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Patterns of genetic differentiation and the footprints of historical migrations in the Iberian Peninsula

Now you are shattered by the seas in the depths of the waters . I will bring you to a terrible end, and you will exist no more. You will be looked for, but you will never again be found. Says the Lord GOD. Ezekiel 27-34 and 26-21

It is through the delusions of the monotheists (her, Ezekiel ) of antiquity that I introduce a study on the genetics of the populations of the Iberian Peninsula carried out by a team of Oxford researchers.

These monotheists who in their religious madness, plagiarise in all details the religion of the pharaohs which, it, tells a tragedy that really happened on the island of Amentet, located in the Atlantic Ocean.
With tailor-made adaptations to their customers, these monotheists who had had access to the library of the pharaohs of Alexandria before they burned it, nevertheless reveal certain facts of the first history of humanity, in particular the submersion of the island located in the Atlantic Ocean. This explains the strangeness of the finding of researchers on the genetics of the populations of the Iberian Peninsula, the strong presence of the gene of what the researchers believe to be exclusively North African in Galicia, north of the ‘Spain.

The mythology of Islam which made us believe that the North Africans on behalf of the beduins without civilization or history, invaded the South of Spain, particularly the region of Andalusia, is reduced to nothing by the science of genetic. The North African gene in this region of Islam in Spain is very weak. The North African gene in this region exists only through the natural relationship of the neighborhood, in the same way as is the more marked French and Italian gene which represents respectively 67% and 17%.
This study excludes the presence of population from the Middle East and thus destroys the myth of a Bedouin invasion, of which no one has ever witnessed such an event, to impose Islam on Europeans. Islam for those who follow this site know that it is a syncretism of the different beliefs existing on the Mediterranean rim, the main trunk of which was the Arian Unitarian Church.
Islam in Spain is of Arianist Goth origin!

Glory to scientific research that destroys the lies of proto-Islamic Christian Unitarianism and Orthodox Christian.

Perhaps surprisingly, north African ancestry does not reflect
proximity to north Africa, or even regions under more extended
Muslim control. The highest amounts of north African ancestry
found within Iberia are in the west (11%) including in Galicia,
despite the fact that the region of Galicia as it is defined today
(north of the Miño river), was never under Muslim rule34 and
Berber settlements north of the Douro river were abandoned by
741. This observation is consistent with previous work using Ychromosome data11. We speculate that the pattern we see is
driven by later internal migratory flows, such as between Portugal
and Galicia, and this would also explain why Galicia and Portugal
show indistinguishable ancestry sharing with non-Spanish groups
more generally. Alternatively, it might be that these patterns
reflect regional differences in patterns of settlement and integration with local peoples of north African immigrants themselves, or varying extents of the large-scale expulsion of Muslim
people, which occurred post-Reconquista and especially in towns
and cities10,32.
We show that population structure exists at ultra-fine scales in
Galicia (Fig. 3a), particularly in the province of Pontevedra, with
some clusters having geographic ranges of less than 10 Km (rootmean-square distance from cluster centroid). To our knowledge,
these results represent the finest scales over which such structure
has yet been observed in humans. Previously, it has been shown
that by jointly analysing people from a priori defined sampling
locations, subtle differences in group averages at certain genomic
loci can be observed at fine geographic scales..

Clare Bycroft 1, Ceres Fernandez-Rozadilla2, Clara Ruiz-Ponte2, Inés Quintela3, Á
ngel Carracedo2,3,4, Peter Donnelly 1,5 & Simon Myers1,5

The Iberian Peninsula is linguistically diverse and has a complex demographic history,
including a centuries-long period of Muslim rule. Here, we study the fine-scale genetic
structure of its population, and the genetic impacts of historical events, leveraging powerful,
haplotype-based statistical methods to analyse 1413 individuals from across Spain. We detect
extensive fine-scale population structure at extremely fine scales (below 10 Km) in some
regions, including Galicia. We identify a major east-west axis of genetic differentiation, and
evidence of historical north to south population movement. We find regionally varying
fractions of north-west African ancestry (0–11%) in modern-day Iberians, related to an
admixture event involving European-like and north-west African-like source populations. We
date this event to 860–1120 CE, implying greater genetic impacts in the early half of Muslim
rule in Iberia. Together, our results indicate clear genetic impacts of population movements
associated with both the Muslim conquest and the subsequent Reconquista.

1Wellcome Trust Centre for Human Genetics, University of Oxford, Oxford OX3 7BN, UK. 2 Fundación Pública Galega de Medicina Xenómica- CIBERER-IDIS,
Santiago de Compostela, A Coruña, Spain. 3Grupo de Medicina Xenómica, Centro Nacional de Genotipado (CEGEN-PRB2-ISCIII), Universidade de Santiago
de Compostela, A Coruña, Spain. 4 Institute of Forensic Sciences., University of Santiago de Compostela, A Coruña, Spain. 5Department of Statistics,
University of Oxford, Oxford OX1 3LB, UK. These authors jointly supervised this work: Peter Donnelly, Simon Myers. Correspondence and requests for
materials should be addressed to S.M. (email: myers@stats.ox.ac.uk)

