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Intro
Y chromosomal DNA and mtDNA haplogroups have been used to identify information about a population’s deep ancestral roots going back many thousands of years ago. [1]. From a Y-DNA perspective, South Slavs contain the highest frequency of haplogroup I2, postulated to have originated in the Balkans before the Neolithic. Such data is interpreted as evidence of genetic continuity between modern South Slavs and the ancient Balkan inhabitants. In addition, South Slavic men also harbour significant frequencies of E3b, R1a, R1b, and J2; making South Slavs the most genetically diverse Europeans. This highlights the fact that the Balkans has played pivotal role in the paths of many a migration. It should be noted, however, that mtDNA and Y-chromosome based genetic studies do not give a complete picture of the overall genetic structure of a population; but are useful in highlighting ancient migratory patterns. In addition, due to methodological factors, different studies produce often wide variations in the estimated ages of certain haplogroups, thus arriving at potentially different conclusions. Consequently, further studies are needed before we can obtain more accurate and generalisable conclusions.
Background
Historically, the Balkans has been home to a diverse range of peoples. Analysis of extant Balkan populations has so far given interesting insights into the timing and patterns by which Europe was populated.
The colonization of Europe occurred c. 35 to 40 kYa (during the Palaeolithic), when White Man split off from his West Asian cousins and migrated into Europe, with the subsequent development of the Aurignacian culture. One route of entry was via the Strait of Bosporus into South Eastern Europe (the other major route being along the northern contours of the Black Sea, the Pontic steppe) [2]. A drop in the earth’s temperature occurred circa 20 kYa, an event known as the Last Glacial Maximum. This rendered much of Europe uninhabitable, except a few areas in southern Europe which became ice age refugia. One of these refugia was in the Balkans. As the climate began to warm from 14 kYa, Europe was repopulated by offspring of the few survivors, hence making Europeans the most homogeneous of all populations.
Subsequently, the Neolithic Revolution heralded the transition of human life from hunter-gathering to a more settled life of agro-pastoralism. This began in the Levant, over 10 kYa, and is postulated to have spread into Europe, arriving into the Balkans by 7 kYa. One issue which scholars debate is whether this spread occurred via an actual migration of peoples from the Middle East into Europe, or whether it was merely a flow of ideas via cultural contact. Population Genetic Analysis of today’s South Eastern Europeans has been able to shed some light on the issue. The introduction of farming is associated with the development of several Neolithic culture complexes in the Balkans (See ‘’Prehistoric Balkans’’). However, it was not until much later that distinct, recognizable “ethnic” groups arose. The hypothesized arrival of the Proto-Indo-Europeans during the Early Bronze Age, and their subsequent interaction with the Palaeolithic “Old Europeans”, produced the first historically recorded peoples in the Balkans- the Hellenes, the Illyrians and the Thracians[3]. Such a “Kurganization” scenario of ethnogenesis is by no means proven, and intense academic debates continue regarding this theory.
During antiquity, the Balkan peoples were often in contact with other cultures, especially the Celts of central Europe, and subsequently (and perhaps more importantly) the Romans. During the Roman zenith, Roman culture flourished, and historical sources suggest that there were population movements associated with the settlement of Roman veterans, soldiers and traders in the newly conquered Balkan provinces; counterset by the removal, and even systematic extermination, of rebellious Balkan tribes. In the early middle ages, there were numerous invasions by “barbarian” groups such as the Goths, Huns, Gepids, Lombards, Avars, Bulgars, and Slavs, during which time the Balkans was apparently “devastated” and “depopulated”.
The population genetic studies that will be discussed in this article have helped demonstrate a remarkable continuity between all modern European populations and their Palaeolithic forefathers. Indeed, this supports new research in the fields of archaeology and social anthropology which have seriously challenged the theory of “mass invasions” associated with the ’’Völkerwanderung’’.
Y-DNA Haplogroups
Y-DNA haplogroups are genetic loci found on the non-recombining portion of the Y chromosome (the ‘male’ chromosome). They are paternally inherited, and their respective relationships can be established by examining the pattern of single nuclear polymorphism (SNP) mutations which have accumulated over hundreds of thousands of years.
Over ninety per cent of the South Slavic men carry one the following five Y-DNA haplogroups: R1a, R1b, E1b1b (formerly E3b), I2 (formerly I1b), and to a lesser extent, J2.
