Within the European context, the Orkney BA stands in stark contrast as a location, at the northwestern extreme of the continent, where the majority of the genome was overwritten between the Late Neolithic and the end of the EBA but the male lineages somehow persisted. Even so, we can understand this phenomenon in terms of the same patrilocal marriage practices that we see throughout west Eurasia. The ancestral distribution in Orkney demonstrates deliberate marriage patterns involving local men and incoming women. This process of preferential assimilation seems likely to have continued for many generations, given the extent of replacement of the remainder of the Orcadian Neolithic genome.
The existence of a powerful and likely strongly hierarchical strand in Neolithic society has been proposed on the basis of the discovery of an incestuous first-degree union at Newgrange in Ireland (41) and was prefigured by earlier analyses of Ireland and other megalithic cultures in both northwest and central Europe (32, 48). Cassidy et al. (41) argue that it encompassed the whole of Ireland, adding that it may have incorporated the similar megalithic communities of Wales and Orkney, most likely originating in Brittany (1, 49). I2a1b-M423 is seen in both Mesolithic and Neolithic Ireland, and the main cluster seen in Late Neolithic Ireland, I2a2a1a1-M284—found in the putative elite lineage at Newgrange—matches an Orcadian Neolithic lineage from the Isbister Chambered Cairn (“Tomb of the Eagles”) on South Ronaldsay (Fig. 3 and SI Appendix, Fig. S13) (13). Both our data from BA Orkney and the Neolithic circumcoastal distribution of the Y-chromosome I2a1b-M423 haplogroup lend further support to this suggestion. European Neolithic society, at one extreme (but hardly peripheral) edge of its distribution, may have been patrilineal, patrilocal, and hierarchical long before the arrival of the Beaker complex and (most likely) Indo-European speech (27, 28, 31, 50).
Basically, a Y chromosomal lineage assimilated from Iberian hunter-gatherers, and assimilated into Neolithic farmers, seems to have spread over the British Isles and persisted in a part of Orkney as the dominant paternal line for 1,000 years after it was overwhelmed elsewhere.