The first settlers on Orkney ,post glaciation , like much of the UK would have had to have come from mainland Europe or in the case of Orkney maybe mainland Scotland . The Orkney vole is a potential clue ,although possibly belonging to a later immigration than the earlier ones .
Not a lot of Mesolithic bone but plenty of later examples which might provide a clue to the source of the various immigrations . Today there is a big Nordic / Scandinavian influence in the local genes but that might not have been the case with the earliest waves of immigration which may/more likely have come from further south and East , one day we may know for sure .