Sub-Saharan DNA admixture in Europe

Sub-Saharan DNA admixture in Europe refers to the way in which Sub-Saharan African DNA is lightly 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 Sub-Saharan African admixture in Europe today ranges from a few percent in the Iberian Peninsula to almost none 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 Moorish (North Africa) expansion and the African slave trade.

Between 1500 and up to 1900, about four million African slaves were transported to island plantations in the Indian Ocean; eleven million were taken by the Atlantic slave trade to the Caribbean, North America, Central America, and, above all, South America—mainly to Brazil; an estimated eight million were transported north across the Sahara to North Africa by the Arab slave trade. [Pier M. Larson, [http://backintyme.com/rawdata/larson01.pdf Reconsidering Trauma, Identity, and the African Diaspora: Enslavement and Historical Memory in Nineteenth-Century Highland Madagascar] , "William and Mary Quarterly" 56, no. 2 (1999): 335-62.] Of the vast majority shipped by the Atlantic trade, most were sent directly to the Americas as part of an Atlantic triangular trade, and so never saw Europe. Most of the trans-Saharan trade ended at markets in North Africa and the Middle East.

In the same period about 200,000 Africans were sold into Europe via the Atlantic slave trade [Hugh Thomas, "The Slave Trade: The Story of the Atlantic Slave Trade: 1440-1870" (New York: Simon and Schuster, 1997), 804] , and these seem to have "vanished" without a trace. However, they can account for much of the presence of Sub-Saharan African DNA markers in the modern European gene pool, although it is not clear how much (in opposition to traces from pre-historic and medieval migrations). It also must be noted that levels of African DNA from these relatively recent arrivals are too low to have had an appreciable effect on Phenotypes.

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.

It should be noted that differences among the major population groups of the world constitute only 3% to 5% of genetic variation, while within-population differences among individuals account for 93% to 95% of such variation. [ [http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=PubMed&list_uids=12493913&dopt=AbstractPlus Genetic structure of human populations.] Rosenberg NA, et al, Dec 2002]

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. A person born around the year 2000 would have had about slightly more than half a million ancestors alive in the year 1500 alone [If one calculates 20 generations in 500 years, that would make a total of 524,288 individual genealogical positions (without pedigree collapse) in the 20th generation alone; plus an exactly equal number, minus 1, in all the preceding generations (1 to 19th generations). Notice that the number doubles with each generation, thus, in the 21st generation one has 1,048,576 individual genealogical positions in that generation alone.] , if there were no pedigree collapse, but only two of them would have carried that person's 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 tend to be mobile, women stay in place), 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).

mtDNA

A study by Gonzalez et al. 2003 [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=12627534&query_hl=2 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).

In a study by Plaza et al. 2003 ["L haplogroups are relatively infrequent in Italians (with a maximum of 8.1% in South Italians) and Iberians (with a maximum of 6.1% in Central Portuguese). On the contrary, L haplogroups are distributed in all North African populations at high frequencies (from 26% in South Berbers to 43.5% in Mauritanians) with the exception of Mozabites (12.9%) and Moroccan Berbers (3.2%). In fact, the frequency of the L haplogroups in Moroccan Berbers is similar to that found in Iberians and Italians. The frequency of the L haplogroups might represent the sub-Saharan genetic flow into the populations analysed, which has shown to be substantial in NW Africa but very limited in European populations. (...) Three Italian L sequences have been described throughout Africa, and the remaining five are not found in >1,000 sub-Saharan individuals. Thus, the presence of L sequences cannot be attributed to migration from NW Africa, and may instead represent gene flow from other sources, such as the Neolithic expansion or the Roman slave trade.", [http://www3.interscience.wiley.com/cgi-bin/fulltext/118863897/HTMLSTART Plaza et al. (2003), Joining the Pillars of Hercules: mtDNA Sequences Show Multidirectional Gene Flow in the Western ] ] , L haplogroups were found to be "relatively infrequent in Italians (with a maximum of 8.1% in South Italians) and Iberians (with a maximum of 6.1% in Central Portuguese)".