Haplogroup I is ‘unique’ to European populations, enjoying highest frequencies in the western Balkans. Haplogroup I is believed to have originated in Europe before the LGM, between 28-22 kYa, in descendants of men that arrived from the Middle East circa 20-25 kYa [4]. Indeed, its closest phylogenetic relative is Haplogroup J, which dominates the Y-DNA diversity of Near Eastern men. Its genetic age closely corresponds to archaeological evidence of an arising culture in Europe, the Gravettian culture, which was found from Western Europe through to Central Europe and even Russia. With the onset of the LGM, much of Europe became uninhabitable- except a few areas in southern Europe [5]. During this glacial maximum, Western Europe (Iberia) and Central- Eastern Europe (the northwestern Balkans) became genetically and culturally isolated from each other, and an Epi-Gravettian culture persisted in the latter[6]. A population of survivors bearing the haplogroup I lineage survived in the Balkan refuge. With the Allerod Oscillation from c. 14 kYa, the climate began to improve. Subsequently, Europe was re-colonized by the few survivors, during which time haplogroup I2 differentiated from the parent I* group (11.1 kYa according to Pericic, 10.7 kYa according to Rootsi 2004). The bearers of I2 (named the Dinarics) [7] consequently repopulated much of eastern and southeastern Europe, especially during the early Holocene period[8]. {In contrast, the origin of I1 is still debated; Rootsi placed it in the Iberian ice age refuge, where it differentiated from parent haplogroup I. Later studies, however, suggest a younger, post-glacial origin. Either way, I1 is most frequent amongst Scandinavians and Northern Germans, although it is 2nd in frequency to R1b}.
I2 has the highest frequency and diversity in the western Balkans, with a peak of 64% in Herzegovinians. High rates are also found throughout all other republics of the former Yugoslavia (over 30%). The frequency is 23% in Bulgarians, with moderate frequencies throughout the remaining peoples of central and Eastern Europe (10-25%). Its frequency drops sharply as one goes west into Italy (1% in Nth Italians), whereas there is a more gradual tapering as one goes south (18% in Northern Greeks, 8% in central Greeks, 2% in Turks). Thus it can be seen that the Balkans served as an ice age refuge, and its “Dinaric” people re-populated much of South Eastern and Eastern Europe.
R1b (M 343) is dated to have arisen from its parent haplogroup, R1 (M 173), circa 25-30, 000 years ago. The undifferentiated R1 complex underpinning both R1a and R1b prevalence in Europe has been dated to circa 40, 000 years ago, origination in central Asia. Semino links its spread into Europe with the arrival of Homo sapiens sapiens, possibly coinciding with the appearance of the Aurignac culture [9]. Today, R1b is found throughout Europe, as well as western and central Asia. The highest frequencies of R1b are found in Western Europe, although Western European subclades of R1b are dated to be the youngest of all (confirming an Asian origin). Like the situation with haplogroup I2 in the Balkans, the prevalence of haplogroup R1b in Western Europe is due to population bottlenecking and founder effects associated with the LGM. During the LGM, much of the European population died out, reducing genetic diversity. A population bearing R1b managed to survive in Iberia, where R1b achieved genetic homogeneity. Ancestors of this population subsequently repopulated Western Europe, dispersing R1b. Consequently, R1b has very high values in western Europe, especially amongst Atlantic countries (Britain, Spain and Portugal all have frequencies over 80%). Its frequency decreases toward Eastern Europe. Its overall frequency in the Balkans is 9%, with two demographic peaks – the northwest and southeast. In the north, Slovenes (23%) and mainland Croats (15%), and in the south, Bulgarians (17%), Greeks (up to 22%), and Turks (circa 20%), all have higher frequencies compared to the more central Balkanians (10% in Serbs, and less than 5% in Bosnians, Herzegovinians and Macedonians). Such a pattern may be reflective of two separate post-glacial migrations of R1b bearers into the Balkans, one eastward from Iberia, and another westward migration from an Anatolian ice age refuge[10][11].
R1a is considerably younger compared to R1b, dating back to 15, 000 BP. R1a reaches its highest frequency and diversity in Eastern Europe. Sometimes called the “Slavic gene”, highest frequencies of R1a are found amongst Poles, Ukrainians, Russians and Sorbians (all at rates exceeding 50%); and is indeed found in significant frequencies throughout all Slavic speaking countries. However, rates are also high amongst Scandinavians (26% in Norwegians, 19% in Swedes), as well as Hungarians and Romanians (both harbouring rates of circa 20%). Frequency drops to 8% in Greeks, although some studies have shown that Greek Macedonians have a frequency of 35%. To the west, frequency rapidly drops to below 5%, although 10% of German men bear R1a. Outside of Europe, high frequencies are found in some central Asian populations, such as Tajiks and Kyrgyz, possibly reflecting founder effects associated with the spread of Indo-Europeans. The average frequency of R1a amongst South Slavs is reported at 16%, with a north to south gradient. It is more prevalent amongst Slovenians (37%) and mainland Croats (34%) compared to Bulgarians, Serbs, Macedonians and Herzegovinians (all less than 15%). Bosnians have an intermediate frequency of 24% [12].
