Haplogroup X (mtDNA)

Origin

The genetic sequences of haplogroup X diverged originally from haplogroup N, and subsequently further diverged thousands of years ago to two sub-groups, X1 and X2.

Distribution

Haplogroup X is found in approximately 7% of native Europeans, and 3% of all Native Americans from North America.

Overall, haplogroup X is found in around 2% of the population of Europe, the Near East and North Africa. It is especially common among Egyptians inhabiting El-Hayez oasis (14.3%).

Sub-group X1 is much less frequent, and is largely restricted to North Africa, the Horn of Africa and 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 Southern Europe and somewhat less strongly present in the rest of Europe. Particular concentrations appear in Georgia (8%), Orkney (in Scotland) (7%), and amongst the Israeli Druze community (27%). Subclades X2a and X2g are found in North America, but are not present in native South Americans. Many of the early carriers of haplogroup X2a were found in eastern maritime Canada, a prime theoretical landing location for Solutreans. This encouraged adherents to the Solutrean hypothesis. However, more recent discoveries of haplogroup X2a and subgroups have been more widely geographically dispersed. Neither is there a path or ancestral form of X2a found in Europe or the Middle East.

Druze

The greatest frequency of haplogroup X is observed in the Druze, a minority population in Israel, Jordan, Lebanon, and Syria, as much in X1 (16%) as in X2 (11%). The Druze also have much diversity of X lineages. This pattern of heterogeneous parental origins is consistent with Druze oral tradition. The Galilee Druze represent a population isolate, so their combination of a high frequency and diversity of X signifies a phylogenetic refugium, providing a sample snapshot of the genetic landscape of the Near East prior to the modern age.

North 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 bigger haplogroup in northern 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—among the Sioux (15%), the Nuu-chah-nulth (11%–13%), the Navajo (7%), and the Yakama (5%).

Unlike the four main Native American mtDNA haplogroups (A, B, C, D), X is not at all strongly associated with East Asia. The main occurrence of X in Asia discovered so far is in the Altai people in Siberia, and detailed examination has shown that the Altaian sequences are all almost identical (haplogroup X2e), suggesting that they arrived in the area probably from Transcaucasia more recently than 5000 BP. In addition, Zegura et al. 2003 ascertained that only the Altai and southwest Siberian regions "possesses all of the major Native American Y chromosome" and that "mtDNA founding haplogroups, thereby making it the best available candidate for the ancestral source region for the Native American population system."

Earlier (2001) research indicated that Altaians have maintained their native identity and only begun, very recently, to mix with groups (mostly Russians and Kazakhs) who do not show mtDNA haplogroup X. Genetic studies and researchers show, "the analysis of the tribal structure of Southern Altaians has shown that the present-day Altaians have retained their native language and ethnic identity. They have begun to mix with other ethnic groups (mostly Russians and Kazakhs) only recently, so the interethnic admixture is estimated to be <5% (Luzina 1987; Osipova et al. 1997). The haplogroup X mtDNAs have not been found in populations of central Asia, including Kazakhs, Uighurs, and Kirghizs (Comas et al. 1998). Since the frequency of haplogroup X in Russians is extremely low (3 of 336; Orekhov et al. 1999; Malyarchuk and Derenko 2000; authors’ unpublished data), the recent European admixture cannot explain the presence of haplogroup X in the Altaians. Hence, the results of the present study allow us to suggest that haplogroup X was the part of the ancestral gene pool for Altaian populations, being found both in northern and southern Altaians."

In addition, these same (2001) researchers indicated that the mtDNA haplogroup X haplotype present in the Altaians of Siberia is intermediate between Native Americans clades and that of Europeans. As a Russian research group observed, "American Indian and European haplogroup X mtDNAs ... are distantly related to each other". They propose however not an early European colonization of America, but that Altaians contributed to migrants bound for Europe and America; "The network further suggests that the Altaian X haplotypes occupy the intermediate position between European and American Indian haplogroup X mtDNA lineages" However, further research in 2003 indicated that the haplotype present in the Altaians is not intermediate between Native American clades and that of Europeans, and that the Native Americans probably split early from the others, with the split occurring "likely at the very beginning of their expansion and spread from the Near East, ... around, or after, the Last Glacial Maximum when the climate ameliorated".

One theory of how the X Haplogroup ended up in North America is it migrated from central Asia along with the A, B, C, and D Haplogroups, from an ancestor from the Altai Region of Central Asia. Two sequences of haplogroup X2 were sampled further east of Altai among the Evenks of Central Siberia. These two sequences belong to X2* and X2b. It is uncertain if they represent a remnant of the migration of X2 through Siberia or a more recent input.

This relative absence of haplogroup X2 in Asia is one of the major factors used to support the Solutrean hypothesis. However, the New World haplogroup X2a is as different from any of the Old World X2b, X2c, X2d, X2e, and X2f 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 postulates that haplogroup X reached North America with a wave of European migration about 20,000 BP 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.

In a 2008 article in the American Journal of Human Genetics, a group of researchers in Brazil (except for David Glenn Smith, of U.C. Davis) argue against the Solutrean hypothesis, stating: "Our results strongly support the hypothesis that haplogroup X, together with the other four main mtDNA haplogroups, was part of the gene pool of a single Native American founding population; therefore they do not support models that propose haplogroup-independent migrations, such as the migration from Europe posed by the Solutrean hypothesis ... Here we show, by using 86 complete mitochondrial genomes, that all Native American haplogroups, including haplogroup X, were part of a single founding population, thereby refuting multiple-migration models."

An abstract in a 2012 issue of the "American Journal of Physical Anthropology" states that "The similarities in ages and geographical distributions for C4c and the previously analyzed X2a lineage provide support to the scenario of a dual origin for Paleo-Indians. Taking into account that C4c is deeply rooted in the Asian portion of the mtDNA phylogeny and is indubitably of Asian origin, the finding that C4c and X2a are characterized by parallel genetic histories definitively dismisses the controversial hypothesis of an Atlantic glacial entry route into North America."

A 2015 report re-evaluates the evidence. Stating the possibility that evidence might be uncovered that supports a trans-Atlantic migration, they state that "X2a has not been found anywhere in Eurasia, and phylogeography gives us no compelling reason to think it is more likely to come from Europe than from Siberia. Furthermore, analysis of the complete genome of Kennewick Man, who belongs to the most basal lineage of X2a yet identified, gives no indication of recent European ancestry and moves the location of the deepest branch of X2a to the West Coast, consistent with X2a belonging to the same ancestral population as the other founder mitochondrial haplogroups. Nor have any high-resolution studies of genome-wide data from Native American populations yielded any evidence of Pleistocene European ancestry or trans-Atlantic gene flow."

Tree

This phylogenetic tree of haplogroup X subclades is based on the paper by Mannis van Oven and Manfred Kayser Updated comprehensive phylogenetic tree of global human mitochondrial DNA variation and subsequent published research.

Popular culture

In his popular book The Seven Daughters of Eve, Bryan Sykes named the originator of the European-American subset of this mtDNA haplogroup - corresponding to X2 - "Xenia".