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146 Scandinavian genetic ancestry and the beginnings of the Viking era
147 Although VA Scandinavians shared a common cultural, linguistic and material background, there
148 was no common word for Scandinavian identity at that time1. The word ‘Viking’ is used in
149 contemporary sources to mean a ‘pirate’ or ‘sea warrior’2. As such, there is no single ‘Viking world’,
150 but a coalescence of ‘Viking worlds’ marked by rapidly growing maritime exploration, trade, war
151 and colonization, following the adoption of deep-sea navigation among the coastal populations of
152 Scandinavia and the Baltic Sea area7,8. Thus, it is unclear whether the Viking-phenomenon refers to
153 people with a recently shared genetic background and if foreign influence initiated or accompanied
154 the transition from the Scandinavian Iron Age into the Viking era.
Fig 2
155 To assess the genetic relationship of the VA Scandinavians with that of earlier European peoples, we
156 performed genetic clustering using multi-dimensional scaling (MDS) on a pairwise identity-by-state
157 (IBS) sharing matrix, as well as latent mixed-ancestry models (Admixture)9. We find that the majority
158 of our samples broadly cluster within the range of European Bronze Age (BA) and Iron Age (IA)
159 populations, characterized by an ancestry component that is related to pastoralist populations from
160 the Pontic-Caspian steppe (Fig. 2a and Extended Data Fig. 2) entering Europe around 5000 BP10,11.
161 A different dimensionality reduction technique using uniform manifold approximation and projection
162 (UMAP) revealed additional fine-scale genetic structure. European individuals from the Bronze Age
163 and onwards are generally distributed within a broad area anchored by four ancestry clusters across
164 the two UMAP dimensions: Early BA individuals from the Steppe; pre-BA Neolithic Europeans;
165 Baltic BA individuals; and Scandinavian IA and early VA individuals (Fig. 2b). We observe a wide
166 range of distributions for VA individuals within this broad area, with notable differences between
167 geographic regions (Fig. S8.10), indicating complex fine-scale structure among the different groups.
168 Modelling Scandinavian groups from the BA and onwards as mixtures of three ancestral components
169 (Mesolithic hunter-gatherers; Anatolian Neolithic; Steppe early BA), again revealed subtle
170 differences in their composition. We find that the transition from the BA to the IA is accompanied by
171 a reduction in Neolithic farmer ancestry, with a corresponding increase in both Steppe-like ancestry
172 and hunter-gatherer ancestry (Extended Data Fig. 6). While most groups show a slight recovery of
173 farmer ancestry during the VA, there is considerable variation in ancestry across Scandinavia. In
174 particular, we observe a wide range of ancestry compositions among individuals from Sweden, with
175 farmer ancestry proportions (40% or more in individuals from Malmö, Kärda or Öland).
176 Ancestry proportions in Norway and Denmark on the other hand appear more uniform
177 (Extended Data Fig. 6). Finally, we detect an influx of low levels
178 of “eastern” ancestry starting in the early VA, mostly constrained among groups from eastern and
179 central Sweden as well as some Norwegian groups (Extended Data Fig. 6). Testing of putative source
180 groups for this “eastern” ancestry revealed differing patterns among the Viking Age target groups,
181 with contributions of either East Asian- or Caucasus-related ancestry (Supplementary Note 10).
182 Overall, our findings suggest that the genetic makeup of VA Scandinavia derives from mixtures of
183 three earlier sources: Mesolithic hunter-gatherers, Neolithic farmers, and Bronze Age pastoralists.
