Talk:Genetic history of Europe/ Sub-Saharan African admixture in Europe

Sub-Saharan DNA admixture in Europe refers to the presence in Europe of human genetic lineages typical of the peoples of Sub-Saharan Africa. Since the settlement of Europe 45,000 years ago, evidence suggests that there has been gene flow between Africa and Europe in both prehistoric and historic times. . In more recent history, the peoples of Europe and Africa came into contact during the, the exploration and colonisation of Africa and also during the Atlantic slave trade. As a result of these recent contacts, lineages of African descent have also been detected in Europe. In general, African admixture is distributed along a South-to-North cline, with peaks in the Mediterranean region and Iberia.

Defining African admixture
Some DNA polymorphisms are shared by Europeans, West Asians, North Africans and Sub-Saharan Africans. Examples of such variants include the y-chromosomal haplogroup e3b and mitochondrial haplogroup M1. This sharing of polymorphisms is the result of long distance migration of peoples between Sub-Saharan Africa and Eurasia that involved traversing across North Africa and sometimes the Middle East. Consequently, the definitions African, Sub-Saharan African, North African will depend on the time frame of reference or the semantic preferences of any particular scientist. Due to prehistoric migrations in and out of Africa, North African populations tend to exhibit allele frequencies that are intermediate between Sub-Saharan Africa and Eurasia. Due to this complex genetic profile of Africa, African admixture in Europe could be the result of direct contact with Sub-Saharan Africans, or indirectly through contact with North Africans with Sub-Saharan affiliations.

Back migrations between Africa and Eurasia also complicate defining admixture. For example Haplogroup U is a Eurasian haplogroup that entered North and East Africa in the Paleolithic. One clade of haplogroup U, U6a1 is known to have expanded from East Africa back into Europe. The y chromosome haplogroup E1b1b is thought to have originated in East Africa and spread to Eurasia. One clade E-M34 is found in high frequencies in Ethiopia, but its presence there is thought to be the result of a back migration from the Near East. Thus the presence of E-M34 in Europe could either be from East Africa or the Near East.

Geographical influences
The Mediterranean Sea and the Sahara Desert were formidable barriers to gene flow between Sub-Saharan Africa and Europe. But Europe was periodically accessible by Africans due to fluctuations in the size and climate of the Sahara. At the Strait of Gibraltar, Africa and Europe are separated by only 15km of Ocean. At the Suez, Eurasia is connected to Africa forming a single land mass. The Nile river valley, which runs from East Africa to the Mediterranean Sea served as a bidirectional corridor in the Sahara desert, that frequently connected people from Sub-Saharan Africa with the peoples of Eurasia.

Y-DNA
Gene flow from Sub-Sahran Africa is primarily represented by haplogroup E, the most prevalent haplogroup in Sub-Saharan Africa. Haplogroup E entered Europe, predominantly through its subclade E1b1b, in the late pleistocene or early neolithic periods. E1b1b is thought to have emerged about 22,000 years ago in East Africa and is suggested to have migrated to the Middle East by 11,000 years ago. Populations carrying these E lineages have been associated with the spread of farming from the Middle East into Europe during the Neolithic transition. E1b1b lineages are found throughout Europe but are distributed along a South-to-North cline, with peak frequencies in the Balkans. In separate migrations, E lineages appear to have entered Europe from Northwest Africa into Iberia. In a sample of European males, Cruciani et al observed Haplogroup E at a frequencies of 7.2%.