Genetic differentiation within or between human populations (population structure) has been studied using a
variety of approaches over many years1–5. Recently there
has been an increasing focus on studying genetic differentiation at
fine geographic scales, such as within countries6–8. Identifying
such structure allows the study of recent population history, and
identifies the potential for confounding in association studies,
particularly when testing rare, often recently arisen variants9. The
Iberian Peninsula is linguistically diverse, has a complex demographic history, and is unusual among European regions in having a centuries-long period of Muslim rule10.
Previous studies of population structure in Spain have examined either a small fraction of the genome11–13 or only a few regions of Spain14,15, and typically compare groups of individuals
defined a priori using broad ethnic or geographic labels, such as
autonomous community. Using such approaches only limited
population structure within Iberia has been identified15–19. Some
structure within northern Spain has been detected, including
statistically significant differences in frequencies of Ychromosome haplotypes and other genetic markers between the Basque-speaking regions and other parts of Iberia11,12, a result
consistent with a European-wide analysis using autosomal
DNA20. Studies of Spain that used genome-wide data did not
leverage information in correlations between genetic markers14,15,
excepting one study21, which detected a cline of variation broadly
distinguishing samples in País Vasco from other parts of northern
Spain, especially Galicia, but no evidence of sub-structure in
central or southern Spain. Thus the overall pattern of population
structure within Spain—including subtle structure at fine geographic scales—remains uncharacterized.
The cultural and linguistic impact of Muslim rule in Iberia is
well-documented, but the historical record is limited in its ability
to inform about the extent, timing and geographic spread of
genetic mixing between immigrants and indigenous Iberians over
several centuries after the initial conquest22. Previous genetic
studies have reported signals of admixture from sub-Saharan
Africa and/or north Africa into Iberia at some point in the
past23–27. However, estimates of the timing of this admixture vary
greatly, from as long as 74 generations ago (~100 BC)23 to 23
generations ago (~1330 CE)25. Estimates of overall mean proportions of African-like DNA in the Iberian Peninsula also vary, ranging from 2.424 to 10.6%11. Differences within Iberia have also
been reported11,26, based on comparisons between sampled
regions, with higher fractions observed in western regions of
Iberia (e.g. 21.7% in Northwest Castile11) and lower fractions in
the north-east (e.g. 2.3% in Cataluña11). Estimates of the timing
and extent of admixture tend to vary depending on the reference
populations assumed to represent the ancestral mixing groups
(e.g. Moroccan11 or Saharawi26), as well as heterogeneity in the
ancestral make-up of the modern-day Iberian samples used in the
Here we analyse genome-wide genotyping array data for 1413
Spanish individuals sampled from across Spain. By using powerful, haplotype-based statistical methods we identify extensive fine-scale structure down to scales <10 km in some places. We
identify a major axis of genetic differentiation that runs from east
to west across Iberia. In contrast, we observe remarkable genetic
similarity in the north–south direction, and evidence of historical
north–south population movement. Finally, we sought to clarify
the timing and composition of African-like and potentially nonAfrican genetic contributions to the Iberian Peninsula, by jointly analysing genotype data sourced from a wide range of African
and European regions. We show that modern Spanish people
have regionally varying fractions of ancestry from a group most
similar to modern north-west Africans. This African ancestry,
identified without making particular prior assumptions about source populations, results from an admixture event that we date to 860–1120 CE, corresponding to the early half of Muslim rule.
Our results indicate that it is possible to discern clear genetic
impacts of the Muslim conquest and population movements
associated with the subsequent Reconquista.
Extensive fine-scale population structure in Spain. We analysed
phased genotyping array data for 1413 Spanish individuals typed
at 693,092 autosomal single nucleotide polymorphisms (SNPs)
after quality control (Methods). We applied fineSTRUCTURE28
to these data to infer clusters of individuals with similar patterns
of shared ancestry (Methods). fineSTRUCTURE inferred 145
distinct clusters, along with a hierarchical tree describing relationships between the clusters (Fig. 1a; Methods). We used genetic data only in the inference, but explored the relationship
between genetic structure and geography using a subset of 726
individuals for whom geographic information was available and
all four grandparents were born within 80 km of the centroid of
their birthplaces. Figure 1b represents each of these individuals as
a point on a map of Spain, located at the centroid of their
grandparents’ birthplaces and labelled according to their cluster
assignment after combining small clusters at the bottom of the
tree (Methods). Their grandparents were likely to have been born
in the decades either side of 1900 (median birth-year of the
cohort is 1941), so the spatial distribution of genetic structure
described in this study would reflect that of Spain around that
These results reveal patterns of rich fine-scale population
structure in Spain. At the coarsest level of genetic differentiation
(i.e. two clusters at the top of the hierarchy) individuals located in
a small region in south-west Galicia are separated from those in
the rest of Spain. The next level separates individuals located
primarily in the Basque regions in the north (País Vasco and
Navarra) from the rest of Spain. Further down the tree
(background colours in Fig. 1b) many of the clusters closely
follow the east–west boundaries of Spain’s autonomous communities, especially in the north of Spain. However, in the north–south direction several clusters cross boundaries of multiple autonomous communities. Overall, the major axis of genetic differentiation runs from east to west, while conversely there is
remarkable genetic similarity on the north–south direction. In a
complementary analysis that included Portugal, although fewer
SNPs (Methods), Portuguese individuals co-clustered with
individuals in Galicia (Fig. 2a), showing that this pattern extends
across the whole Iberian Peninsula. Indeed, rather than mainly
reflecting modern-day political boundaries (autonomous communities), the broad-scale genetic structure of the region is strikingly similar to the linguistic frontiers29 present in the
Iberian Peninsula around 1300 CE (Fig. 1c). Via more formal
simulation-based testing, we confirmed this: the association of
genetic structure with language is statistically significant (p <
0.008), even after accounting for both physical distance and
autonomous community membership (Supplementary Note 9;
Supplementary Figure 8). Conversely, once physical distance and
language are taken into account, no significant association with
autonomous community remains (p = 0.12).
Although some geographically dispersed clusters (e.g. ‘central’
and ‘west’) remain largely intact at the bottom of the hierarchical
tree (Fig. 2b) many of the clusters that emerge further down the
tree involve greater geographical localisation. By far the strongest
sub-structure is seen within a single province in Galicia,
Pontevedra, which contains almost half of the inferred clusters
in all of Spain (Fig. 1a). This ultra-fine structure is seen across
scales of <10 km and the clusters align with regions defined by hills and/or river valleys (Fig. 3a). This structure is not an artefact
of the denser sampling in this region, as it was still evident in an
analysis after sub-sampling (Supplementary Note 4). Highly
localised structure is also seen in other parts of Spain, including
four clusters within the Basque regions (Fig. 3b), and a cluster that is exclusive to a ~50 km segment of the River Ebro in La Rioja (Fig. 3c).
To further understand the relationships between the clusters
inferred by fineSTRUCTURE, we examined patterns in the
matrix of ancestry sharing (coancestry) between each pair of 1413

Fig. 1 Spanish individuals grouped into clusters using genetic data only. a Binary tree showing the inferred hierarchical relationships between clusters inferred using genotype data of 1413 individuals (fineSTRUCTURE analysis A). The colours and points correspond to the clusters shown on the map, and the length of the coloured rectangles is proportional to the number of individuals assigned to that cluster. We combined some small clusters (Methods) and the thick black branches indicate the clades of the tree that we visualise in the map. Clusters are labelled according to the approximate location of most of their members, but geographic data was not used in the inference. b Each individual (n = 726) is represented by a point placed at (or close to, <24 Km) the centroid of their grandparents’ birthplaces. We only plot the individuals for whom all four grandparents were born within 80 km of their average birthplace, although the data for all individuals were used in the fineSTRUCTURE inference. The background is coloured according to the spatial densities of each cluster at the level of the tree where there are 14 clusters (Methods). The colour and symbol of each point corresponds to the cluster the individual  was assigned to at a lower level of the tree, as shown in a. Spain’s autonomous communities are also shown. c A representation of changes in the linguistic and political boundaries in Iberia from ~930 to 1300 CE, adapted with permission from maps by Baldinger29. Different linguistic areas are shown with the colors and shading, and political boundaries with white borders (in the far right map only). Only the colors and labels of the Christian kingdoms have
been added to aid visualization

individuals (Fig. 4a). In general, coancestry between individuals within a cluster is higher than between individuals in different clusters, reflecting genetic drift unique to each cluster. This effect is strongest for highly localised clusters, such as those in Galicia and País Vasco and La Rioja (Fig. 4b). These clusters also tend to
have greater certainty in their cluster assignment (Supplementary
Figure 1b). In contrast, the cluster labelled ‘central’ (shown with
yellow triangles in Fig. 1b) shows no clear drift signal. In fact,
individuals in this cluster have—on average—more coancestry
with the members of Basque-centred clusters (blue squares and
triangles) than they do with other individuals in their own cluster
(p < 0.02; Fig. 4c). Theoretical arguments predict (Methods) that
this effect can only occur if admixture from a highly drifted group
into another population takes place. That is, the effect could not
be explained by Basques inheriting DNA from ancestors of the
central group (although this may have happened in addition).
Thus, this signal provides evidence of admixture into the ‘central’
cluster from a group related to the Basque populations.

