Showing posts with label aDNA. Show all posts
Showing posts with label aDNA. Show all posts

April 8, 2018

Luxmanda: a 3,000 years-old proto-Horner in Tanzania

I knew, more intuitively than rationally, that the Horner (Ethiopian, Somali, Eritrean) type of Afro-Eurasian admixture was very old but no idea it was so much. I knew that West Eurasian Upper Paleolithic had an impact on Africa (LSA) but I did not know it went so deep South nor that it had left such a massive legacy as ancient DNA reveals.

Pontus Skoglund et al. Reconstructing Prehistoric African Population Structure. Cell 2017 (open access). DOI:10.1016/j.cell.2017.08.049

The data analysis speaks volumes by itself:

Figure 1 - Overview of Ancient Genomes and African Population Structure


Figure S2 - Ancient Individuals and African Population Structure

Figure 2 - Ancestral Components in Eastern and Southern Africa

We show bar plots with the proportions inferred for the best model for each target population. We used a model that inferred the ancestry of each target population as 1-source, 2-source, or 3-source mixture of a set of potential source populations.


So much that I don't really know what else to say. Of course this is just a sample of what there is in the paper, read it. I'm sure there will be plenty of comments even if the study was published months ago.

Regarding haploid DNA, I don't see anything outstanding but, as I know there is generally quite a bit of interest, these are screenshots of the ancient lineages found (full data in the supp. materials of the study):

Ancient Y-DNA (screenshot)
Ancient mtDNA (screenshot)

Related: No Iberian in Iberomaurusian.

Correction: I first titled this "a 30,000 years-old...". That was a major error on my part and I apologize for any confusion it may have caused. Thanks to Capra Internetensis for spotting it.

No Iberian in Iberomaurusian

After almost a century of controversy on the matter, it seems that archaeogenetics solved the riddle. Not in the sense I thought it would but it did anyhow.


Ancient DNA samples from Taforalt (Iberomaurusian or Oranian culture, Upper Paleolithic of North Africa) show no trace of Paleoeuropean ancestry (WHG), however they show strong affinity to West Asians of Palestinian type, showing also some significant amount of African Aboriginal ancestry, probably closer to East African Hadza and Sandawe and ancient Mota than to West African types. The result is something roughly similar to Afars but not quite the same in any case. 

Fig. S8 - Taforalt individuals on the top PCs of present-day African, Near Eastern and South European populations.


Fig. S11 - ADMIXTURE results for a few informative K values.

So the conclusion must thus be that the Eurasian influence in North African Upper Paleolithic (call it Iberomaurusian, Oranian or my personal unorthodox preference: Taforaltian) arrived from West Asia with whichever intermediate stage in Egypt and Cyrenaica, where that influence is quite apparently much older in the archaeological record. This seems contradictory to the chronology of Taforaltian, with Western sites producing older radiocarbon dates but the genetic data seems overwhelming. 

I must say I wish they would have contrasted with older (and available) Paleoeuropean samples than WHG (Epipaleolithic) but I guess that some WHG influence would have shown up if there was some older European influx because the various Paleoeuropean layers are not disconnected. But it is still something someone should test, just in case. 


Haploid DNA


The Taforalt sample was rich in mtDNA U6a, with also one instance of M1b:


All six male samples carried Y-DNA E1b1b, with most of them being well defined as E1b1b1a1-M78 (see table S16 for details).


Related: Luxmanda: a 3,000 years-old proto-Horner in Tanzania.

March 17, 2018

Oldest known Iberian R1b-S116 (and DF27) is NOT at all Indoeuropean

This study is very interesting but it is very wrongly argued, maybe in an attempt to fit their findings with what has sadly become the mainstream current of "explanation" about the origins Y-DNA haplogroup R1b-S116 (also P312, etc.)

Cristina Valdiosera, Thorsten Günther et al. Four millennia of Iberian biomolecular prehistory illustrate the impact of prehistoric migrations at the far end of Eurasia. PNAS 2018. DOI:10.1073/pnas.1717762115

The issue is that they found the very first known carrier of R1b-S116 (and R1b-DF27, the main Iberian haplogroup) in an individual of the Bronze Age of Lower Rioja (Cueva de Los Lagos, Alhama de Cervera), belonging very clearly to the Central Iberian culture of Cogotas I, even if it is at its very northeast margin.

What is wrong? Well, the very title is wrong. It is nothing but an artifact produced by forced (supervised) results of Admixture within the simplistic 3-population model. Even then their result is in fact so weak that it immediately cried to me as "artifact" (noise or whatever you want to call it) and it is effectively nothing but that. 

And to demonstrate it is as simple as digging into the supplementary materials and look at the unsupervised Admixture run (dataset S03), whose optimal columns (lowest CV scores) are K=16-19 (all four are optimal, what is fine with me but makes explanation and understanding a bit more dense). 

As that unsupervised admixture is massive, with lots of global populations ancient and modern, I made a selection using only the four optimal K-values (K=16 to K=19, from left to right):

Click to expand (labels at bottom are mine)

And it is absolutely clear from K=16 to K=18 that there is not a speck of the Caucasus component which is absolutely universal in all the true Indoeuropean samples. There is a tiny speck of it in the K=19 column but there even Sardinians and some Anatolian Neolithic individuals have it at much greater values and thus cannot anymore be automatically interpreted as Indoeuropean marker, but just as extra Caucasus affinity present in some Neolithic-derived populations or individuals more than others since the very beginning of mainline (Vasconic) European Neolithic at the Aegean.

And this is it. Quod erat demonstrandum (Q.E.D): R1b-S116, at least in Iberia, has nothing to do with Indoeuropean expansion, nothing at all: it is absolutely clear that it is a pre-Indoeuropean thing. And it has been present in Lower Rioja since at least the Bronze Age.

