Sub-Saharan DNA admixture in Europe

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Sub-Saharan African DNA is scattered throughout the European continent.

Not every population has been studied yet, but enough have so that a picture is starting to emerge. The amount of Black admixture in Europe today ranges from a few percent in the Iberian Peninsula to almost nil around the Baltic. It seems to show a decreasing cline from the Southwest to the Northeast, which corresponds with the areas most affected by the African slave trade or the Moorish (North Africa) expansion.

Between 1500 and up to 1900, about four million African slaves were transported to island plantations in the Indian Ocean; eleven million were taken to Central America and, mainly, to South America; half a million were taken to North America; and eight million were transported north across the Sahara to North Africa.^[1]

In the same period about 200,000 Africans were sold into Europe via the Atlantic slave trade.^[2] These seem to have vanished without a trace. And yet, the traces are present in the modern European Gene pool. Though it must be noted that levels of African DNA are too low to have had an appreciable effect on Phenotypes.

1 Approaches to detecting admixture
2 The Arnaiz-Villena controversy
3 Footnotes
4 See also
5 External links

[edit] Approaches to detecting admixture

There are three main approaches to detecting continent-of-ancestry admixture: gender-specific markers (Mitochondrial DNA and Y-chromosome DNA), neutral autosomal markers, and adaptive autosomal markers.

Each approach has strengths and weaknesses in distinguishing ancient sub-Saharan markers (from our species' common origin in Africa) from more recent ones. Some approaches are more quantifiable than others.

[edit] Gender-specific markers

Mitochondrial DNA (mtDNA) and Y-chromosome DNA trace individual lineages, Matrilineal and patrilineal, respectively.

They do not mix or recombine at each generation. Hence, they can identify different population migrations. The descendants of the sub-Saharan Africans who first began the Great Diaspora about 70 millennia ago can be distinguished from the sub-Saharan groups who helped to re-colonize Europe after the glaciers melted 16 millennia ago, and from sub-Saharan people who crossed or went around the Mediterranean in Ancient Egyptian or Roman times or thereafter as slaves, soldiers, settlers, or traders.

Although mtDNA and Y-DNA can quantifiably estimate a modern population's overall admixture, the approach cannot measure an individual's genealogy. You had about a million ancestors alive in the year 1500, but only two of them carried your mtDNA and Y-DNA. Differences between the patterns of mtDNA and Y-DNA can suggest why populations migrated: military conquest tends to propagate Y lineages but leave mtDNA lineages in place (men conquer, women get raped), mass migrations in search of a new homeland tend to propagate mtDNA and Y lineages equally, and a slave trade tends to propagate mtDNA lineages but leave Y lineages in place (female slaves are encouraged to propagate, males are not).

According to a summary study by Pereira et al. 2005, sub-Saharan mtDNA L haplogroups were found at rates of 0.62% in a German-Danish sample, 1% in the British, 3.83% in Iberians (Portuguese and Spanish), 2.38% in Albanians, 2.86% in Sardinians and 0.94% in Sicilians.

Another study by Gonzalez et al. 2003 found L haplogroups at rates of 0.1% in Scotland, 0.4% in England, 0.7% in North Germany, 1.4% in France, 2.9% in Galicia, 2.2% in Northern Portugal, 4.3% in Central Portugal, and 8.6% in Southern Portugal (Alentejo and Algarve) (note that these figures do not count the L3 lineage, which may be of ancient introduction and so remains ambiguous). For comparison, sub-Saharan mtDNA runs 21.8% in North Africa.

Sub-Saharan African Y-chromosomes are much less common in Europe, for the reasons discussed above. However, Haplogroups E(xE3b) and Haplogroup A spread to Europe due to migrations from Northeast Africa, rather than the slave trade. The haplotypes have been detected in Portugal (3%), Spain (0.42%), Germany (2%), Austria (0.78%), France (2.5% in a very small sample), Italy (0.45%), Sardinia (1.6%) and Greece (0.27%). By contrast, North Africans have about 5% paternal black admixture.^[3]

[edit] Neutral autosomal markers

Neutral autosomal markers are odd fragments of DNA that do not affect a person's physical traits.

Because they are autosomal (within the Nuclear DNA that is subject to Meiosis), such markers reflect the recombination of paternal and maternal DNA with each generation. Hence, they are less useful than mtDNA or Y-DNA in tracking migrations and they are less precise as to time.