The genetic impact of historical migrations. Next, we sought to
characterize the relationship between Iberians (combining
Spanish and Portuguese individuals) and non-Iberian groups, to
understand the extent to which recent migrations from outside
Iberia have influenced modern-day DNA in Spain. We constructed a combined dataset (300,895 SNPs) of 2919 individuals
from Spain, Europe, north Africa30 and sub-Saharan Africa31
(Methods). We used fineSTRUCTURE to identify 29 non-Iberian
donor groups (Methods). We extended the fineSTRUCTURE
model to re-cluster individuals within Iberia, now based only on
their levels of ancestry sharing across these 29 groups (Methods).
These clusters capture the impact of migration into and across
Spain, removing the effects of simple isolation events.
Using this approach we inferred six distinct clusters within
Iberia (Fig. 5a), many fewer than in the Spain-only analysis
(Fig. 1a), implying that much of the fine-scale structure seen
within Spain is a result of regional genetic isolation. The six
clusters still associate with geographical regions, predominantly
in the east–west direction rather than north–south. Notably, the
extensive sub-structure in Pontevedra disappears, and indeed
these individuals now co-cluster with Portuguese individuals.
Therefore, the extensive fine-scale structure in Galicia is most
likely explained by local drift effects. In contrast, a distinct cluster
still occurs within the Basque region. This indicates that alongside
regional isolation, distinctive levels of ancestry sharing with nonSpanish groups contribute to fine-scale structure in this region.
To characterise the genetic make-up of these six Iberian
clusters we estimated their ancestry profiles: we fitted each cluster
as a mixture of (potentially) all 29 donor groups to approximate
the unknown ancestral groups that actually contributed to
modern-day Iberian individuals (Methods). This approach accounts for the stochasticity in ancestral relationships along the genome and was previously shown to be informative in the
context of the British Isles6. Only six of the 29 donor groups show
a contribution >1% in Iberia, and all are located in Western and
Southern Europe, and north-west Africa (Fig. 6). For all six
Iberian clusters the largest contribution comes from France
(63–91%), with smaller contributions that relate to present-day
Italian (5–17%) and Irish (2–5%) groups. With the exception of
the Basque cluster, these three donor groups contribute
proportionally similar amounts throughout Iberia, so probably
represent ancient ancestry components rather than recent
migration. In contrast, north Moroccan ancestry shows strong
regional variation (Fig. 5c, Methods). See Supplementary Note 7
for a fuller discussion of the ancestry profiles.
To distinguish between possible scenarios that could produce
these patterns, we applied the GLOBETROTTER method25 to
each of our six clusters (Methods). GLOBETROTTER infers dates
of admixture and the make-up of the source populations, and
tests whether admixture patterns are consistent with a simple
mixing of two groups at a single time in the past, compared to
more complex alternative models. GLOBETROTTER found
strong evidence (p < 0.01) of admixture for all six clusters
(Methods; Supplementary Table 3a). For all six clusters, an
extremely similar event was inferred (Fig. 5b), in a tight timerange of 860–1120 CE, and with similar source groups, present in varying proportions (4–10% for the minor group). The major
source was inferred to contain almost exclusively European donor
groups, and the minor source is made up of mainly north-west
African donor groups, including Western Sahara, and to a lesser
extent west Africans (YRI), consistent with the overall ancestry
profiles. The ‘Portugal-Andalucia’ cluster shows the greatest YRI
contribution, and also shows some evidence of a second
admixture date, with a more recent event involving only subSaharan-African-like and European-like source groups (see Supplementary Figure 7 and Supplementary Note 8.2). This
indicates a recent pulse of sub-Saharan African DNA, independent of the north African component. For the other five clusters,
the dates are more precise than any previous estimate that used
north African haplotypes in the analysis20,25,26. In our results any
one 95% confidence interval (CI) spans no more than 11
generations (~300 years) and all confidence intervals combined
span less than 14 generations (< 400 years).
GLOBETROTTER shows a subtle preference for Western
Sahara as a source of north African DNA, as opposed to north
Morocco. This might be explained if modern-day north
Moroccan haplotypes are more similar to present-day Spanish
individuals than the historical source population was. Indeed, a
mixture analysis we performed of the north Moroccan group
itself (Supplementary Figure 4; Methods) shows that this group
has a non-trivial proportion of European-like ancestry while

Fig. 2 Clustering analysis including Portuguese individuals; and large clusters at the bottom of the tree. a This map and tree show clusters inferred by fineSTRUCTURE (analysis B) that included data from Portuguese individuals but using a smaller set of SNPs (Methods). As in Fig. 1b we show the level of tree such that all clusters contain at least 15 individuals (39 clusters). Points representing 843 individuals are shown on this map but, as with analysis A, data for all Portuguese and Spanish individuals (1530) were used in the inference. Positions of points and background colours are determined using the same procedure as for Fig. 1b (Methods), with the exception of Portugal. No fine-scale geographic information was available for these individuals, so we placed them randomly within the boundaries of Portugal and show a single background colour. b This map shows geographic spread of the three large clusters that remain at the bottom of the tree inferred in the Spain-only fineSTRUCTURE analysis (see main text; Fig. 1a). These clusters each contain more than 100 individuals out of the full set of 1413. The accompanying tree highlights the three clusters within the full tree structure. The width of the coloured
rectangles is proportional to the number of individuals belonging to each cluster (yellow = 222; orange = 165; red = 123)

Fig. 3 Ultra-fine-scale genetic structure within Spain. Points representing individuals are placed on each of the magnified maps and coloured as described in
Fig. 1, with short dark lines pointing to their precise locations (the average birthplace of their grandparents). The three magnified maps show local elevation, rivers, and water bodies, as well as borders of autonomous communities (solid black lines) and provinces (dashed lines and text). a Locations of individuals (44) within the genetic clusters centred in Galicia. Note that we show this region at a higher level of the tree (14) as the lower level yields clusters with fewer than three individuals with fine-scale geographic location data. b Locations of individuals (60) within the clusters centred in the Basque-speaking
regions of País Vasco (Basque Country) and Navarra. For visual clarity we only show the individuals that are within the clade coloured blue and green in Fig. 1. This clade makes up the majority of all individuals located in this region, and a majority of this clade is located in this region (60 of 64 with geographic data). c Locations of individuals (16) who almost all comprise a single cluster exclusive to a ~50-km-wide region along the banks of the River Ebro in La Rioja, just south of País Vasco and Navarra

the Western Sahara donor group has none. Previous work showed
similar results30. If this European-like ancestry had arrived more
recently than the detected admixture event, the north Moroccan
donor group would be a poor proxy for the historical source
population and GLOBETROTTER would use a better alternative.
Since GLOBETROTTER detects admixture based on the DNA
received by the target population (Iberia) this would not affect the
date estimates25.
Our earlier results imply the incorporation of Basque-like DNA
elsewhere within Spain. We next incorporated the clade labelled
‘Basque1’ as a potential donor group, to characterise and date this
event (Methods). Ancestry profiles show contributions of Basquelike DNA (Fig. 5d) highest in places immediately surrounding the main location of the Basque donor group (País Vasco), and much
higher southwards than to the east and west. GLOBETROTTER
yields congruent results, and inferred dates for the arrival of
Basque-like DNA in the range 1190–1514 CE, more recent than the
north-west African influx (Fig. 7)
Our observation that genetic differences are small in the
north–south direction within Spain, and evidence of gene flow
preferentially in this direction, are most straightforward to
interpret in the light of historical information regarding the
Reconquista, during which Christian-controlled territory in the
north moved gradually southwards from the mid-8th Century,
following the Muslim conquest of Iberia (711CE). By 1249,
almost all of Iberia was under Christian rule, and the Battle of
Granada in 1492 marks the end of Muslim rule in Iberia. There is
historical evidence of migration of peoples from the northern
Christian kingdoms into newly conquered regions during the
Reconquista10,32. The east–west boundaries of the clusters we see
in the north of Spain correspond closely to the regions of broad
linguistic differences in the Christian-ruled north, which date
back to at least the first 200 years of Muslim rule (Fig. 1c), and we
date the southwards movement of Basque DNA later within the
Reconquista itself. Thus, it appears that present-day population

structure within Spain is shaped by population movements within
this key period.
We also detect a genetic footprint of the Muslim conquest, and
subsequent centuries of Muslim rule. Following the arrival of an
estimated 30,000 combatants33, a civilian migration of unknown
numbers of people occurred, thought to be mainly Berbers from
north Morocco and settling in many parts of the peninsula33. Our
analysis confirms and refines previous findings11,20,26 of a substantial and regionally varying genetic impact, narrowing to a
period spanning < 400 years. Crucially, unlike previous genetic

