Corded Ware Culture (CWC) Genetics

Discussion
All individuals studied here are associated to burials of the CWC and BBC complexes. Based on the geographic location and the strontium isotope analysis we identified four different territorial sub groups, and based on the genetics we identified at least two different groups within our sample set. The mtDNA and the Y-chromosome data provide a slightly different picture of the genetic variation in the region contrasting earlier studies of individuals from corresponding archaeological contexts from other regions of Central Europe. In contrast to observations by Juras et al.12 we did not find mitochondrial lineages specifically linked to Yamnaya pastoralists, instead most of mtDNA lineages found in our sample may be associated with European Neolithic farming groups as is the case for the Western Corded Ware sample in the earlier study12. Our results would indicate a stronger continuity with earlier Neolithic populations than previously observed. In other words, our study detected traces of an evident “incorporation” of local individuals into the migrating groups. However, the funerary rituals seem to have been affected in limited extent as the burials exhibit the typical CWC pattern in all cases examined. The Y chromosome haplogroup lineage R1b-M269 or R-L11 are characteristic of Yamnaya and Bell Beaker individuals5,10 and they were particularly widespread throughout Eurasia in the Bronze Age and thereafter38. Curiously, the haplogroup is uncommon among other published Corded Ware Complex individuals from Europe (Germany, Poland, Bohemia, Estonia, Lithuania)6 and is associated with the later Bell Beaker communities5. We see the inclusion of the Yamnaya genetic signals but again in a different manner than what has been shown in adjacent regions. These results indicate a higher level of CWC continuity with earlier Neolithic individuals than those previously studied. The result also shows that the CWC groups exhibit an influence of the Steppe world, i.e. in the individuals with specific Y chromosome. Later the influence of the BBC communities was stronger.

The PCA revealed that despite geographical proximity there is a distinct genetic separation between CWC and BBC individuals from southern Poland. The genetic variation of CWC individuals from southern Poland overlaps with the majority of previously published CWC individuals from Germany while the eight published CWC individuals from the Polish lowland10,11 more closely resemble BBC individuals (Fig. S21). This fact is not unexpected if we consider the CWC communities in Polish lowlands as representatives of north-western parts of the CWC world called as the Single-Grave culture (see supplementary information). The genetic variation of BBC individuals from south-eastern Poland overlaps with the broad variation of BBC individuals from Central Europe (Bohemia, Moravia, Germany, south-western Poland and Hungary) (Fig. S22) which corresponds well with archaeological data. The results are in line with Admixture analyses. To determine whether the structuring can be detected on even more regional level we divided our sample into regional subgroups to test their relations.

According to f4-statistics on individual level CWC individuals from south-eastern Poland are equally different to published CWC and Yamnaya individuals, while the three Pełczyska individuals tend to select German, Estonian, Lithuanian and Polish western CWC to the exclusion of Yamnaya (Table S16). This however is not the case for CWC individuals from the Kuyavia region published by Fernandes11. CWC (but not BBC) from south-eastern Poland tends to pick Yamnaya over CWC from Kuyavia region. Our results emphasize the different impacts the Yamnaya migration event had on different populations across Europe, i.e. the genetic legacy that the Yamnaya process left varies greatly between regions and cultures.

We obtained statistically significant values for group II being closer to group I & IV to the exclusion of group III confirming that BBC Pełczyska are a distinct population group. This suggests structuring not only within the Eneolithic in southern Poland but also between groups representing the same (Archaeological) cultural horizon. Compared to the Early and Middle Neolithic samples it seems that the CWC groups I, II and IV are equally distant from the Yamnaya pastoralists and from most of the earlier published Corded Ware groups from Estonia, Germany, Lithuania and central Poland (Table S16). The south-eastern Polish CWC individuals are significantly more closely related to Yamnaya than to CWC individuals from the Polish lowland supporting the differentiation between various CWC groups from Central Europe. This is in coincidence with archaeological finds that show differences between lowlands and uplands materials of CWC23. Bell Beaker individuals from Pełczyska mostly favour German, Polish (lowland) and Estonian CWC as well as German and Czech Bell Beaker populations over Steppe ancestors (Table S16). Interestingly, in contrast to CWC individuals from south-eastern Poland (Group I, II and IV), they share significantly closer affinity to Neolithic Iberian, Italian, Hungarian, Swedish, Polish TRB and Brześć Kujawski group populations (and nonsignificant but positive affinity to Polish Globular Amphora) than Yamnaya, pointing to possible continuity between this group and earlier populations. The genetic specificity of the population associated with this process shows similarity to the features of the BBC complex in Central Europe dated ca. 150-200 years later5.

