The Longue Durée of Genetic Ancestry: Multiple Genetic Marker Systems and Celtic Origins on the Atlantic Facade of Europe
2004; Elsevier BV; Volume: 75; Issue: 4 Linguagem: Inglês
10.1086/424697
ISSN1537-6605
AutoresBrian McEvoy, Martin Richards, Peter Forster, Daniel G. Bradley,
Tópico(s)Genetic diversity and population structure
ResumoCeltic languages are now spoken only on the Atlantic facade of Europe, mainly in Britain and Ireland, but were spoken more widely in western and central Europe until the collapse of the Roman Empire in the first millennium a.d. It has been common to couple archaeological evidence for the expansion of Iron Age elites in central Europe with the dispersal of these languages and of Celtic ethnicity and to posit a central European “homeland” for the Celtic peoples. More recently, however, archaeologists have questioned this “migrationist” view of Celtic ethnogenesis. The proposition of a central European ancestry should be testable by examining the distribution of genetic markers; however, although Y-chromosome patterns in Atlantic Europe show little evidence of central European influence, there has hitherto been insufficient data to confirm this by use of mitochondrial DNA (mtDNA). Here, we present both new mtDNA data from Ireland and a novel analysis of a greatly enlarged European mtDNA database. We show that mtDNA lineages, when analyzed in sufficiently large numbers, display patterns significantly similar to a large fraction of both Y-chromosome and autosomal variation. These multiple genetic marker systems indicate a shared ancestry throughout the Atlantic zone, from northern Iberia to western Scandinavia, that dates back to the end of the last Ice Age. Celtic languages are now spoken only on the Atlantic facade of Europe, mainly in Britain and Ireland, but were spoken more widely in western and central Europe until the collapse of the Roman Empire in the first millennium a.d. It has been common to couple archaeological evidence for the expansion of Iron Age elites in central Europe with the dispersal of these languages and of Celtic ethnicity and to posit a central European “homeland” for the Celtic peoples. More recently, however, archaeologists have questioned this “migrationist” view of Celtic ethnogenesis. The proposition of a central European ancestry should be testable by examining the distribution of genetic markers; however, although Y-chromosome patterns in Atlantic Europe show little evidence of central European influence, there has hitherto been insufficient data to confirm this by use of mitochondrial DNA (mtDNA). Here, we present both new mtDNA data from Ireland and a novel analysis of a greatly enlarged European mtDNA database. We show that mtDNA lineages, when analyzed in sufficiently large numbers, display patterns significantly similar to a large fraction of both Y-chromosome and autosomal variation. These multiple genetic marker systems indicate a shared ancestry throughout the Atlantic zone, from northern Iberia to western Scandinavia, that dates back to the end of the last Ice Age. Family resemblances among the Celtic languages were first identified by Edward Lhuyd at the end of the 17th century (Renfrew Renfrew, 1987Renfrew C Archaeology and language. Jonathan Cape, London1987Google Scholar; Cunliffe Cunliffe, 1997Cunliffe B The ancient Celts. Oxford University Press, Oxford, United Kingdom1997Google Scholar; James James, 1999James S The Atlantic Celts. British Museum Press, London1999Google Scholar). The surviving insular Celtic (as opposed to the extinct continental Celtic) languages are usually subdivided into two groups on the basis of a single consonantal shift. Welsh, Breton, and the extinct Cornish language are referred to as “Brythonic,” or “P-Celtic,” whereas Irish, Scottish Gaelic, and the extinct Manx language are thought to be more archaic and are referred to as “Goidelic,” or “Q-Celtic” (Trask Trask, 1996Trask RL Historical linguistics. Arnold, London1996Google Scholar). The continental Gaulish language, formerly spoken in northern and central France, was more closely related to the insular Brythonic group but has no surviving descendants. Traces also survive of Celtiberian, a Celtic language spoken in antiquity in western and central Iberia alongside the non-Indo-European Basque language of northeastern Spain and Aquitaine, and of Lepontic, which was spoken in northern Italy from the 6th century. Lhuyd named the languages “Celtic,” after one of the names applied to the people of northern France in Greek and Roman ethnography dating from about the 5th century b.c. Conflating the historical ethnographic “Celts” and 17th-century “Celtic speakers,” Lhuyd followed up his discovery by proposing that the two forms of the Celtic language had been brought to Britain and Ireland by waves of invasion from the continent, and, in the years that followed, antiquarians and archaeologists suggested that these migrations had their “heartland” in the elite La Tène cultural package of the central European Iron Age. Despite the adoption of La Tène art styles, however, archaeological evidence for large-scale Iron Age migrations into the British Isles has been singularly lacking. In Ireland, for example, La Tène artifacts are relatively rare and are almost always of indigenous manufacture rather than of