Fig. 4 Estimates of shared ancestry between Spanish individuals and across fineSTRUCTURE clusters. a Matrix of coancestry values used in cluster inference. Each of 1413 individuals is represented as a row, where each element is the coancestry (in cM) shared with each of the other individuals (see Methods for the definition of coancestry). In order to visualise the bulk of the variation, values equal to or above the 90th percentile (7.7 cM) are coloured black. The tree is as shown in Fig. 1a, and the horizontal black lines demarcate the clusters at the lower level of the tree, and labelled with points. b The distribution of the mean coancestry between individuals in the same cluster for 200 bootstrap resamples (Methods). Clusters are ordered by their median value, and coloured/labelled according to those shown in a. One cluster (part of the clade labelled ‘Galicia_central’) was excluded from this analysis as it
only contains 9 individuals. c Evidence for excess of coancestry with a source cluster compared to within-cluster coancestry. Each row of this matrix is a cluster inferred in the fineSTRUCTURE analysis as labelled in a. For each recipient cluster (rows) we tested whether the mean coancestry among individuals within the recipient cluster is smaller than their mean coancestry with individuals in each of the other clusters (columns). Each element is
coloured according to –log10(p), where p-values are based on 200 bootstrap resamples using the same sample size (13 individuals) for all clusters (Methods). Dark borders indicate source-recipient pairs with a p-value < 0.02 (not Bonferroni corrected). d Illustration of demographic scenarios leading to high coancestry between two different clusters. The symbols α and β represent clusters of individuals today, and α′ and β′ represent their ancestral
populations. Arrows represent mixing of one ancestral population into the other at some time (or times) in the past. In the left two scenarios individuals in β will have—on average—higher coancestry with each other than with individuals in α. In the right two scenarios it is possible for individuals in β to have higher coancestry with individuals in α than with each other (see Supplementary Note 5 for a fuller discussion)

studies of admixture in Iberia11,24,26, we avoid strong assumptions about the genetic make-up of the historical admixing
groups. Instead of specifying in advance the modern-day sources
that we assume best represent the historical populations that
came together in the past, we infer the best mixture of modernday populations from a large set of possible groups. Our GLOBETROTTER results suggest that amongst the six potential
African populations in our study, the best match to the predominant group involved in the actual admixture event is northwest African. Moreover, admixture mainly, and perhaps almost
exclusively, occurred within the earlier half of the period of
Muslim rule (Fig. 5b). Within Spain, north African ancestry
occurs in all groups, although levels are low in the Basque region
and in a region corresponding closely to the 14th-century Crown
of Aragon (compare Figs 1c, 5c). Therefore, although genetically
distinct22,23, north African-like ancestry in the Basque region
could be explained through genetic interactions between the
Basque groups and other parts of Spain within the past 1300
Perhaps surprisingly, north African ancestry does not reflect
proximity to north Africa, or even regions under more extended
Muslim control. The highest amounts of north African ancestry
found within Iberia are in the west (11%) including in Galicia,
despite the fact that the region of Galicia as it is defined today
(north of the Miño river), was never under Muslim rule34 and
Berber settlements north of the Douro river were abandoned by
741. This observation is consistent with previous work using Ychromosome data11. We speculate that the pattern we see is
driven by later internal migratory flows, such as between Portugal
and Galicia, and this would also explain why Galicia and Portugal
show indistinguishable ancestry sharing with non-Spanish groups
more generally. Alternatively, it might be that these patterns
reflect regional differences in patterns of settlement and integration with local peoples of north African immigrants themselves, or varying extents of the large-scale expulsion of Muslim
people, which occurred post-Reconquista and especially in towns
and cities10,32.
We show that population structure exists at ultra-fine scales in
Galicia (Fig. 3a), particularly in the province of Pontevedra, with
some clusters having geographic ranges of less than 10 Km (rootmean-square distance from cluster centroid). To our knowledge,
these results represent the finest scales over which such structure
has yet been observed in humans. Previously, it has been shown
that by jointly analysing people from a priori defined sampling
locations, subtle differences in group averages at certain genomic
loci can be observed at fine geographic scales. For example, subtle
differences in blood group frequencies have been observed among
villages in Italy’s 70-km-long Parma Valley35. Our results go
beyond this, showing that by leveraging information genomewide, it is possible to detect subtle genetic structure at fine geographic scales, without utilizing prior geographic information. It
will be interesting to identify whether (and if so which) other
parts of the world show similar patterns. Pairs of individuals
within these clusters show high levels of coancestry relative to the
rest of Spain (Fig. 4b). In contrast, when we only consider their
patterns of coancestry with non-Iberian groups this structure
disappears: individuals from Pontevedra are indistinguishable
from those from Portugal and other parts of western Spain
(Fig. 5a). Therefore, the very strong population stratification
observed in Galicia can most easily be explained through very
recent geographic isolation, occurring subsequent to major
migrations into the region (see Supplementary Note 4 for further
discussion). It is worth noting that differences in the amount of
population structure observed in different regions may be
sample-dependent (see Supplementary Note 2.3 for discussion).
In principle, if sampling is differentially biased towards, for
example, rural verses urban areas in different parts of Spain it
could potentially lead to differences in detected patterns of
structure. This might mask structure in some regions, but crucially, our approach would not be able to find structure if it was
not there.
Overall, the pattern of genetic differentiation we observe in
Spain reflects the linguistic and geopolitical boundaries present
around the end of the time of Muslim rule in Spain, suggesting
this period has had a significant and long-term impact on the
genetic structure observed in modern Spain, over 500 years later.
In the case of the UK, similar geopolitical correspondence was
seen, but to a different period in the past (around 600 CE)6.
Noticeably, in these two cases, country-specific historical events
rather than geographic barriers seem to drive overall patterns of
population structure.
It appears that within-country population structure occurs
across the world6,36, and in this study is observed down to scales
of <10 km. Such strong, localised genetic drift predicts the existence of geographically localised rare mutations, including
pathogenic ones. For example, cases of a specific form of inherited
ataxia (SCA36) cluster within a specific region of Galicia37. Our
results imply this phenomenon will tend to happen in specific
areas, and can arise even where population density is high and in
the absence of obvious strong geographic barriers, such as Galicia
in the case of Spain.

