Haplogroup I2 is the most common paternal lineage in former Yugoslavia, Romania, Bulgaria and Sardinia, and a major lineage in most Slavic countries. Its maximum frequencies are observed in Bosnia (55%, including 71% in Bosnian Croats), Sardinia (39.5%), Croatia (38%), Serbia (33%), Montenegro (31%), Romania (28%), Moldova (24%), Macedonia (24%), Slovenia (22%), Bulgaria (22%), Belarus (18.5%), Hungary (18%), Slovakia (17.5%), Ukraine (13.5%), and Albania (13.5%). It is found at a frequency of 5 to 10% in Germanic countries.
I2 (M438/P215/S31) is thought to have originated during the Late Paleolithic, around the time of the Last Glacial Maximum, some 22,000 years ago. Its region of origin is undetermined at present. It could have been one of the Last Glacial Maximum refugia or somewhere in Anatolia or around the Caucasus. Three hypotheses are consequently possible.
The first scenario is that I2 originated in Europe. When the ice sheets started receding to the north from 20,000 to 12,000 years ago, the I2 hunter-gatherers re-expanded from their LGM refugium and colonised vast parts of western, central and Eastern Europe. In this hypothesis I2 would be associated with mtDNA haplogroups H1, H3, U5 and V, among others.
In the second scenario I2 originated in West Asia, but also colonised Europe when the ice sheets receded. In this hypothesis I2 would be mostly associated with mtDNA haplogroups J and T.
In the third and least likely scenario, I2 originated in West Asia but did not come to Europe until the Neolithic. There seems to have been several independent migrations of Neolithic farmers and herders from the Middle East to Europe, bringing lineages such as G2a, E1b1b, J and T. It is not yet clear at present whether each group brought only one or perhaps two haplogroups, or whether most migrations already comported blends of many haplogroups. In this hypothesis I2 could be associated with mtDNA haplogroups N1a, R, HV, H (various subclades), J, T, K and X.
In the two first cases I2 would have been absorbed by Neolithic farmers in Southeast Europe (M423), Central Europe (P214, L596), and the western Mediterranean (M26). The relative success of specific branches of I2 seem to be linked to the diffusion of agriculture. The south-western I2a1a (M26) branch was absorbed by Neolithic farmers of the Printed-Cardium Pottery culture (5000-1500 BCE), whose descendants are found mostly in modern Sardinians and Basques.
The eastern I2a1b (M423) is probably linked to the Cucuteni-Trypillian culture (4800-3000 BCE), which was the most advanced Neolithic culture in Europe before the Indo-European invasions in the Bronze Age. In contrast, central, northern and Western European I2 lineages (such as L38, M223, L1286, L1294 and L880) only survived at low frequencies. The reason could be that I2 hunter-gatherers adopted agriculture too late and were not numerous when the wave of Indo-European took over central, northern and Western Europe (=> see R1b history).
Haplogroup I2a1 is by far the largest branch of I2 and the one most strongly linked to Neolithic cultures in south-east, south-west and north-west Europe.
I2a1a was known as I1b2 until 2005, I1b1b in 2006-7, and I2a1 from 2008 to 2010. It is found in all Western Europe, and reaches maximum frequencies among the Sardinians (37.5%) and the Basques (5%), two population isolates. M26 is geographically restricted to the British Isles, the Low Countries, France, western Germany, Switzerland, Sardinia, Sicily, the west coast of Italy, Iberia and the Mediterranean coast of the Maghreb. The only M26 negative for the L160 mutation are confined to Ireland.
I2a1a-M26 was probably one of the main paternal lineages of the Megalithic cultures of Western Europe during the Neolithic and Chalcolithic periods.
I2a2 was known as I1c until 2005 and I2b until 2010. It is associated with the pre-Celto-Germanic people of north-Western Europe, such as the megaliths builders (5000-1200 BCE). The wide variety of STR markers within I2a2 could make it as much as 13,000 years old.
I2a2 is found in all Western Europe, but apparently survived better the Indo-European invasions (=> see R1b) in northern Germany, and was reintroduced by both the La Tène Celtic expansion (5th to 1st century BCE) and the Germanic invasions (3rd to 6th century CE). Nowadays, I2a2 peaks in central and northern Germany (10-20%), the Benelux (10-15%) as well as in northern Sweden. It is also found in 3 to 10% of the inhabitants of Denmark, eastern England, and northern France. It is rarer in Norway, except in the south, where the Danish influence was the strongest historically.