African admixture in Europe

African admixture in Europe refers to the European presence of human genetic polymorphisms, which are considered to be evidence for movements of people from Africa to Europe in both the prehistoric and historic past, often indirectly via the Middle East.

Migratory times and vectors

First Settlers

While there is still a possibility that some archaic homo sapiens genes contributed to the modern European genepool, most of the genetic makeup of Europeans are thought to be traceable to migrations Out of Africa by anatomically modern humans (homo sapiens sapiens). These people were first seen in Africa, starting 150,000-200,000 years ago, and started entering Europe after 45,000 years ago, most likely chiefly via routes through Western Asia and the Balkans. (See Peştera cu Oase.) At this time Neanderthals lived in Europe and areas of Southwest and Central Asia. Anatomically speaking, these first anatomically modern Europeans are the so called "Cro Magnon".

Duration within refuges

From this early period humans progressed westward and by 24,000 years ago as all traces of Neandertals disappearred from Europe, humans settled in for the long duree of the Last Glacial Maximum a period between 25,000 years ago until at least 19,000 years ago when most of Europe was uninhabitable most of the time. At times after this, and until the end of the Younger Dryas, much of Europe may have also been inhospitable, during these periods it is believed that the ancient Europeans spent most of their lives in refuges, Southern France and Iberia, Italy, the Balkans and Greece, and regions East of the Carpathian mountains in Eastern Europe.

There is archaeological evidence that the warmer climate of the Epi-Paleolithic and/or Mesolithic periods (the terms are not always clearly distinguished) was another period of important new African and Asian influxes, which are thought to have had a major impact on the genetic make-up of the human population still in Europe. During this period most of Western Europe's ancient peoples lived along the coasts or lowland rivers or swamps, the diet was rich in shellfish, indicating they had maritime capability and thus the potential for gene flow between North Africa and Iberia. There is evidence of gene flow both from Africa and to Africa during this period.

Neolithic to the end of the prehistoric

During Africa's late stone age, a significant migration from Africa across the Sinai appears to have occurred around the time of the Levantine Natufian culture, which in turn was one of the sources of the farming cultures which developed in the Fertile Crescent and then spread into Europe via the Balkans. Triticeae cultivation and pastoralism traveled quickly from SE Europe via at least two migration routes: one was along the Danube basin in central Europe and reaching Britain by 5100 BC, the largest expansion phase of this culture is known as the "Linear Pottery" or LBK culture. Early genetic studies of Europe this period suggest this migration was important in defining Europe's modern genetic population. However signs of African contribution are now most evident in Europeans living around the Eastern Mediterranean. In this region the most important expansion of Neolithic people is associated with the Impressed or Cardial Ware culture. In contrast to the LBK, these cultures were more focused on caprids. The Cardial culture was sea faring and linked Northern Africa with Southern Europe. Some if these animals and cereals (such as millet) may have come from North Africa. Domesticated cattle in Southern Iberia as late as the bronze age carried African mitochondrial DNA.

Historic period

In historical times, there has been a well-documented period of North African influence in Southern Europe, especially southern Iberia and Southern Italy, during the time of the Islamic Golden Age. The genetic effect of this period on modern European populations is the subject of on-going discussion (see below). In more recent history, the peoples of Europe and Africa came into contact during the, the exploration and colonisation of Africa and as a consequence of 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.

Assessing African genetic contributions in non-Africans

During the primary migration out of Africa, the exiting population entered a bottleneck that isolated them partially from the diversity of sub-Saharan Africa. As people entered Eurasia and migrated away from the earliest areas of settlement they further isolated themselves from the core areas of diversity. These two process resulted in genetic drift of the most extreme migrants relative to their sub-Saharan counterparts. In addition mutations and recombination caused further divergence between Eurasians and Africans. Modern Europeans exist in both North South and East West genetic clines. For example, with HLA the people of Ireland have the highest allele and haplotype frequencies in Mainland Europe (indicating the greatest loss of diversity, See AH8.1), the frequencies of the major haplotypes decline over hundreds of miles, such that by the time one reaches Japan, no aspect of these haplotypes remain. Similar haplotype clines are evident from East Asia westward.

These genetic clines are evidence of genetic drift and selective evolution that has taken place and despite diffusion the geographic distances and barriers are sufficient to maintain regional genetic character. Whenever gene flow occurs into populations that have lost diversity, this gene flow increases diversity, however this increase in diversity may not reveal the direction. Consequently genetic studies investigate relevant markers, such as a specific allele, haplotype, or tandem repeat that is enriched in 1 of several potential sources of gene flow.

Examples of gene flow

Some DNA polymorphisms are shared by Europeans, West Asians, North Africans and Sub-Saharan Africans. Examples of such variants include the y-chromosomal haplogroup E1b1b and mitochondrial haplogroup M1.

This sharing of polymorphisms is the result of long distance movements of peoples between Sub-Saharan Africa and Eurasia that involved traversing 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.

In some cases, genes found in Africa and Europe may have a common origin in Asia (for example possible Y haplogroups R1 and T), or else have moved from Europe directly into Africa (for example mitochondrial haplogroups H1, H3. Such back migrations between Africa and Eurasia also complicate defining admixture.