Data and quality control. For the Spain-only analysis we used genotype data that
was originally collected and typed for a colorectal cancer GWAS38. Biological
samples were sourced from a variety of hospitals across Spain as well as the Spanish 

Fig. 5 Characterising genetic contributions to Iberia. a Geographic distribution of 843 Iberian individuals grouped into six clusters based on haplotype sharing with external populations (Methods). More individuals (1530) were used in the inference, but only those with adequate geographic data are shown on the map. Background colours and the positions of points on the map are determined using the same procedure as for Fig. 1b, with the exception of individuals of Portuguese origin. No fine-scale geographic information was available for these individuals, so we placed them randomly within the boundaries of Portugal and show a single background colour (Methods). b Admixture dates and mix of admixing groups in single-date, two-way admixture events, as inferred using GLOBETROTTER (n = 541 individuals). On the left are the donor groups inferred to best represent the two ancestral populations involved in the admixture event (separated by a dashed line), along with the inferred admixture proportions of the smaller side (for donor groups contributing at least 1%). Estimated dates and 95% bootstrap intervals are shown on the right, for each target Iberian cluster as shown in a. The white
vertical dashed lines show the time of the initial Muslim conquest (711 CE) and the Siege of Seville (1248 CE), between which around half (or more) of Iberia was under Muslim rule. The admixture dates assume a 28-year generation time, and a current generation date of 1940 (the approximate average birth-year of this cohort). Detailed results of this GLOBETROTTER analysis are tabulated in Supplementary Tables 3a and 4. c, d We estimated ancestry profiles for each point on a fine spatial grid across Spain (Methods). The background colour shows the fraction contributed from a particular donor group, as defined by the scale bar. Grey crosses show the locations of the Iberian individuals used in the estimation: 843 in c, 793 in map d. Map c shows the fraction contributed from the donor group ‘NorthMorocco’. Map d shows the fraction contributed from the donor group ‘Basque1’, which we defined based on the Spain-only fineSTRUCTURE analysis (Fig. 1a). Maps for other donor groups are shown in Supplementary Figure 5

National DNA Bank. All samples were assayed together by Affymetrix (now
Thermo Fisher Scientific) in the same facility. See ref. 38 for full details of sample
collection and genotype calling. We used both cases and controls in Phase I of the
GWAS study, which totalled 1548 individuals prior to quality control. Individuals
from all 17 of Spain’s autonomous communities are represented in this dataset, but
the Spanish territories of Melilla, Ceuta, and the Canary Islands are excluded
from analyses involving geographic labelling due to limited sampling in these areas
(four samples).
After applying a series of quality control filters (Supplementary Note 1.1) we
phased the genotype data using SHAPEIT v239 with a reference panel and
recombination map from Phase I 1000 Genomes40. Close relatives (kinship
coefficient > 0.1), and individuals with evidence of recent, non-Spanish ancestry
were included in the phasing, but excluded from the fineSTRUCTURE analysis.
This procedure resulted in 1413 samples and 693,092 SNPs for our main analysis.
For all analyses involving individuals from outside of Spain we combined four
sources of genotype data: European samples from POPRES41, north African
samples from ref. 30, sub-Saharan African samples from Hapmap Phase 331, and
the Spanish samples described above. The POPRES and north African samples
were typed on the Affymetrix 500K array, which overlaps substantially with the

Fig. 6 Locations of donor groups and ancestry profiles of Iberian clusters. a Locations of individuals (n = 1503) within 30 non-Spanish genetic groups inferred using fineSTRUCTURE (Methods). Each point represents an individual, placed at their country-level location of origin, and coloured according to their inferred genetic group. Individuals from the same location (country) have been randomly jittered for visual clarity. Names are assigned to clusters based on where the majority of the individuals in the clusters are located. Where a cluster was split more evenly across two regions, a double-barrel name is used. All groups shown here, except ‘Portugal’, were used as donor groups in the analyses of Iberia. b Each column shows the ancestry profile for each of the inferred clusters shown in Fig. 5a. The heights of the bars show the proportion of each cluster’s ancestry which is best represented by that of the labelled non-Iberian donor group (Methods). Note that each row has a different y-axis range for visibility of the smaller components. Error bars show the range of the inner 95% of 1000 bootstrap resamples (Methods), and donor groups are only shown if at least one cluster has a range not including zero and a point estimate >0.001. The exact values plotted here and cluster sample sizes are in Supplementary Table 1
Affymetrix 6.0 array. We first merged these data and then applied quality filters to
the combined dataset of 6617 individuals, which we then phased altogether. After
excluding related individuals, those with self-reported mixed ancestry, and subsampling some heavily sampled regions (e.g. Switzerland) the dataset comprised
2919 individuals and 300,895 SNPs, which we used in our joint analyses. See
Supplementary Note 1.2 for full details of quality control and phasing.

Clustering using fineSTRUCTURE. We inferred clusters of individuals based on
genetic data only by applying the fineSTRUCTURE method28, which uses a modelbased approach to cluster individuals with similar patterns of shared ancestry.
Within the fineSTRUCTURE framework, shared ancestry is measured as the total
amount of the genome (in centiMorgans (cM)) for which individual i shares a
common ancestor with individual j, more recently than all the other individuals in
the sample. This is estimated for each pair of individuals i and j, defining a square
matrix referred to as the coancestry matrix (e.g. Fig. 4a). This matrix is then used to
cluster individuals into groups with similar patterns of coancestry, i.e. similar rows
and columns in the matrix.

We applied fineSTRUCTURE using the procedure
recommended by the authors, except in one aspect: to measure shared ancestry
(coancestry) we used the total amount of genome (in cM) for which individual i
shares a common ancestor with individual j, more recently than all the other
individuals in the sample. The software default coancestry measure is the number
of contiguous segments (chunks) rather than the total amount of genome, but we
found the alternative measure to be more robust to artefacts such as genotyping
error (Supplementary Figure 2; Supplementary Note 2.1).
Using the coancestry matrix, fineSTRUCTURE applies a Markov chain Monte
Carlo (MCMC) procedure to find a high posterior probability partition of
individuals into a set of clusters. The number of clusters is not specified in advance,
but rather estimated under the fineSTRUCTURE probability model. Having found
a set of clusters, fineSTRUCTURE then infers a hierarchical tree by successively
merging pairs of clusters whose merging gives the smallest decrease to the posterior
probability (of the merged partition) among all possible pairwise merges.
We ran several fineSTRUCTURE analyses using phased haplotypes from the
following sets of individuals:
(A) Spanish individuals
(B) Spanish and Portuguese individuals
(C) Non-Spanish individuals
In all cases we used CHROMOPAINTER software (v2)28 to estimate the
coancestry matrix, followed by fineSTRUCTURE’s clustering and tree-building
procudures. We used a previous successful application of fineSTRUCTURE6 as a
guide for the number of iterations in the MCMC, and other required parameters
(see Supplementary Note 2 for full details). We also checked that the MCMC
samples were largely independent of the algorithm’s initial position by visually
comparing the results of two independent runs starting from different random
seeds. Good correspondence in the pairwise coincidence matrices of the two runs
indicates convergence of the MCMC samples to the posterior distribution28. See,
for example, Supplementary Figure 1a showing the two independent runs for
analysis A. Without loss of generality, we used the first of these two runs in our
main analysis.
The statistical uncertainty of cluster assignments.