In a study by Pereira et al. 2005, [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=16201138&dopt=Abstract Pereira et al. (2005) African female heritage in Iberia: a reassessment of mtDNA lineage distribution in present times] ] sub-Saharan mtDNA L haplogroups were found at rates of 0.62% in a German-Danish sample, 0.94% in Sicilians, 1% in the British/Irish, 2.38% in Albanians, 2.86% in Sardinians. This paper which provides a deeper and more global insight into the African female influence in Iberia shows that the mean frequency reaches 3.83% in Iberians. The frequency is clearly higher in Portugal (32 sequences in 549 individuals; 5.83% with a high frequency of 11% in southern Portugal) than in Spain (8 out of 496; 1.61% with a higher frequency of 3.26% in Galicia) and without parallel in the rest of Europe.

For comparison, in North Africa, in a study by Rando et al. 1998, levels of sub-Saharan mtDNA ran from 4% in Moroccan Berbers, 17% in Algerian Berbers, 21% in non-Berber Moroccans to 44% in Saharans and Mauritanians, and 82% in Tuareg ["The mitochondrial data of the NorthwestAfrican populations (Berbers from Morocco and Algeria, Moroccans, West-Saharans, Mauritanians, Tuareg) show a mosaic composition of mtDNA types, with a pronounced gradient of sub-Saharan lineages from north to south: at the one extreme, the Berbers from Morocco have a predominantly European (Iberian) affinity, while at the other extreme, the Tuareg are closely related to sub-Saharan West Africans as represented by several Senegalese groups in this study, whereas the West-Saharans and Mauritanians are somewhat intermediate. It is remarkable that the Tuareg bear little mitochondrial resemblance to the Berber populations, although they speak a Berber language.Despite the seemingly European make-up of the Berber mtDNA pool, there are a few lineages which are specific to North Africans and of a probable early Near Eastern origin", [http://www3.interscience.wiley.com/cgi-bin/fulltext/119102385/PDFSTART Rando et al. (1998), Mitochondrial DNA Analysis of Northwest African Populations Reveals Genetic Exchanges with European, Near-Eastern, and Sub-Saharan populations. Ann Hum Genet; 62:531-550] ] .

Y-DNA

For the reasons outlined above, sub-Saharan Y-DNA markers are much less common in Europe. The small presence of the Haplogroups E(xE3b) (i.e. clades of E other than E3b) and Haplogroup A in Europe is almost exclusively attributable to the slave trade, as these haplogroups are characteristic of western, central and southern Africans and are barely observed elsewhere. [Sanchez et al. (2005). "High frequencies of Y chromosome lineages characterized by E3b1, DYS19-11, DYS392-12 in Somali males". European Journal of Human Genetics; 13:856–866] The haplotypes have been detected in Portugal (3%), France (2.5% - in a very small sample), Germany (2%), Sardinia (1.6%), Austria (0.78%), Italy (0.45%), Spain (0.42%) and Greece (0.27%). By contrast, North Africans have about 5% paternal black admixture. [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?holding=npg&cmd=Retrieve&db=PubMed&list_uids=15042509&dopt=Abstract Cruciani et al. 2004] ,
[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15280900&query_hl=13&itool=pubmed_docsum Flores et al. 2004] ,
[http://www3.interscience.wiley.com/cgi-bin/abstract/112139846/ABSTRACT Brion et al. 2005] ,
[http://dx.doi.org/10.1016/j.forsciint.2005.06.002 Brion et al. 2004] ,
[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11078479&dopt=Abstract Rosser et al. 2000] ,
[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15069642 Semino et al. 2004] ,
[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12927125&dopt=Abstract DiGiacomo et al. 2003] .]

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 studies by Rosenberg et al. 2002 [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12493913&dopt=Abstract Rosenberg et al. 2002] ] and Wilson et al. 2001 [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11685208&query_hl=15&itool=pubmed_docsum Wilson et al. 2001] ] failed to detect any sub-Saharan admixture in Scots, Russians, Basques, Frenchmen or Italians, while 1% was observed in Norwegians.