external origin (Raftery Raftery, 1994Raftery B Pagan Celtic Ireland: the enigma of the Irish Iron Age. Thames and Hudson, London1994Google Scholar), leading archaeologists and historians to question the accepted idea of Celtic migration to Ireland (Ó’Donnabháin Ó’Donnabháin, 2000Ó’Donnabháin B An appalling vista? The Celts and the archaeology of later prehistoric Ireland.in: Desmond A Johnson G McCarthy M Sheehan J Shee Twohig E New agendas in Irish prehistory. Wordwell, Bray, Ireland2000: 189-196Google Scholar). More generally, Renfrew (Renfrew, 1987Renfrew C Archaeology and language. Jonathan Cape, London1987Google Scholar), among others, proposed that the roots of insular Celtic identity lay within the region in which the Celtic languages were historically spoken, in the diffusion of Indo-European speakers into Britain and Ireland with the arrival of the Neolithic in ∼4000 b.c. Cunliffe (Cunliffe, 2001Cunliffe B Facing the ocean: the Atlantic and its people. Oxford University Press, Oxford, United Kingdom2001Google Scholar) appears to go further, describing the coalescence of the Celtic languages along the coastline of the Atlantic facade of Europe, from southern Iberia to the Shetland Islands, via maritime networks that reach back into the late Mesolithic period. The similarities in prehistoric monumental architecture and the spread of the early–Bronze Age “Beaker package,” to take two examples, attest to the likely sharing of beliefs and attitudes through social networks that extended from one end of the Atlantic zone to the other. This view implies an uncoupling of the link, established by Lhuyd, of a necessary connection between the various aspects of what in the past 200 years has come to be thought of as a “Celtic package”—including, in particular, the peoples encountered and described as “Celts” by the classical authors, the producers of Iron Age La Tène art and their descendants, and speakers of Celtic languages. Modern Celtic speakers should, by this view, be thought of rather as “Atlantic Celts,” whose putative continental Iron Age ancestry is open to question (James James, 1999James S The Atlantic Celts. British Museum Press, London1999Google Scholar; Cunliffe Cunliffe, 2001Cunliffe B Facing the ocean: the Atlantic and its people. Oxford University Press, Oxford, United Kingdom2001Google Scholar). At the same time, many archaeologists and a seeming majority of historians retain the traditional view with some vigor (Megaw and Megaw Megaw and Megaw, 1996Megaw JVS Megaw MR Ancient Celts and modern ethnicity.Antiquity. 1996; LXX: 175-181Google Scholar, Megaw and Megaw, 1998Megaw JVS Megaw MR The mechanism of (Celtic) dreams?: a partial response to our critics.Antiquity. 1998; LXXII: 432-435Google Scholar). Genetic evidence has recently lent some support to the suggestion of a shared ancestral heritage among the human populations of Atlantic Europe. Y-chromosome analysis has highlighted similarities between the Pyrenean populations of northern Spain and western population samples from the British Isles (Hill et al. Hill et al., 2000Hill EW Jobling MA Bradley DG Y-chromosome variation and Irish origins.Nature. 2000; 404: 351-352Crossref PubMed Scopus (104) Google Scholar; Wilson et al. Wilson et al., 2001Wilson JF Weiss DA Richards M Thomas MG Bradman N Goldstein DB Genetic evidence for different male and female roles during cultural transitions in the British Isles.Proc Natl Acad Sci USA. 2001; 98: 5078-5083Crossref PubMed Scopus (130) Google Scholar). More specifically, a modal haplotype defined by SNP and STR markers (the “Atlantic modal haplotype” in haplogroup R1b [Y Chromosome Consortium Y Chromosome Consortium, 2002Y Chromosome Consortium A nomenclature system for the tree of human Y-chromosomal binary haplogroups.Genome Res. 2002; 12: 339-348Crossref PubMed Scopus (657) Google Scholar]) is present at an unusually high frequency in each population. This has been interpreted as a common Paleolithic genetic legacy that was relatively undisturbed at the edge of the European peninsula by subsequent dispersals from the east, such as those suggested to have taken place during the spread of the Neolithic (Wilson et al. Wilson et al., 2001Wilson JF Weiss DA Richards M Thomas MG Bradman N Goldstein DB Genetic evidence for different male and female roles during cultural transitions in the British Isles.Proc Natl Acad Sci USA. 2001; 98: 5078-5083Crossref PubMed Scopus (130) Google Scholar). 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We then isolated DNA from buccal cells and sequenced the mtDNA HVS-I in both forward and reverse directions. To allow comparison with previously published Irish data, only variation in positions 16030–16394 (with respect to the Cambridge reference sequence [CRS] [Anderson et al. Anderson et al., 1981Anderson S Bankier AT Barrell BG de Bruijn MH Coulson AR Drouin J Eperon IC Nierlich DP Roe BA Sanger F Schreier PH Smith AJ Staden R Young IG Sequence and organization of the human mitochondrial genome.Nature. 1981; 290: 457-465Crossref PubMed Scopus (7178) Google Scholar]) was considered. Four SNP markers were also typed by use of RFLP analysis: 12308, 4580, 7028, and 73, distinguishing haplogroups U, V, H, and pre-HV, respectively (Torroni et al. Torroni et al., 1996Torroni A Huoponen K Francalacci P Petrozzi M Morelli L Scozzari R Obinu D Savontaus ML Wallace DC Classification of European mtDNAs from an analysis of three European populations.Genetics. 