Haplogroup X (mtDNA)

In human mitochondrial genetics, Haplogroup X is a human mitochondrial DNA (mtDNA) haplogroup which can be used to define genetic populations. The genetic sequences of haplogroup X diverged originally from haplogroup N, and subsequently further diverged about 20,000 to 30,000 years ago to give two sub-groups, X1 and X2. Overall haplogroup X accounts for about 2% of the population of Europe, the Near East and North Africa. Sub-group X1 is much less numerous, and restricted to North and East Africa, and also the Near East. Sub-group X2 appears to have undergone extensive population expansion and dispersal around or soon after the last glacial maximum, about 21,000 years ago. It is more strongly present in the Near East, the Caucasus, and Mediterranean Europe; and somewhat less strongly present in the rest of Europe. Particular concentrations appear in Georgia (8%), the Orkney Islands (in Scotland) (7%) and amongst the Israeli Druze (26%); the latter are presumably due to a founder effect.

North and South America
Haplogroup X is also one of the five haplogroups found in the indigenous peoples of the Americas. Although it occurs only at a frequency of about 3% for the total current indigenous population of the Americas, it is a major haplogroup in northeastern North America, where among the Algonquian peoples it comprises up to 25% of mtDNA types. It is also present in lesser percentages to the west and south of this area -- in North America among the Sioux (15%), the Nuu-Chah-Nulth (11%–13%), the Navajo (7%), and the Yakima (5%), and in South America among the Yanomami people (12%) in eight villages in Roraima in northwestern Brazil.

Unlike the four main Native American haplogroups (A, B, C, and D), X is not at all strongly associated with East Asia. The sole occurrence of X in Asia discovered so far is in Altaia in South Siberia (Derenko et al, 2001), and detailed examination (Reidla et al, 2003) has shown that the Altaian sequences are all almost identical, suggesting that they arrived in the area probably from the South Caucasus more recently than 5000 BC.

This absence of haplogroup X2 in Asia is one of the major factors causing the current rethinking of the peopling of the Americas. However, the New World haplogroup X2a is as different from any of the Old World X2b-f lineages as they are from each other, indicating an early origin "likely at the very beginning of their expansion and spread from the Near East".

The Solutrean Hypothesis posits that haplogroup X reached North America with a wave of European migration about 20,000 BC by the Solutreans, a stone-age culture in south-western France and in Spain, by boat around the southern edge of the Arctic ice pack.