A major expansion of peoples throughout Sub-Saharan Africa occurred after the introduction of Agriculture 5,000 years ago. During the Bantu expansion people carrying Haplogroup E(xE3b) lineages dispersed across much of Sub-Saharan Africa from their original homeland near the border between Nigeria and Cameroon. The haplogroup most often associated with this expansion is E1b1a,which constitutes up to 48% of the African male gene pool. The presence of E1b1a lineages outside Africa can typically be associated with events that occurred after the Bantu Expansion, such as the trade in African slaves or the Moorish occupation of Iberia. In much of Europe Frequencies of Ex(E3b) are low, usually less than 1%. Cruciani et al 2004, report E(xE3b) lineages at frequencies of 4% in Northern Portugal, 2% in Southern Portugal, 2.9% in Istanbul and 4.3% among Turkish Cypriots. E1b1a is closely related to E1b1b, the most frequent clade in Europe. E lineages that are not E1b1a or E1b1b could therefore reflect either a recent expansion associated with E1b1a or ancient population movements associated with E1b1b.

Haplogroups A and B, are thought to have been the predominant haplogroups in Central and Southern Africa prior to the Bantu Expansion. Currently these haplogroups are less common than E lineages. In a sample of 5,000 African men, haplogroup A had a frequency of 5%. Haplogroup A has rare occurrencies in Europe, but recently the haplogroup was detected in 7 males with the same surname were in Britain.

MtDNA
Haplogroup L lineages are relatively infrequent(less than 1%) throughout Europe with the exception of Iberia, where frequencies as high as 11.7% percent have been reported. These lineages had previously been associated with the African slave trade. However an analysis by Gonzalez et al revealed that most of the L lineages matched Northwest African L lineages rather than contemporary Sub-Saharan L lineages. The authors suggest this pattern indicates that most of the Sub-Saharan L lineages entered Iberia in prehistoric times rather than in more recent periods.

Haplogroup L lineages are present at low frequencies in Eastern Europe. Though a high diversity of African mtDNA lineages have been detected, few lineages have accumulated enough mutations in Europe to form monophyletic clusters. Malyarchuk et al 2005 detected only two monophyletic clusters, L1b and L3b, in Russians with an estimated age no greater than 6,500 years. Malyarchuk et al 2008 identified African L1b, L2a, L3b, L3d and M1 clades in slavic populations at low frequencies. L1b, L3b and L3d had matches with West African populations, indicating that these lineages probably entered Europe through Iberia. One lineage L2a1a, appeared to be much older, indicating that it may have entered Europe in prehistoric times. This clade was possibly related to L2a1 clades identified in 10 individuals of Ashkenazi heritage from France, Germany, Poland, Romania and Russia. L2a lineages are widespread throughout Africa, as a result, the exact origins of this lineage are uncertain.

Haplogroup M1 is also found in Europe at low frequencies, but it is not uncommon in Southern Europe. In a study by Gonzalez et al 2007, haplogroup M1 had an overall frequency of 0.3% in the samples that were analyzed. The highest frequencies were found in Sicily where 3.8% of the population were members of haplogroup M1. The origins of haplogroup M1 have yet to be conclusively established. However, one clade of haplgroup M1, M1a is widely accepted to be of East African origin. About 40% of all clades of M1 found in Europe are M1a and consequently of recent East African origin.

Autosomal

 * Measures of genetic distance between Europe and Sub-Saharan are generally smaller than Genetic distances between Africa and other continental populations. Cavalli-Sforza states that the relatively short genetic distance is likely due to prehistoric admixture.
 * A 2009 study by Auton et al found a North-South Cline of Hapmap Yoruba haplotypes (YRI) in Europe. The study determined that South and Southwest subpopulations had the highest proportion of YRI This distribution is indicative of recurrent gene flow into Europe from both the Southwest and the Middle East. The authors suggest that the haplotype sharing between Europe and the YRI are suggestive of gene flow from Africa, albeit from West Africa and not necessarily North Africa.
 * A 2007 study conducted at Penn State University found low levels of African admixture(2.8-10%) that were distributed along a North South cline. The authors suggest that the distribution of this African admixture mirrors the distribution of haplogroup E3b-M35(E1b1b).
 * A principal component analysis of data from Human Genome Diversity Project by Reich et al detected a West-to-East gradient of Bantu related ancestry across Eurasia. The authors suggest that after the Out of Africa migration, there was most likely a later Bantu-related gene flow into Europe.