Building on the idea that the CWC complex identity is founded on the type of burial rituals performed, the study of graves and the double burials in particular can be illuminating for interpretations of the local social structures of the CWC groups regarding possible kinship structures. Positive results were obtained from 3 double graves, all containing individuals of the same sex. Kinship was observed in the graves from Chłopice grave 11 (pcw211 and pcw212) and from Święte, grave 408 (pcw061 and pcw062). The former burial represented a second-degree kinship while the latter was a first degree one. The two young boys buried in the double grave from Łubcze (pcw361 and pcw362) were not closely related according our READ analysis. However, their similar Y chromosome haplogroup (Haplogroup R1b1a1a2a1a) leaves room for a possible shared ancestry on the male linage. In Chłopice both young females died and were buried at the same time but the cause of their death could not be established. Skeletons from Łubcze were too badly preserved to provide information on time or cause of death. Interestingly in Święte (grave 408), the younger female (pcw061) probably had died earlier than the older one (pcw062) as her remains probably were exhumed from another place and added to the grave with the older female. The niche construction made “revisits” and secondary depositions possible, however this practice was part of an uncommon funeral rite in which the kinship between the two females might have had relevance. Interestingly, these closely related females spent their childhood in different places as according to strontium isotopes analysis pcw061 was a non-local and pcw62 – a local individual in the area. The observations are particularly surprising and significant considering the CWC burial customs and also social organisation. They show that in some cases close relatives were buried together in spite of different time of death. Closely related individuals were also found in graves 40 A (pcw040) and 43 (pcw041) in Święte, although not placed in a double grave they had been placed in burials in close proximity of each other. Both individuals were non-locals. The first-degree kinship identified here most likely is the father and his son, as they both have the same radiocarbon date. The close proximity of the graves indicate that kinship was an important part of the society and may have been manifested even in the funeral customs of the Corded Ware Complex communities.

Conclusion
Social processes in prehistory are hard to identify let alone interpret. Using ancient DNA and genomic analysis we can detect several levels of structuring and population dynamics within the CWC complex as well as between the CWC and BBC cultures present in Eneolithic south-eastern Poland. By evaluating the admixture between groups, the impact of earlier known demographic events like the Steppe expansions can be detected in our CWC individuals. Not surprisingly we also discover strong connections to the population in Central Germany and the CWC subpopulation living on the German lowlands. These ties are not new but they might indicate an interesting southern affiliation that has not been as clear in other previously studied samples from Poland. Furthermore, we detect less association between CWC groups from the Kuyavia region and southern Poland revealing fine scale routes of the spread of CWC traditions. The most unusual signal identified is the one between the CWC and the Afanasievo complex. This genetic incorporation from a Steppe population further east than the Yamnaya culture, is novel for these parts and suggests a CWC population structure and history more complex than previously thought. Although, our results should be treated with caution due to not just low number of samples, but the appearance of the same signal in individuals that predate steppe expansions, and are geographically more widespread. Our findings are in alignment with recent archaeological reviews suggesting lesser impact of Yamnaya event than estimated in earlier genomic studies (e.g.16,17). The CWC ancestry exhibits links both to common mtDNA linages of the initial Neolithic but also to those assimilating and replaced by the Yamnaya pastoralists. Moving further forward in time we detect genetic reminiscence of south-eastern CWC in BBC gene pool. This region was an important social area in the 3rd Millennium BCE, a true prehistoric melting pot of human groups with different origin, which may have witnessed emergence of typical BBC genomics almost 200 years earlier than in other parts of Europe5.

By using strontium isotopes, we can detect locals and non-locals in our material, which can shed light on how different subgroups within the CWC interacted. Using these characterisations as a foundation allows for deeper look into kinship amongst the buried individuals. The identification of three different types of kinship amongst the individuals studied and especially in the context of an unusual burial customs expands our understanding of rituals and traditions underlying the social structures of the CWC complex.

www.nature.com/articles/s41598-020-63138-w