As far as South Slavs are concerned:
“At least three major episodes of gene flow might have enhanced R1a variance in the region: early post-LGM recolonizations expanding from the refugium in Ukraine, (Indo-European) migrations from the northern Pontic steppe between 3000 and 1000 B.C., as well as possibly massive Slavic migration from A.D. 5th to 7th centuries.”[13]
Haplogroup E1b1b (formerly E3b), characterised by the M35 mutation, originated in North Eastern Africa, circa 24 kYa[14]. The most prevalent sublcade of E1b1b in Europe is that of the E1b1b1a2 clade (characterised by the M-78, V-13 mutation), which enjoys highest frequencies in the Balkans, although its diversity (age) is higher in Anatolia. Formerly, the presence of E1b1b1 in South-Eastern Europe was proposed to represent the spread of Neolithic farmers out of Anatolia [15]. However, a newer study by Cruciani suggests that whilst it originated in Anatolia c. 10 kYa, its expansion throughout the Balkans was actually the result of a population expansion originating from within the Balkans c 5.3 kYa, possibly associated with the Balkan Bronze Age[16] . Pericic links the spread of Balkan E1b1b to archaeological evidence of trade networks along the Morava-Danube-Vardar river system. {Whilst over 93% of E1b1b haplogroups found in the Balkans are of the V-13 variety, some 4% (on average) Spanish and southern Italian men carry the V-12 (E1b11a1) variety, which represents an ancient, possibly direct trans-Mediterranean, migration from northern Africa to Southern Italy and Spain, c. 13 kYa. In addition, another subhaplogroup - E1b1b1b (M 81)- is also found in Southern Spaniards, representing gene flow brought in by the migration of Berbers and Sephardic Jews during the Middle Ages. These latter two E haplogroups are not found in the Balkans}. Haplogroup E1b1b is found at frequencies of approximately 20% in Bulgarians, Macedonians, and Serbians. In contrast, it is found at frequencies of less than 10% in Bosnians, Herzegovinians, Croats and Slovenes. Amongst other Balkan peoples, it is also very frequent amongst Greeks, Albanians and Romanians. North of the Carpathians, frequencies are low, often less than 5%. Moderately high frequencies exist in Southern Italy also, perhaps reflecting movements from Greece.
The dispersal of E3b reflects a more localized expansion of farmers/ traders originating from within the Balkans during and after the Neolithic. In contrast, R1b, R1a and I2a reflect older, stronger dispersals over wider geographic areas, coinciding with the re-population of a de-populated, post-glacial, Europe.
Bulgarians, Greeks, Turks and Albanians all have J2 frequencies of over 20%. J2 frequency drops sharply as one goes progressively northward from these populations. Excepting Bulgarians, South Slavs have rather low frequencies of J2, less than 10%.
J2 is proposed to have originated in the northern Levant or Anatolia[17], prior to the Neolithic. It is found in significant proportions along coastal Mediterranean regions such as southern Italy, the southeastern Balkans, and, to a lesser extant, parts of Spain. Such a pattern is suggestive of maritime migration out of the Near East [18]. In addition, King and Underhill described that the distribution of J2 closely corresponds to the distribution of painted pottery and anthropomorphic figurines associated with the spread of agriculture during the Neolithic[19]. {One particular sublcade, J2e (aka J2b by some authors) has the highest frequency and diversity in central Greece. It has been postulated that this represents a later, separate migration pattern out of Greece, linked to the Bronze Age Greek culture, a period which saw the establishment of Greek colonies along the Black Sea, eastern Anatolian coastline, Southern Dalmatia, Southern Italy and parts of Spain. J2e is the predominant J subhalogroup amongst South Slavs, although Bulgarians harbour J2f, J2a, as well as J1}. This suggests that there was a demic diffusion of Neolithic farmers out of the Levant into southern Europe- particularly the southeastern Balkans and southern Italy. Subsequently farming spread throughout the rest of Europe only by means of cultural contact, since the frequency of this haplogroup outside the abovementioned areas drops rapidly.
In an attempt to categorise the populations of Europe and analyse their relative genetic backgrounds, Semino performed a principal component analysis (using five autonomic protein markers and Y-DNA haplogroups). It showed that Europeans clustered into three major groups – Western Europeans (including Iberians, northern Italians, Dutch, Germans), Eastern Europeans (Slavic-speaking countries, Hungarians, Romanians), and Mediterranean (Turks, Lebanese, Southern Italians, Greeks, Albanians). Certain populations were more borderline than others. For example, Germans and Czechs are almost midway between eastern and western European cluster groups. Greeks, whilst tending to cluster with the Mediterranean group, were closer to Europeans than Syrians, Lebanese and Turks. Naturally, southern Europeans (such as Italians, French and Andalusians) have a closer genetic relationship to the Mediterranean cluster than northern Europeans[20].