Geographical influences

The Mediterranean Sea and the Sahara Desert are 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 15 km 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

distribution of Haplogroup E (Y-DNA)

Post LGM Y chromosome flow from Africa to Europe is primarily represented by haplogroup E. Haplogroup E entered Europe, predominantly through its subclade E1b1b1, which is thought to have emerged about 22,000 years ago in East Africa and branches of it are thought to have migrated to the Middle East by 11,000 years ago during the late Pleistocene or early Neolithic periods.

Entering the late mesolithic Natufian culture, the E1b1b1a2 (E-V13) sub-clade has been associated with the spread of farming from the Middle East into Europe either during or just before the Neolithic transition. E1b1b1 lineages are found throughout Europe but are distributed along a South-to-North cline, with a E1b1b1a mode in the Balkans.

In separate migrations, E lineages in the form of the E1b1b1b subclade 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%. The timing of this movement is one given widely variant estimates at this time.

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(not including E1b1b) 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 E lineages which are not E1b1b are very low, usually less than 1%. For example Cruciani et al. 2004, report such lineages at 2% in Southern Portugal, 4% in Northern 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 occurrences in Europe, but recently the haplogroup was detected in seven males with the same surname who were in Britain.

X chromosome

In recent times it has been proposed that X chromosal haplotypes are particularly useful for locating some of the oldest surviving non African lineages in humanity.

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 have been associated with the historical events, such as the African slave trade. Alternatively they may represent prehistoric migrations from North Africa. Gonzalez et al 2003 revealed that most of the L lineages in Iberia 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. However, according to Pereira et al. (2005), the sub-Saharan lineages found in Iberia matched lineages from diverse regions in Africa. They suggest this pattern is more compatible with recent arrival of these lineages after slave trade was initiated in the 15th century. According to the study, alternative scenarios, that invoke much older and demographically more significant introductions (Gonzalez et al. 2003) or that claim a substantial role of the Roman and/or Islamic periods on the introduction of sub-Saharan lineages, seem unlikely. The study detected L lineages at an average frequency of 3.83%(40 out of 1,045) in Iberia. .

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 ten 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.

HLA

Distribution of the B18-DR3 component of the A30-Cw5-B18-DR3-DQ2 haplotype in the Old World

One of the most convincing evidences of gene flow from Africa is the A30-B18-DR3 haplotype, this or component haplotypes reaches a peak frequency in Sardinia with components of ~20%. Three independent studies conclude this haplotype is paleo North African in origin. Several HLA haplotypes and alleles that appear to be of recent Africa origin are found in Europe with frequencies declining from SW to north east similar to A30-B18-DR3. A number of other alleles including, Cw*0501, Cw*0701 and Cw*1601 appear restricted to European and African populations. This pattern has been described as being suggestive of a close historical relationship between Europeans and Africans.

Paleoanthropology

An analysis of human remains from the Natufian culture in Israel detected Sub-Saharan influences. The Natufian culture is often considered the source of the European Neolithic. Some studies suggest that the Neolithic in Europe occurred by a "demic diffusion" of farmers from the Near East. According to Brace et al. (2008), there is clear evidence of Sub-Saharan influences in the Natufian samples. They further argue that these influences would have been diluted by the interbreeding of the Neolithic farmers from the Near East with the indigenous foragers in Europe. Ricaut et al. (2008) associate the Sub-Saharan influences detected in the Natufian samples with the migration of E1b1b lineages from East Africa to North Africa and then to the Levant.

See also

• Archaeogenetics
• Archaeogenetics of the Near East
• Caucasoid
• European ethnic groups
• Human genetic variation
• Population genetics
• White people
• Genetic history of the British Isles
• Genetic history of Europe

References

• Brace; et al. (2005), The questionable contribution of the Neolithic and the Bronze Age to European craniofacial form, doi:10.1073/pnas.0509801102 {{citation}}: Explicit use of et al. in: |last= (help)
• Cavalli-Sforza (1997), "Genes, Peoples and Languages", PNAS, retrieved 2009-07-22
• Cherni; et al. (2008), Post-Last Glacial Maximum Expansion From Iberia to North Africa Revealed by Fine Characterization of mtDNA H Haplogroup in Tunisia, doi:10.1002/ajpa.20979 {{citation}}: Explicit use of et al. in: |author= (help)
• Ennafaa; et al. (2009), "Mitochondrial DNA Haplogroup H structure in North Africa", BMC Genetics (8) {{citation}}: Explicit use of et al. in: |author= (help)
• Lancaster, Andrew (2009), "Y Haplogroups, Archaeological Cultures and Language Families: a Review of the Multidisciplinary Comparisons using the case of E-M35" (PDF), Journal of Genetic Genealogy, 5 (1)
• Ricaut; et al. (2008), "Cranial Discrete Traits in a Byzantine Population and Eastern Mediterranean Population Movements", Human Biology 80(5):535-564., doi:10.3378/1534-6617-80.5.535 {{citation}}: Explicit use of et al. in: |last= (help)