For analysis A, we measured
uncertainty in the assignment of individuals to clusters by using a procedure
described formally in ref. 6, which uses the information from multiple samples of
the fineSTRUCTURE MCMC. Informally, the procedure measures the overlap
between a cluster k, and individual i’s assigned cluster in each of the MCMC
samples within a single fineSTRUCTURE run. This can take values between 0 and
1, and sums to 1 across all clusters for a given individual. It provides a measure of
certainty about the assignment of individual i to each cluster k in the final set of
clusters. The cluster assignment certainty for an individual is the value corresponding to the final cluster assignment, and will be close to 1 if they are assigned
to a cluster with largely the same set of individuals in each MCMC sample. This
measure can be obtained for different layers of the hierarchical tree by summing
the values for the clusters that merge at each layer.

Fig. 7 Variation and timing of Basque-like genetic contributions in Iberia. Fraction contributions from the Basque-like donor group in ancestry profiles
(Methods) and Basque-like admixture dates (GLOBETROTTER) for each cluster inferred in the Spain-only analysis (as shown in Fig. 1a) plus Portugal. The clade labelled ‘Galicia_Pontevedra’ in Fig. 1a was combined into one group for this analysis. The admixture dates are for a two-way admixture event involving a Basque-like side and an European-like side, and shown with 95% bootstrap intervals (Methods). The dates shown assume a 28-year generatio time, and a current generation date of 1940. Detailed results of this GLOBETROTTER analysis are in Supplementary Table 3b
For fineSTRUCTURE analysis A, cluster assignments at higher levels of the
hierarchical tree are typically more certain than lower-level clusters. At the broader
level of the tree shown (14 background colours in Fig. 1b) 94% of individuals have
a cluster assignment certainty >0.7; and at the finer level (shown with points in
Fig. 1b), this level of certainty is reached by 89% of individuals (see Supplementary
Figure 1b). Furthermore, the clusters with the highest certainty tend to be those
with greater geographic localization, e.g. those labelled ‘LaRioja’ and ‘Baleares’
(Fig. 1).

Selection of levels of the Spanish tree to analyse. In the fineSTRUCTURE
analysis A 145 clusters were inferred. In order to examine properties of these
clusters, such as their geographical locations, we focused on two different levels of
the tree, thus highlighting both broad-scale and fine-scale structure. There is no
right level of the tree to pick, but we chose them based on the sizes of the clusters.
To examine broad-scale structure we chose a level (14 clusters) such that all
clusters were larger than size 20. Recall that moving up the tree, at each level a pair
of clusters is merged. At the base of the tree, newly separated groups typically
contain few (<15) individuals, with higher clustering uncertainty. To avoid the
presence of such minor clusters, we traversed up the tree until the first time a
merge occurred between two larger clusters (>15 individuals). This occurs at the
level of 49 clusters. However, at this level over half (28) of the clusters are within
the clade involving individuals mostly from south-west Galicia (labelled ‘Galicia_Pontevedra’ in Fig. 1a), and for many of these clusters fine-scale geographic
information was only available for one or two individuals, and/or the cluster
contained fewer than 15 individuals. Therefore, to aid visualization in Fig. 1b and
Fig. 3a we only show the clusters at the higher level of the tree (level 14) for this
clade, although the full tree is still shown in Fig. 1a.

Map-based data visualisation. Geographic information was available for individuals in the Spanish cohort, along with their age at collection, sex, and genotyping
plate (controls only) and batch used in genotype calling. The geographic information includes region of origin (autonomous community) for all individuals; for
959 individuals (68%) the birthplace (town) of at least one grandparent was
available, and for 883 of these the birthplace of all four grandparents was available.
We assigned each individual to a geographic coordinate by matching the text (e.g.
‘Barcelona’) to a municipal region as defined by the Spanish Statistical Office
(www.ine.es, 2014) and coding them to the centre of the matching region. Some
locations were not themselves a municipality, so we coded these individuals to the
centre of the nearest municipality, identified by using Google Maps. However, in the fineSTRUCTURE analysis we clustered all individuals, i.e. also included those
individuals for whom the exact birthplaces of their grandparents was not known,
but these are not used in interpreting the spatial distribution of the inferred genetic
In figures showing a map of Spain (e.g. Fig. 1b) each individual is represented
by a point placed at the average coordinate (centroid) of their grandparents’
birthplaces (coordinates were derived as described above). To minimise the effects
of very recent (20th Century) migration within Spain, only those individuals (726)
with all four grandparents born within 80 km of each other are shown on the maps.
See Supplementary Table 6 for the effect of this filtering in different regions of
Spain. Where many individuals have the same coordinate, such as in Barcelona,
points have been randomly shifted, by no more than 24 km, to aid visualisation. To
visually represent the discrete assignment of individuals to clusters by
fineSTRUCTURE, the members of each cluster are represented using the same
colour and symbol. We also coloured the background of the maps using a Gaussian
kernel smoothing procedure on a regular grid of 3-km-wide squares across Spain.
Informally, each square in the grid is coloured according to the relative
contributions of each cluster, where contributions are measured by Gaussian
densities centred on the location of each individual. See Supplementary Note 3 for a
formal description.
The linguistic maps in Fig. 1c are based on scanned images of previously
published work (Baldinger, K. La Formación de los Dominios Lingüísticos en la
Península Ibérica, Spain: Gredos, 1963, maps 6-9, 10(29)), which we adapted in the
following ways using Adobe Illustrator. Colours were added to help distinguish the
different linguistic groups and the political borders (from Map 10 by Baldinger29)
were overlaid onto the right-most map.

Signals of drift and admixture in the coancestry matrix. Recall that coancestry
(as we have used it) measures the amount of genome (in cM) for which an individual i shares its most recent common ancestor with another individual j, out of
all the individuals in the sample. Properties of this matrix are informative of
patterns of drift and admixture within and across clusters inferred by fine structure. Specifically, excess coancestry between individuals in the same cluster
(within-cluster coancestry) is a natural measure of genetic drift of that cluster
relative to all the other clusters28. In general, individuals are often observed to have
the highest levels of coancestry with other individuals in their assigned cluster. This
is not a constraint of the fine structure model; rather it is because if two
individuals have similar patterns of shared ancestry, they are naturally also likely
have more recent shared ancestry between them. However, it is possible for this not to be the case, and this is informative of admixture (see Supplementary Note 5 for
We looked for such signals in the case of Spain by testing whether a cluster
(inferred by fine structure) has a within-cluster coancestry that is, on average,
smaller than its coancestry with another cluster. We used the 26 clusters (indicated
with symbols on the axes in Fig. 4a), which contained at least 13 individuals. To
avoid potential bias due to uneven sizes of the clusters, we estimated within-cluster
coancestry levels by randomly sub-sampling (without replacement, as coancestry is
only defined between two different individuals) each of the 26 clusters such that
there were of equal size (13). We re-computed the coancestry matrix using
CHROMOPAINTER, and the same set of parameters as in fine structure
analysis A, but using this smaller subset. We repeated this 200 times and used these
resamples to compare coancestry levels across clusters. For each resample and each
cluster we computed the mean of the coancestry values within that cluster
(excluding zeros on the diagonals), and with each of the other clusters. We then
computed a p-value using the number of resamples (S) for which the mean withincluster coancestry is smaller than the mean coancestry with each of the other
clusters. That is: ¼ Sþ1
201 . Results are shown in Fig. 4b, c.

