.
THE PROJECT’S INTERNATIONAL DIMENSIONS, SUCH AS CONTACTS AND MATERIAL
We have developed new collaborations both within the presented project by inviting participants but also by participation in ancient genomic papers utilizing kinship as means of understanding past burial traditions (Yaka et al. 2021, Linderholm et al. 2020, Krzewinska et al. 2018a; Krzewinska et al. 2018b). Those studies were critical as they focused on adopting novel kinship identification techniques which were later used in realisation of current project (Yaka et al. 2021). At the same time some collaborations directly resulted in a number of follow up studies. One such study is currently being realised as a master project at Centre for Paleogenetics (CPG) dealing with newly generated genomic data from a previously described Bronze Age Russian site in southern Urals (Krzewinska et al. 2018). The master project is focusing on identification of kinship patterns and pedigree reconstruction as means of explaining variable traditions and practices at a single Bronze Age Russian burial site. Another currently realised project is a direct result of previous collaboration which among others based on kinship analyses allowed for identification of novel burial traditions among Polish Corded Ware Culture individuals (Linderholm et al. 2020). The new study focuses on the role of women in Early Bronze Age societies in Southern Poland (Narodowe Centrum Nauki, grant no 2020/39/B/HS3/00454) and will do so by systematic analyses of whole burial site, an optimal approach as confirmed by this project. Finally, invested interests in kindship analyses and pedigree reconstruction led to participation in the NEOMATRIX program of the HORIZON2020 (Grant agreement ID: 952317).
HOW THE PROJECT TEAM HAS DISSEMINATED THE RESULTS TO OTHER RESEARCHERS AND GROUPS OUTSIDE THE SCIENTIFIC COMMUNITY
Dissemination of results outside the scientific community was somewhat restricted due to Covid-19 and fewer opportunities to participate in public presentation. However, the thus far published results from the study were highlighted through interviews and publications in such journals, sites and news outlets as Atlas Obscura, Archaeology - A publication of the Archaeological Institute of America, Science Direct, Science Daily, Wikipedia and others. The results and proceedings from the project were used in teaching at "Paleoecology, genomics and human prehistory" course at Stockholm University meeting with much interest from the students and were also used by all project participants during various public lectures and presentations.