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Haplogroup I1 is the most common type of haplogroup I in northern Europe. It is found mostly in Scandinavia and Finland, where it typically represent over 35% of the Y chromosomes. Associated with the Norse ethnicity, I1 is found in all places invaded by ancient Germanic tribes and the Vikings. After the core of ancient Germanic civilization in Scandinavia, the highest frequencies of I1 are observed in other Germanic-speaking regions, such as Germany, Austria, the Low Countries, England and the Scottish Lowlands, which all have between 10% and 20% of I1 lineages. Haplogroup I is the oldest major haplogroup in Europe and in all probability the only one that originated there (apart from very minor haplogroups like C1a2 and deep subclades of other haplogroups). Haplogroup IJ would have arrived from the Middle East to Europe some 35,000 years ago, then developed into haplogroup I soon afterwards. It has now been confirmed by ancient DNA test that the first Homo sapiens to colonize Europe during the Aurignacian period (45,000 to 28,000 years ago), belonged to haplogroups CT, C1a, C1b, F and I. It is estimated that the I1 branch bifurcated from the rest of haplogroup I some 27,000 years ago. I1 is defined by over 300 unique mutations, which indicates that this lineage experienced a serious population bottleneck. Most of the Late Glacial and Mesolithic remains tested to date belonged to haplogroup I* or I2. It is not yet clear in which part of Europe I1 originated. It has been speculated that I1 evolved in isolation in Scandinavia during the late Upper Paleolithic and Mesolithic periods, when hunter-gatherers from southern Europe recolonised the northern half of the continent from their Last Glacial Maximum refugia. The oldest attested evidence of postglacial resettlement of Scandinavia dates from 11,000 BCE with the appearance of the Ahrensburg culture. However, five Y-DNA samples from Mesolithic Sweden, dating from c. 5800 to 5000 BCE and tested by Lazaridis et al. (2013) and Haak et al. (2015) all turned out to belong to haplogroup I2. The earliest sign of haplogroup I1 emerged from the testing of Early Neolithic Y-DNA from western Hungary (Szécsényi-Nagy et al. (2014)). A single I1 sample was identified alongside a G2a2b sample, both from the early Linear Pottery (LBK) culture, which would later diffuse the new agricultural lifestyle to most of Poland, Germany and the Low Countries. This means that haplogroup I1 was present in central Europe at the time of the Neolithic expansion. It is therefore possible that I1 lineages were among the Mesolithic European hunter-gatherers that were assimilated by the wave of East Mediterranean Neolithic farmers (represented chiefly by Y-haplogroup G2a). There is also evidence from the samples of the Early Neolithic Starčevo culture and Cardium Pottery culture that haplogroup I2a lived alongside G2a farmers both in south-eastern and south-western Europe. The most likely hypothesis at present is that I1 and I2 lineages were dispersed around Europe during the Mesolithic, and that some branches prospered more than others thanks to an early adoption of agriculture upon contact with the Near Eastern farmers who were slowly making their way across the Balkans and the Mediterranean shores. The small groups of farmers from the early LBK culture in Hungary surely included a majority of G2a men accompanied by other minor haplogroups assimilated along the way over the centuries, including I1 men. Yet distinct families would have spread in different directions and met varying successes in their expansion. It would appear that a founder effect in the northern LBK population led to a sudden explosion of I1 lineages, perhaps in part thanks to their better knowledge of the Central European terrain and fauna (since hunting was typically practised side by side to agriculture to complement the farmers' diet). I1 would later have spread to Scandinavia from northern Germany.
