X is one of the rarest mitochondrial haplogroups in terms of frequency, yet one of the most widespread geographically. It is found throughout Europe, North Africa, the Near/Middle East, in most of Central Asia, parts of North Asia, and among Native North Americans (X2a). X is currently divided in four top branches (X1, X2, X3 and X4). 99% of Europeans belong to X2, which is further divided in 15 subclades (X2a to X2o). The most common subclades in Europe are X2b and X2c.
The highest frequencies of X have been observed among the Druzes of Lebanon (15%), among the Abkhazians of Georgia (10%), on the island of Orkney (7%) in northern Scotland, in Lombardy (6%), and in most ethnic groups of the North Caucasus (4-5%).
Haplogroup X seems to have first come to Europe during the Neolithic period. It has been found in Early Neolithic sites in Iberia and southern France (Cardium Pottery culture), in Megalithic sites in the Basque country, Navarre and Brittany, as well as in all Neolithic cultures derived from the Linear Pottery culture (LBK) in Central Europe.
The R1b branch of the Indo-Europeans probably carried a small percentages of X2 lineages based on its strong presence around the Caucasus, but also on the fact that X2 was found in Central Asia, the Altai and in the modern population of the Tarim basin in Northwest China, where 20% of the men belong to Y-haplogroup R1b.

Note the relative resemblance with the distribution of the Caucasian admixture from Dodecad.
UPDATE: a detailed page about the origins, history, distribution and subclades of haplogroup X is now available here.
The highest frequencies of X have been observed among the Druzes of Lebanon (15%), among the Abkhazians of Georgia (10%), on the island of Orkney (7%) in northern Scotland, in Lombardy (6%), and in most ethnic groups of the North Caucasus (4-5%).
Haplogroup X seems to have first come to Europe during the Neolithic period. It has been found in Early Neolithic sites in Iberia and southern France (Cardium Pottery culture), in Megalithic sites in the Basque country, Navarre and Brittany, as well as in all Neolithic cultures derived from the Linear Pottery culture (LBK) in Central Europe.
The R1b branch of the Indo-Europeans probably carried a small percentages of X2 lineages based on its strong presence around the Caucasus, but also on the fact that X2 was found in Central Asia, the Altai and in the modern population of the Tarim basin in Northwest China, where 20% of the men belong to Y-haplogroup R1b.

Note the relative resemblance with the distribution of the Caucasian admixture from Dodecad.
UPDATE: a detailed page about the origins, history, distribution and subclades of haplogroup X is now available here.
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