184 Intriguingly, our results also indicate ongoing gene flow from the south and east into Iron Age
185 Scandinavia. Thus, these observations are consistent with archaeological claims of wide-ranging
186 demographic turmoil in the aftermath of the Roman Empire with consequences for the Scandinavian
187 populations during the late Iron Age12,13. We caution, however, that our sampling for the periods
188 preceding the VA is still sparse, and hence do not provide a full picture of the genetic diversity across
189 Scandinavia during that period.
191 Genetic structure within Viking-Age Scandinavia
192 By the end of the Iron Age in the 8th century CE, Scandinavia formed a patchwork of conflicting and
193 competing kingdoms with a shared cultural background. For centuries, a political economy based on
194 raiding, trading and gifts had been common5. However, the cause for the development of this
195 economic and political system into the more organized maritime society of the Viking era remains
196 debated5. It is commonly argued that seafaring8,14 contributed to create a densely interlinked
197 Scandinavia during the Viking era2,15,16.
198 To disentangle the fine-scale population structure within VA Scandinavia, we performed genotype
199 imputation on a subset of 300 individuals with sufficient coverage (>0.5X) and inferred genomic
200 segments shared via identity-by-descent (IBD) within the context of a reference panel of 1,464
201 present-day Europeans, using IBDseq. We find that VA Scandinavians on average cluster into three
202 groups according to their geographic origin, shifted towards their respective present-day counterparts
203 in Denmark, Sweden and Norway (Fig. 3a). Closer inspection of the distributions for the different
204 groups reveals additional complexity in their genetic structure (Fig. S10.1). We find that the
205 ‘Norwegian’ cluster includes Norwegian IA individuals, who are distinct from both Swedish and
206 Danish IA individuals which cluster together with the majority of central and eastern Swedish VA
207 individuals. Many individuals from southwestern Sweden (e.g. Skara) cluster with Danish present208
day individuals from the eastern islands (Funen, Zealand), skewing towards the ‘Swedish’ cluster
209 with respect to early and more western Danish VA individuals (Jutland). Some individuals have
210 strong affinity with Eastern Europeans, particularly those from the island of Gotland in eastern
211 Sweden. The latter likely reflects individuals with Baltic ancestry, as clustering with Baltic BA
212 individuals is evident in the IBS-UMAP analysis (Fig. 2b) and through f4-statistics (Extended Data
213 Fig. 4).
147 Although VA Scandinavians shared a common cultural, linguistic and material background, there
148 was no common word for Scandinavian identity at that time1. The word ‘Viking’ is used in
149 contemporary sources to mean a ‘pirate’ or ‘sea warrior’2. As such, there is no single ‘Viking world’,
150 but a coalescence of ‘Viking worlds’ marked by rapidly growing maritime exploration, trade, war
151 and colonization, following the adoption of deep-sea navigation among the coastal populations of
152 Scandinavia and the Baltic Sea area7,8. Thus, it is unclear whether the Viking-phenomenon refers to
153 people with a recently shared genetic background and if foreign influence initiated or accompanied
154 the transition from the Scandinavian Iron Age into the Viking era.
Fig 2
155 To assess the genetic relationship of the VA Scandinavians with that of earlier European peoples, we
156 performed genetic clustering using multi-dimensional scaling (MDS) on a pairwise identity-by-state
157 (IBS) sharing matrix, as well as latent mixed-ancestry models (Admixture)9. We find that the majority
158 of our samples broadly cluster within the range of European Bronze Age (BA) and Iron Age (IA)
159 populations, characterized by an ancestry component that is related to pastoralist populations from
160 the Pontic-Caspian steppe (Fig. 2a and Extended Data Fig. 2) entering Europe around 5000 BP10,11.
161 A different dimensionality reduction technique using uniform manifold approximation and projection
162 (UMAP) revealed additional fine-scale genetic structure. European individuals from the Bronze Age
163 and onwards are generally distributed within a broad area anchored by four ancestry clusters across
164 the two UMAP dimensions: Early BA individuals from the Steppe; pre-BA Neolithic Europeans;
165 Baltic BA individuals; and Scandinavian IA and early VA individuals (Fig. 2b). We observe a wide
166 range of distributions for VA individuals within this broad area, with notable differences between
167 geographic regions (Fig. S8.10), indicating complex fine-scale structure among the different groups.
