667 Finally, we investigated the
fine-scale genetic structure in southern Scandinavia after the
668 introduction of Steppe-related ancestry using a temporal transect of 38 Late Neolithic and Early
669 Bronze Age Danish and southern Swedish individuals. Although the overall population genomic
670 signatures suggest genetic stability,
patterns of pairwise IBD-sharing and Y-chromosome
671 haplogroup distributions indicate at least three distinct ancestry phases during a ~1,000-year time
672 span: i) An
early stage between ~4,600 BP and 4,300 BP, where Scandinavians cluster with early
673 CWC individuals from Eastern Europe, rich in Steppe-related ancestry and males with an R1a Y674 chromosomal haplotype (Extended Data Fig. 8A,
; ii) an intermediate stage until c. 3,800 BP,
675 where they cluster with central and western Europeans dominated by males with distinct sub676 lineages of R1b-L51 (Extended Data Fig. 8C, D; Supplementary Note 3b) and includes Danish
677 individuals from Borreby (NEO735, 737) and Madesø (NEO752) with distinct cranial features
678 (Supplementary Note 6); and iii) a final stage from c. 3,800 BP onwards, where a distinct cluster of
679 Scandinavian individuals dominated by males with I1 Y-haplogroups appears (Extended Data Fig.
680 8E). Using individuals associated with this cluster (Scandinavia_4000BP_3000BP) as sources in
681 supervised ancestry modelling (see “postBA”, Extended Data Fig. 4), we find that it forms the
682 predominant source for later Iron- and Viking Age Scandinavians, as well as ancient European
683 groups outside Scandinavia who have a documented Scandinavian or Germanic association (e.g.,
684 Anglo-Saxons, Goths; Extended Data Fig. 4). Y-chromosome haplogroup I1 is one of the dominant
685 haplogroups in present-day Scandinavians,s, and we document its earliest occurrence in a ~4,000-
686 year-old individual from Falköping in southern Sweden (NEO220). The rapid expansion of this
687 haplogroup and associated genome-wide ancestry in the early Nordic Bronze Age indicates a
688 considerable reproductive advantage of individuals associated with this cluster over the preceding
689 groups across large parts of Scandinavia.