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 Arabian Peninsula shortly after the invention of AgricultureFact|date=June 2007 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.

The Arnaiz-Villena controversy

An often-cited study from 2001 by Antonio Arnaiz-Villena et al. [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11260506&query_hl=20 Arnaiz-Villena et al.] ] which maps 28 world population based on the HLA DRB1 locus, concluded 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. [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11543891&query_hl=11&itool=pubmed_docsum Abstract] ] This second paper dealt specifically with the relatedness of Palestinians and Israelis and was subsequently "deleted from the scientific literature" because, according to the editor-in-chief Nicole Suciu-Foca, it "confounded the elegant analysis of the historic basis of the people of the Mediterranean Basin with a political viewpoint representing only one side of a complex political and historical issue". [Human Immunology, Vol: 62, Issue: 10, October, 2001, pp1063]

Erica Klarreich's report on the controversy further quotes Suciu-Foca as saying that the reaction against the paper was so severe that "We would have had mass resignations and the journal would have been destroyed if this paper were allowed to remain." [ [http://www.nature.com/doifinder/10.1038/35106696 "Nature"] ] The controversy was further reported on in numerous locations including The Observer. [ [http://observer.guardian.co.uk/international/story/0,6903,605798,00.html "The Observer"] ]

Shortly after this, three respected geneticists, Luca Cavalli-Sforza, Alberto Piazza and Neil Risch, argued that the scientific limitations of Arnaiz-Villena's methodology. [ [http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v415/n6868/full/415115b_fs.html "Nature"] ] 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."

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 [ [http://www.goodrumj.com/PC-HGHG.jpgMDS plot] ] ). Then, Ayub et al. 2003 [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=14533184 Ayub et al. 2003] ] did the same thing using 182 loci (view dendrogram [ [http://dienekes.angeltowns.net/articles/greekadna/mfig001.gifdendrogram] ] ).

Another study was conducted in 2004 at Skopje's University of Ss. Kiril and Metodij, using high-resolution typing of HLA-DRB1 according to Arnaiz-Villena's methodology. Contrary to his earlier conclusion, no sub-Saharan admixture was detected in the Greek sample. [http://www.blackwell-synergy.com/links/doi/10.1111%2Fj.1399-0039.2004.00273.x High-resolution typing of HLA-DRB1 locus in the Macedonian population] ]

A 2006 study by Tunisian scientists again asserted the relatedness of the Greeks to sub-Saharans by calculating genetic distances at the DRB1 locus, [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16473309&query_hl=25&itool=pubmed_docsum HLA genes in Southern Tunisians (Ghannouch area) and their relationship with other Mediterraneans.] ] the same marker used in the controversial Arnaiz-Villena paper. Both papers interpreted those results as suggesting an admixture occurred due to the displacement of Egyptian-Ethiopic people during the Pharaonic period. However, the Tunisian scientists failed to analyze any new Greek genetic material, relying solely on the data contained in the earlier Arnaiz-Villena paper, and no Greek laboratory contributed to their research. [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16473309&query_hl=25&itool=pubmed_docsum HLA genes in Southern Tunisians (Ghannouch area) and their relationship with other Mediterraneans.] ]

The credibility of Arnaiz-Villena was seriously damaged after he was suspended without pay from the Hospital Doce de Octubre in Madrid, where he heads the department of immunology and molecular biology, after being charged with embezzlement of funds. [ [ http://www.bmj.com/cgi/content/full/324/7339/695 Controversial immunologist faces court case] ] In addition to this charge, Dr Arnaiz-Villena is facing allegations of "moral harassment" at the Universidad Complutense de Madrid, where he chairs a research and teaching immunology unit.

Footnotes

ee also

*Human genetic variation
*Race
*Race (historical definitions)
*White (people)
*Caucasoid, Negroid, Mongoloid, Australoid, Capoid
*Social interpretations of race

External links

* [https://www3.nationalgeographic.com/genographic/index.html The Genographic Project] - The Genographic Project's web site
* [http://www.scs.uiuc.edu/~mcdonald/WorldHaplogroupsMaps.pdf World Haplogroups Maps]