Principal components and Fst. For both PCA and FST analyses we used a set of
143,599 LD-pruned SNPs (r2 < 0.2) by applying the ‘–indep-pairwise r2’ command
in PLINK (v1.7)42, and excluded regions of long-range LD derived from43. We
computed principal components of the genotype calls using the software Shellfish44,
and FST between different groups of individuals using EIGENSOFT (v5.0.1)45. We
conducted two FST analyses: one using autonomous community as group labels,
and the other using clusters inferred by fineSTRUCTURE as group labels.
In the FST analysis using autonomous community as group labels the strongest
differentiation (but still weak, at 0.002) is between the Basque-speaking regions and
all other regions, and between Galicia and other regions (Supplementary
Figure 9b). The principal components analysis revealed a similar pattern,
separating the same regions (Supplementary Figure 9a). The range of pairwise FST
values is higher when grouping individuals by the clusters inferred by
fine-structure (0–0.008) (Supplementary Figure 9c). This highlights
fine structures ability to find clusters of individuals who share genetic drift,
and reveals two highly drifted clusters within Galicia and the Basque region.

Defining donor groups. In order to define a set of donor groups using the
combined European, north African, and sub-Saharan African individuals, we
applied fine structure as described above in several rounds, each using the
following sets of individuals:
(CI) All individuals combined (excluding Spanish but including Portuguese)
(CII) Individuals from North Africa only
(CIII) Individuals from Europe only
In analysis CI, fine structure cleanly split the three main groups
corresponding to Europe, north Africa, and sub-Saharan Africa, as well as inferring
finer sub-structure. However, in order to maximise the power to detect finer scale
structure28, we obtained fine structure results for the north African and
European groups independently. That is, using coancestry matrices that only allow
copying within each set of individuals in CII and CIII, respectively. We then
defined a set of donor groups based on the clusters and hierarchical trees inferred
by fine structure in these three analyses. We considered the following factors in
defining donor groups. Ideally, each donor group would contain about the same
number of samples, and not be too small. Donor groups should also be relatively
homogeneous with respect to their shared ancestry with the population of interest
(in this case Iberia), although could be heterogeneous within themselves. We
therefore prioritised donor group size over capturing finer scale structure that
might exist within donor groups themselves. See Supplementary Note 6 for full
details. Our procedure resulted in a total of 29 donor groups with median size 30,
and minimum size 16, totalling 1386 individuals (not including Portuguese). Their
locations are shown in Fig. 6a. Labels of the inferred groups are based on the
sampling locations of most of the individuals in a given group. In some cases the
majority of individuals were split across two locations, and this is indicated by a
multi-region label (e.g. Germany–Hungary)

Treatment of Portugal. One cluster in the fine structure analysis CIII overlaps
significantly (98%) with the individuals with grandparental origins in Portugal as
reported by the data source (POPRES). For the purposes of the analyses in this
chapter (and fine structure analysis B), this group of 117 individuals is referred
to as ‘Portugal’ or ‘Portuguese individuals’ (e.g. in Fig. 6a). The strong genetic
similarity between individuals from Portugal and Spanish individuals, especially
those located in Galicia (Fig. 2a), means they are likely to share a similar admixture
history, and including Portugal as a donor group would mask the signal from those
shared events. We therefore excluded them from the set of donor groups and
instead treated them in the same way as the Spanish individuals, bringing the total
number of Iberian samples we analyzed to 1530. This is analogous to the rationale
for excluding Ireland as a donor group in the British Isles study6.
Clustering based on haplotype sharing with external groups. Here we describe
the method we used to infer clusters of Iberian individuals with distinct patterns of haplotype sharing with external groups. We used the fine structure clustering
algorithm but with a modified version of the input coancestry matrix. Specifically,
we compute the coancestry (using CHROMOPAINTER) between each Iberian
individual and each of the non-Iberian individuals, as described above, but only
allowing Iberian individuals to copy from non-Iberian individuals. This results in a
rectangular matrix, X, with N rows and M columns, where N is the number of
Iberian individuals and M the number of non-Iberian individuals. We then constructed an (N + M)×(N + M) square matrix C, such that, and matrix Y contains zeros, except for each of the (block) diagonal entries corresponding to pairs of individuals within the same donor population k. These
entries each take the value gk, which is determined such that the mean of all the
entries corresponding to donor population k are the same for the sub-matrix X as
the sub-matrix Y. The zeros in the matrix C have the effect of not allowing any
copying from or to the Iberian individuals to contribute to the fine structure
likelihood. We then run fine structure algorithm using the force file option
(-F), where each ‘continental’ group is a donor group, thus only allowing splits and
merges to take place among Iberian individuals. We used a c-factor of 0.0579,
which was computed in the manner described in Supplementary Note 2.2, but
using segments of DNA from a CHROMOPAINTER run where we only allowed
Iberian individuals to copy from non-Iberian individuals. 

Estimating ancestry profiles. We estimated ancestry profiles for each of the
Iberian clusters using the procedure described previously6. Briefly, we use
CHROMOPAINTER to compute a coancestry vector for each Iberian individual,
where we only allow them to copy from haplotypes in the donor groups (as with
matrix X above), and then average the coancestry vectors within each cluster and
donor group. For each Iberian cluster i, we then find a smaller set of donor groups
which together (as a non-negative linear mixture whose coefficients sum to 1) best
explains its cluster-averaged coancestry vector, yi (see Supplementary Note 7.1 for
full details). The vector of coefficients in the linear mixture is the ancestry profile
for cluster i, and its elements sum to 1. Results for six Iberian clusters are shown in
Fig. 6b. We also performed a complementary analysis where we treated each donor
group in turn as yi, after removing their corresponding elements in the coancestry
vectors and re-normalising (Supplementary Figure 4; Supplementary Note 7.1).
We measured uncertainty in these ancestry profiles by re-estimating the clusteraveraged coancestry vectors using a set of pseudo individuals. Each pseudo
individual is formed by randomly selecting an individual in cluster i for each
chromosome, and summing the observed chromosome-level coancestry vectors
across all chromosomes. We then compute 1000 such re-estimations and report the
range of the inner 95% of the resulting bootstrap distribution.

Spatially smoothed ancestry profiles. The availability of fine-scale geographic
information for many of the Spanish individuals allowed us to estimate the spatial
distribution of shared ancestry (Fig. 5c, d; Supplementary Figure 5). Instead of
averaging coancestry over individuals within a cluster, we average across geographic space using a Gaussian kernel smoothing method that varies the kernel
band-width depending on the density of available data points (see Supplementary
Note 7.2 for details). This gives a set of coancestry vectors, ys, for each grid-point s
in a fine spatial grid across Spain. We then compute ancestry profiles for each of
the grid points in the same way as for the Iberian clusters (described above), but
setting yi ¼ ys instead of a cluster-averaged coancestry vector. We visualize the
results by colouring each grid-point according to the value of its coefficient for a
single donor population of interest (e.g. NorthMorocco in Fig. 5c). For any gridpoint s and individual i located within the borders of Portugal we set all ys to be the
average coancestry vector across these individuals, because we have no fine-scale
geographic information for them. This means in Portugal there is always one
colour plotted.