168 Modelling Scandinavian groups from the BA and onwards as mixtures of three ancestral components
169 (Mesolithic hunter-gatherers; Anatolian Neolithic; Steppe early BA), again revealed subtle
170 differences in their composition. We find that the transition from the BA to the IA is accompanied by
171 a reduction in Neolithic farmer ancestry, with a corresponding increase in both Steppe-like ancestry
172 and hunter-gatherer ancestry (Extended Data Fig. 6). While most groups show a slight recovery of
173 farmer ancestry during the VA, there is considerable variation in ancestry across Scandinavia. In
174 particular, we observe a wide range of ancestry compositions among individuals from Sweden, with
175 farmer ancestry proportions (40% or more in individuals from Malmö, Kärda or Öland).
176 Ancestry proportions in Norway and Denmark on the other hand appear more uniform
177 (Extended Data Fig. 6). Finally, we detect an influx of low levels
178 of “eastern” ancestry starting in the early VA, mostly constrained among groups from eastern and
179 central Sweden as well as some Norwegian groups (Extended Data Fig. 6). Testing of putative source
180 groups for this “eastern” ancestry revealed differing patterns among the Viking Age target groups,
181 with contributions of either East Asian- or Caucasus-related ancestry (Supplementary Note 10).
182 Overall, our findings suggest that the genetic makeup of VA Scandinavia derives from mixtures of
183 three earlier sources: Mesolithic hunter-gatherers, Neolithic farmers, and Bronze Age pastoralists.
184 Intriguingly, our results also indicate ongoing gene flow from the south and east into Iron Age
185 Scandinavia. Thus, these observations are consistent with archaeological claims of wide-ranging
186 demographic turmoil in the aftermath of the Roman Empire with consequences for the Scandinavian
187 populations during the late Iron Age12,13. We caution, however, that our sampling for the periods
188 preceding the VA is still sparse, and hence do not provide a full picture of the genetic diversity across
189 Scandinavia during that period.
191 Genetic structure within Viking-Age Scandinavia
192 By the end of the Iron Age in the 8th century CE, Scandinavia formed a patchwork of conflicting and
193 competing kingdoms with a shared cultural background. For centuries, a political economy based on
194 raiding, trading and gifts had been common5. However, the cause for the development of this
195 economic and political system into the more organized maritime society of the Viking era remains
196 debated5. It is commonly argued that seafaring8,14 contributed to create a densely interlinked
197 Scandinavia during the Viking era2,15,16.
198 To disentangle the fine-scale population structure within VA Scandinavia, we performed genotype
199 imputation on a subset of 300 individuals with sufficient coverage (>0.5X) and inferred genomic
200 segments shared via identity-by-descent (IBD) within the context of a reference panel of 1,464
201 present-day Europeans, using IBDseq. We find that VA Scandinavians on average cluster into three
202 groups according to their geographic origin, shifted towards their respective present-day counterparts
203 in Denmark, Sweden and Norway (Fig. 3a). Closer inspection of the distributions for the different
204 groups reveals additional complexity in their genetic structure (Fig. S10.1). We find that the
205 ‘Norwegian’ cluster includes Norwegian IA individuals, who are distinct from both Swedish and
206 Danish IA individuals which cluster together with the majority of central and eastern Swedish VA
207 individuals. Many individuals from southwestern Sweden (e.g. Skara) cluster with Danish present208
day individuals from the eastern islands (Funen, Zealand), skewing towards the ‘Swedish’ cluster
209 with respect to early and more western Danish VA individuals (Jutland). Some individuals have
210 strong affinity with Eastern Europeans, particularly those from the island of Gotland in eastern
211 Sweden. The latter likely reflects individuals with Baltic ancestry, as clustering with Baltic BA
212 individuals is evident in the IBS-UMAP analysis (Fig. 2b) and through f4-statistics (Extended Data
213 Fig. 4).