Mitochondrial DNA
Mitochondrial DNA (mtDNA) is the DNA found from mitochondria, organelles found in Eukaryotic cells. A mitochondrion has its own DNA, separate from that found in the cell nucleus. Such mtDNA is maternally inherited.
A study examining the frequency of mtDNA haplogroups in South Slavs revealed that remarkable homogeneity with the rest of the European population. Specifically the study participants predominantly harboured Haplogroup H, at frequencies of 40-45 % [21]. Indeed, such frequencies are found in the general European population. Haplogroup H is estimated to have arrived into Europe from western Asia/ middle East c. 30, 000 years BP, possibly corresponding to the spread of the Gravettian culture[22].
Haplogroup U5 occurs at an average frequency of 10%. It is dated to 50 kYa, postulated to have also originated in the Middle East. Today, Finnic and Sami populations harbour the highest frequencies of U5, due to founder effects of post-LGM recolonization.
The next most significant mtDNA haplogroup is that of J, found at frequencies of 7-12 %. It has been associated as a marker of the spread of farmers from the Near East.
Analysing the mtDNA data, one can see two major differences from the pattern of Y-DNA haplogroups. Firstly, the mtDNA groups are (mostly) older and, secondly, they have a Middle Eastern origin, in contrast to the predominance of the central Asian R1 complex in the Y-DNA pool of European men. Such a difference may be the result of different migratory behaviours between men and women. As Semino suggests:
“These discrepancies may be due in part to the apparent more recent molecular age of Y chromosomes relative to other loci (27), suggesting more rapid replacement of previous Y chromosomes. Gender-based differential migratory demographic behaviours will also influence the observed patterns of mtDNA and Y variation (24).”[23]
Conclusions
The various studies highlight that the paternal and maternal lineages of South Slavic people are, on the whole, consistent with the typical European Y-chromosomal and mtDNA gene pool, respectively. However, the pattern of Y-DNA haplogroup frequency in South Slavic men is unique. Unlike Western Europe, which is heavily dominated by R1b, collectively, the Balkan Slavs harbour multiple haplogroups (E3b, I2, R1a, J2 and R1) in significant frequencies, making them the most genetically diverse Europeans[24].
There are, both, similarities and regional peculiarities amongst the South Slavic Y-DNA haplogroup pool. There is a significant element of common genetic heritage, as seen by the pervasive presence of haplogroup I2. It is viewed as the genetic marker of the migratory path taken by the “Dinarics”, who took refuge in the Balkan Peninsula during the LGM and subsequently repopulated much of South-Eastern and Eastern Europe.
At face value, there seem to be discernable patterns when looking at the frequency of the remaining haplogroups (R1a, R1b, E1b1b, and J2). Post-glacial migratory movements into the Balkans emanating from Ukraine are marked by R1a, which is more prevalent in the northern Balkans. Additional post-glacial movements from Iberia and, possibly, Anatolia are suggested by a regional high diversity of R1b in the Balkans, with two frequency peaks- Slovenians and Croats in the northwest, and Greeks and Bulgarians in the southeast. From an mtDNA perspective, the Glacial Maximum also reduced mtDNA diversity, reflected by the dominance of haplogroups such as H and U which also expanded from the south European refugia. Neolithic and post-Neolithic migrations were more pronounced in the southern and eastern regions of the Balkans, being geographically contiguous to the Near East/ Levant. Indeed, the arrival of agriculture into Europe during the Neolithic has been linked by some scholars with the mtDNA haplogroup J and the Y-DNA haplogroups J2 and E1b1b, although the expansion of the latter may actually represent a population expansion from within the Balkans. Finally, Pericic and Semino envisage an Early Bronze Age genetic contribution from the expanding Kurgan culture, also marked by haplogroup R1a, which may provide genetic evidence for the spread of Indo-European peoples from the urmeheit in modern Ukraine. Further studies will be required to substantiate the haplogroup frequency patterns found so far, and elucidate their meaning.
Ancient, bi-directional gene flows between the Balkans and Eastern Europe account for the clustering of South Slavs with all other Slavs, as well as the non-Slavic Hungarians and Romanians. In addition, we see that of all the Eastern European peoples, Bulgarians have a closer relationship with Mediterranean populations.
Ultimately, the genetic patterns we have encountered testify to the key role the Balkans has played in structuring the genetic, cultural and linguistic landscape of Europe [25]. In addition, the studies indicate that there is significant genetic continuity between the autochthonous Balkan population and modern South Slavs. As such, later migrations of Romans, Huns, Goths, Slavs and Bulgars may have been too small to have significantly affected the genetic composition of Balkan people. However, DNA analysis of burials from, both, autochthonous and early Slavic peoples is required to illustrate whether the events of the Great Migrations period had a significant ‘biological’ impact on the Balkan inhabitants. Such analysis is, unfortunately, complicated owing to the technical difficulties associated with extracting ancient DNA.
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