Estimating admixture dates and source populations. We used the GLOBETROTTER algorithm to estimate dates, proportions and configurations of
admixture events25. Briefly, GLOBETROTTER uses ‘paintings’ from the CHROMOPAINTER algorithm to construct coancestry curves. These curves measure the
rate of decay of linkage disequilibrium with genomic distance, between sites with
ancestry from a pair of source populations. The parameters of exponential functions fitted to these curves (decay rates and intercepts) are used to estimate
admixture dates, admixture proportions, and the best fitting mix of modern-day
groups that characterise the ancestral populations involved in an admixture event.
We conducted two analyses using GLOBETROTTER. The first (gtA) was
designed to detect admixture event(s) in the history of Iberia that might involve
any combination of non-Iberian source populations, without any prior
assumptions on the nature of the event. The second analysis (gtB) was designed to
detect only admixture event(s) involving a Basque-like source population, i.e. based
on a prior hypothesis. In each case we defined a set of target groups within which to
look for an admixture event; a set of donor groups, which we allow to be donors in
the initial painting; and a set of surrogate populations, which we allowed GLOBETROTTER to consider as components of any admixture event
(Supplementary Table 2).
After identifying the presence of admixture based on criteria recommended by
the authors, we next evaluated the evidence for more complex admixture events
(e.g. multiple dates or more than two source populations). GLOBETROTTER
automatically tests for these using a series of criteria based on how well the
coancestry curves fit the models for different types of admixture scenarios25. Using
GLOBETROTTER’s automated criteria, in analysis gtB there was no evidence that
a two-date admixture model fitted better than a one-date model. However, for
some target populations in analysis gtA there was some evidence for a two-date
admixture event, although the one-date event fit well in all cases (Supplementary
Table 3a). Given the potentially complex nature of admixture in Iberia, we further
evaluated the evidence for a two-date admixture event by considering the model fits
for each coancestry curve separately (Supplementary Figure 7; Supplementary
Note 8.2). Notably, only the coancestry curves involving a sub-Saharan African
surrogate group fit better to a two-date admixture event. The improved fit for the
curve for the sub-Saharan African surrogate group ‘Nigeria.YRI1’ is visually
apparent in the coancestry curve shown in Supplementary Figure 7. We therefore
consider the one-date admixture event to be a better fit overall, but that there is
some evidence for a second event involving sub-Saharan African-like DNA mixing
with European-like DNA, with the strongest evidence for this in the Iberian cluster,
‘Portugal-Andalucia’. In the target groups where there is evidence of this,
GLOBETROTTER infers dates in the range 1370–1700 CE (assuming a 28-year
generation time).

Ethics statement. The collection of the Spanish genotype data was approved by
the “Comité Ético de Investigación Clínica de Galicia”, and each of the institutional
review boards of the participating hospitals. All samples were obtained with written
informed consent reviewed by the ethical board of the corresponding hospital, in
accordance with the tenets of the Declaration of Helsinki. A previous analysis of
these data was published in 201327. All other data used in the paper were previously published and are publicly available for research use.
Code availability. Some specialist code used in the paper, which is not already
publicly available, will be made available for academic research on request. We
used the following versions of software: Birdsuite v1.4, KING v1.4, QCTOOL v1,
GLOBETROTTER v3, EIGENSOFT v5.0.1. Details of parameters used in
particular analyses are in the relevant section of Methods or Supplementary
Data availability
Data for the Spanish cohort were previously published38 and the individual-level
genotype data were used here with permission from those authors. While these data
are not yet made openly accessible, they can be made available for academic
research on request. All other data sets used in the paper have been previously
published, and are publicly available for research use. Specifically, European samples
were sourced from POPRES41 (dbGaP Study Accession: phs000145.v4.p2; approved
project number: 6507); north African samples were sourced from a previous
publication30 (accessed via http://www.biologiaevolutiva.org/dcomas/north-africanaffy-6–0-data-henn-et-al-submitted/); sub-Saharan African samples from Hapmap
Phase 331 (accessed via ftp://ftp.ncbi.nlm.nih.gov/hapmap/phase_3/).

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We acknowledge support from the Wellcome Trust (203141/Z/16/Z, 090532/Z/09/Z,
098387/Z/12/Z, 095552/Z/11/Z, 212284/Z/18/Z) and Fondo de Investigación Sanitaria
(Grants PI13/01136 and PI16/01057). We thank G. Hellenthal, D. Lawson and G. Busby
for advice on the use of fineSTRUCTURE and GLOBETROTTER. Also G. Hellenthal for
providing computer code for the analysis of cluster assignment uncertainty in the
fineSTRUCTURE analysis. We also thank M. Robinson, F. Dubert García, and R. Villares
Paz for providing background on the history of the Iberian peninsula and advice on
historical sources. The support of the Spanish National Gentyping Center (CEGENPRB2) is also acknowledged with appreciation.
Author contributions
A.C., P.D. and S.M. initiated and designed the study. C.F.-R., C.R.-P. and I.Q.-G. collected the data for the Spanish cohort and carried out genotype calling under the
direction of A.C. C.B. performed the analyses under the joint supervision of S.M. and
P.D. A.C. provided historical information. C.B., A.C, P.D. and S.M. w 

Additional information
Supplementary Information accompanies this paper at https://doi.org/10.1038/s41467-
Competing interests: S.M. is a director of GENSCI limited. P.D. is a director and Chief
Executive Officer of Genomics plc, and a partner of Peptide Groove LLP. The remaining
authors declare no competing interests.
Reprints and permission information is available online at http://npg.nature.com/
Journal peer review information: Nature Communications thanks the anonymous
reviewers for their contribution to the peer review of this work. Peer reviewer reports are
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1 Comment

  • According to two recent studies, prehistoric migration impacted the genetic history of the Iberian Peninsula. Findings suggest that North Africans called Iberia home more than 4,000 years ago.
    The Iberian Peninsula had an interesting history, that was related to North Africa thousands of years before the Muslim conquest. Migration and trade were two factors that brought Moroccans and other North African inhabitants to Iberia.

    These findings were based on two recent scientific studies that shed light on the genetic history of the region, known in modern day as Spain and Portugal.

    Ancient DNA studies conducted by a group of researchers, led be Inigo Olalde, a geneticist from the Harvard Medical School, and published, Thursday, in peer-reviewed journal Science, shows that Iberia was home to North Africans 4,400 years ago.

    Tracing back the genetic history of the peninsula, the study revealed that «archaeological human remains dating from about 7000 years ago to the present elucidate the genetic impact of prehistoric and historic from migrations from Europe and North Africa».

    Researchers found out that migration from the two continents brought changes, throughout all these years, to the DNA of those who once inhabited the European peninsula. The study, quoted by New York Times, shows that a 4,400-year-old skeleton of a man who was buried in a central Spain grave is «100 percent North African».

    According to Harvard Medical School geneticist David Reich, who is a co-author in the study, the findings looked «crazy». «We double-checked it because it was so weird». However, further discoveries confirmed this theory. Studying the DNA of a 3,500-year-old skeleton of a woman, researchers realized that she had a «North African grandparent».

    These striking findings unveiled an unknown chapter in the history of the region. To put it in other words, Iberia was inhabited by North Africans, 3,000 years ago, before the Roman Empire took control of the peninsula.

    North African ancestry kept growing in the region, during the Iron Age, according to the study’s findings and Mediterranean trade was seen as a factor that pushed North Africans to settle down in Iberian towns. The journey of North African ancestry did not stop there but was extended even after Iberia became part of the Roman empire.

    This study was supported by a second one entitled «Patterns of genetic differentiation and the footprints of historical migrations in the Iberian Peninsula». Conducted by a group of researchers, the study showed that North African DNA survived, even after Muslims left the region.

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