This makes it hard to tell if any particular marker dates from the 1500-1800 slave trade, or from the post-glacial re-colonization of Europe, or from some time in between. On the other hand, neutral autosomal markers are useful for individual genealogies since they reflect just how much of an individual's genome came from which population group. Two recent studies by Rosenberg et al. 2002 and Wilson et al. 2001 failed to detect any sub-Saharan admixture in Scots, Russians, Basques, Frenchmen or Italians, while 1% was observed in Norwegians.

[edit] Adaptive autosomal markers

Adaptive autosomal markers are those that evolved and spread because they enhance survivability.

The best-known example is HbS, which produces the sickle-cell trait. This Allele emerged in Western Equatorial Africa shortly after the invention of Agriculture and spread to Europe because it confers near immunity to the most lethal form of Malaria.

There are many other such traits and they have two main advantages for population studies: #First, they have been well-studied for centuries, so different strains are easily identified and tracked. "Second, because their adaptive advantages are known, their dates of origin and spread are also known to reasonable precision. The main disadvantage of adaptive autosomal markers is that they cannot tell what fraction of a population came from which ancestry. That HbS is found in, say, 10 percent of some European population does not mean that ten percent have sub-Saharan ancestry; it may simply be that many of those lacking the trait in the past died without progeny due to malaria.

[edit] Horn African influences

The neutrality and factual accuracy of this section are disputed.
Please see the relevant discussion on the talk page.

There are a number of genetic markers which are characteristic of Horn African and North African populations (the latter acquired from Horn Africans) which are to be found in European populations signifying ancient and modern population movements across the Mediterranean. These markers are to be found particularly in Mediterranean Europe but some are also prevalent, at low levels, throughout the continent. Y-chromosome haplogroup E-M81, a derivative of Haplotype E3b which arrived in North Africa from the Horn of Africa in the Neolithic, is specific to North African populations and absent in Europe except for Iberia (Spain and Portugal) and Sicily.^[4] The general parent Y-chromosome Haplogroup E3b, which originated in modern day Somalia, is by far the most common in North and Northeast Africa, and is present throughout the majority of Europe, particularly in Mediterranean and South Eastern Europe, reaching its highest concentration in Sicily (27.3%), Albania (25.0%) and Greece (23.8%), but also with an important presence in countries like Germany, Austria or England[1].^[4] The spread of E3b can be traced to migrations in the Neolithic from the Horn of Africa throughout North Africa and the Middle East as well as to more recent exchanges across the Mediterranean.^[4]

[edit] The Arnaiz-Villena controversy

There's an often-cited study from 2001 by Arnaiz-Villena et al. which maps 28 world population based on the HLA DRB1 locus, concluding that "the reason why Greeks did not show a close relatedness with all the other Mediterraneans analyzed was their genetic relationship with sub-Saharan ethnic groups now residing in Ethiopia, Sudan, and West Africa (Burkina Faso)." Later that year, the same data was used in another study by the same author published in a different journal.[2] This second paper dealt specifically with the relatedness of Palestinians and Israelis and was subsequently retracted, drawing a lot of attention. Erica Klarreich accused the journal of pulling the study because of its politically sensitive content,[3] creating a controversy that was reported on in The Observer.[4]

Shortly after, three respected geneticists - Luca Cavalli-Sforza, Alberto Piazza and Neil Risch - countered this accusation by pointing out the scientific limitations of Arnaiz-Villena's methodology.[5] They stated that "Using results from the analysis of a single marker, particularly one likely to have undergone selection, for the purpose of reconstructing genealogies is unreliable and unacceptable practice in population genetics.", making specific allusion to the findings on Greeks (among others) as "anomalous results, which contradict history, geography, anthropology and all prior population-genetic studies of these groups."

Indeed, though adaptive markers of sub-Saharan origin like HbS have been detected in Greece,[6] no multiple-marker analysis has ever duplicated Arnaiz-Villena's results. In The History and Geography of Human Genes (Princeton, 1994), Cavalli-Sforza, Menozzi and Piazza grouped Greeks with other European and Mediterranean populations based on 120 loci (view MDS plot). Then, Ayub et al. 2003 did the same thing using 182 loci (view dendrogram).

The disputed data continues to be cited all over the Internet, mostly by White Supremacists, Afrocentrists and Macedonian nationalists who have political motivations to relate modern or ancient Greeks to black Africans. However, it's no longer referenced by population geneticists in contemporary research, mainly due to the criticism of Cavalli-Sforza et al.