1996; 144: 1835-1850PubMed Google Scholar; Macaulay et al. Macaulay et al., 1999Macaulay V Richards M Hickey E Vega E Cruciani F Guida V Scozzari R Bonne-Tamir B Sykes B Torroni A The emerging tree of west Eurasian mtDNAs: a synthesis of control-region sequences and RFLPs.Am J Hum Genet. 1999; 64: 232-249Abstract Full Text Full Text PDF PubMed Scopus (505) Google Scholar). We observed a total of 155 haplotypes among the 300 Irish individuals studied (including 100 from previous studies), with all but one sample falling into the main western Eurasian haplogroups: U, HV, JT, I, W, and X (Richards et al. Richards et al., 1998Richards MB Macaulay VA Bandelt H-J Sykes BC Phylogeography of mitochondrial DNA in western Europe.Ann Hum Genet. 1998; 62: 241-260Crossref PubMed Google Scholar). Full results and additional supplementary information are available from the authors' Web site. A χ2 test of mtDNA haplogroup frequencies in samples from eastern (n=127) and western (n=128) Ireland showed no significant differences (the remaining 45 samples did not fall into the eastern or western region). In addition, the genetic distance (ΦST value) between the two regions, on the basis of HVS-I, is small and not significantly greater than zero. This contrasts with the Y-chromosome pattern in Ireland, where eastern and western complements have been shown to be substantially different. This difference between eastern and western Irish Y chromosomes has been attributed to the preferential settlement of subsequent migrants to the accessible east coast after initial colonization (Hill et al. Hill et al., 2000Hill EW Jobling MA Bradley DG Y-chromosome variation and Irish origins.Nature. 2000; 404: 351-352Crossref PubMed Scopus (104) Google Scholar). Founder analysis (Richards et al. Richards et al., 2000Richards M Macaulay V Hickey E Vega E Sykes B Guida V Rengo C et al.Tracing European founder lineages in the Near Eastern mtDNA pool.Am J Hum Genet. 2000; 67: 1251-1276Abstract Full Text Full Text PDF PubMed Scopus (745) Google Scholar) dated the entry of different mtDNA lineages into Europe by examining the levels of nucleotide diversity accumulated around haplotypes that have matches in the Near East. Roughly 20% of Europeans, principally those belonging to haplogroups J, T1, and U3, are proposed to descend from Neolithic settlers, with the remainder attributed to earlier Late Paleolithic/Mesolithic inhabitants. About 13% of Irish mtDNAs belong to putative Neolithic clusters, a value that is toward the lower end of the range found in Europe and similar to areas such as Scandinavia and the western Mediterranean (Iberia). This observation is consistent with the progressive dilution of the genetic impact of these migrants toward the north and west of Europe. Furthermore, there is an even distribution of putatively Neolithic haplogroups around the island, suggesting that females who arrived after the initial settlement were not restricted to east-facing regions. There are two potential explanations for this: either they were more mobile after arrival in the east or other regions of the island were in direct contact with the continental source populations. By contrast, however, Y-chromosome lineages of putative Near Eastern Neolithic origin (Semino et al. Semino et al., 2000Semino O Passarino G Oefner PJ Lin AA Arbuzova S Beckman LE De Benedictis G Francalacci P Kouvatsi A Limborska S Marcikiae M Mika A Mika B Primorac D Santachiara-Benerecetti AS Cavalli-Sforza LL Underhill PA The genetic legacy of Paleolithic Homo sapiens sapiens in extant Europeans: a Y chromosome perspective.Science. 2000; 290: 1155-1159Crossref PubMed Scopus (621) Google Scholar) appear to be virtually absent from the west of Ireland (Hill et al. Hill et al., 2000Hill EW Jobling MA Bradley DG Y-chromosome variation and Irish origins.Nature. 2000; 404: 351-352Crossref PubMed Scopus (104) Google Scholar). We first examined broad affinities at the population level. Control-region sequences from 8,733 individuals were assembled from the current and previous studies and were grouped into 45 geographically defined population samples. Each of these was checked for quality, as recommended by Bandelt et al. (Bandelt et al., 2002Bandelt H-J Quintana-Murci L Salas A Macaulay V The fingerprint of phantom mutations in mitochondrial DNA data.Am J Hum Genet. 2002; 71: 1150-1160Abstract Full Text Full Text PDF PubMed Scopus (215) Google Scholar) (results available on the authors' Web site). Samples from some small and/or isolated populations were excluded from our analysis because of the possibility of unusually strong genetic drift that might confound any broader phylogeographical patterns. These included the Western Isles of Scotland, Orkney, and Skye (Helgason et al. Helgason et al., 2001Helgason A Hickey E Goodacre S Bosnes V Stefansson K Ward R Sykes B mtDNA and the islands of the North Atlantic: estimating the proportions of Norse and Gaelic ancestry.Am J Hum Genet. 2001; 68: 723-737Abstract Full Text Full Text PDF PubMed Scopus (165) Google Scholar). We estimated genetic distances between all populations as linearized ΦS
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