Furthermore, when we look at the Central European Bell Beaker (Central BB) samples and compare them with their immediate chronological precursors of (definitely Indoeuropean) Corded Ware culture, we must admit that there is a decrease of the Caucasus component and an increase of the Vasconic Neolithic (light blue) element. This also speaks against the Indoeuropean "explanation" for the expansion of R1b-S116 into Central Europe, because the first known such ancient carriers are from the Bell Beaker period and not a moment earlier, and these clearly express an anti-Indoeuropean tendency in their autosomal genomes.

There is however a sizable Indoeuropean component in modern non-Basque Iberians, smaller than in most other European populations but very clear nevertheless. This must have arrived at later times: (1) with the Celts, who arrived to Catalonia at the end of the Bronze Age, later expanding into Central and Western Iberia, (2) with the Romans, (3) maybe also to some extent with the Germanic invaders of the late Roman period. None of these expansions seem particularly associated with R1b-S116, however the c. 1% R1a and the c. 8% J2 (with plausible Italo-Roman origin) should be related to it, along with an assortment of other haplogroups. 

For those willing to dig in the details, there is also a small treasure trove of other ancient Y-DNA, mostly I (which underlines the Paleoeuropean influence in Neolithic Iberia, regardless of whether this is local or was carried on from further East by the Neolithic settlers), as well as one instance of unspecific R1b, another of G and another of H.

Someone may ask, which is then the origin and means of expansion of R1b-S116, if not Indoeuropean? Good question to which I don't have yet a well defined answer. But my tentative explanation is that it should be related to two ultimately related processes within Western European "Neolithic" (Late Neolithic and Chalcolithic): 
  1. The well documented phenomenon of increase, in most areas at least, of the Paleoeuropean component time passes, this may be to some extent because of simple absorption of local subneolithic "hunter-gatherers" but it probably also produced different subpopulations within the Western Neolithic and in some cases we do see these peripheral "Second Neolithic" groups expanding at the expense of the "First Neolithic" peoples. This is most clear in Central Europe with the expansion of Funnelbeaker cultures from, probably, Denmark and nearby areas of Low Germany. In fact Michelsberg culture and its close relative in France Seine-Oise-Marne basically wipe out the first farmers of LBK (Linear Pottery) at what I usually describe as the Chalcolithic but is often described as Middle or Late Neolithic in other sources.
  2. Clearly Bell Beaker had something to do: we see their impact in Germany, Britain and Ireland and one could argue that Cogotas I is somehow derived from the Bell Beaker of Ciempozuelos, although in this I'm going to remain neutral and a bit skeptic until more evidence shows up. 
But what seems very apparent tome is that R1b-S116 should have expanded from somewhere in France, probably towards the South. And we do need better genetic studies, including archaeogenetic ones, on the Hexagon before we can jump to conclusions. France is not the most affected area by Bell Beaker, so I am cautious about attributing too much weight to only Bell Beaker and I would rather think on a complex succession of expansions associated to various cultures. 

Of great interest here should be the ill-known but fascinating Artenacian culture, which expanded in all West France and Belgium from a core at Dordogne before the BB period and coincident with the Corded Ware expansion in Central Europe. Like Bell Beaker folk, they were adept at bowmanship but their area is not densely affected by Bell Beaker later on (although there is indeed a scatter of findings). I do wonder if somehow Bell Beaker is derived from Artenac, even if it is clearly not the same thing. Food for thought.


Update (March 18): small steppe-like noise appears in diverse Iberian samples since the Late Neolithic/Chalcolithic.

This has arisen in the discussion below (h/t to MZ): when the supervised (forced assignment to rigid three populations) is used, the appearance of "steppe" ancestry is found here and there also before the Bronze Age. As we see above, this is not real: it does not happen in the unsupervised model at all but mere "noise" or "artifact" produced by the excessive simplicity of the three populations model.



This does not make the three populations model "wrong": it is still approximately right but "evidence" produced  ONLY from rigidly applying this model is not evidence of anything, just a hint to be confirmed or rejected via wider analysis at best.

March 16, 2018

Ancient DNA from the Balkans

This study has been for several months around but I have not discussed until now and is well worth a mention.

Ian Mathieson et al., The Genomic History Of Southeastern Europe. BioRXiv (pre-pub) 2017. doi:10.1101/135616

There is a lot of ancient autosomal DNA from the region but it basically says one thing: everything was almost exactly as expected from archaeology. The Karanovo-Gumelnita people, famed for inventing the Bronze Age a whole millenium earlier than anyone else, and nearby related cultures, were within the mainstream (Vasconic) Neolithic genetic grouping. This changed however with the kurgan invasion expressed primarily in the Ezero culture, which I've been told should be considered direct precursors of Thracians. But the change is not something radical: more genetic affinity with the steppe is visible than before and more generalized through all samples. 

Fig. 1-D - Supervised ADMIXTURE plot, modeling each ancient individual (one per row), as a mixture of
populations represented by clusters containing Anatolian Neolithic (grey), Yamnaya from Samara (yellow), EHG (pink) and WHG (green).
  (click to expand)


Well, it is not exactly everything what is as expected, if we consider Polish Globular Amphorae culture, which I would have expected to be at least somewhat steppary, Indoeuropean, already but were not. Thus it seems I have to concede on this culture and its precursors being still part of the Vasconic Neolithic. This makes Indoeuropean penetration into Central-North Europe a much more sudden episode and one directly tied to Corded Ware culture and nothing else. It must have been perceived by its victims like a massive catastrophe, because it was a huge area which they conquered and to a large extent colonized in a very short span of time.


Lots of R1b in Epipaleolithic Balkans

Most interesting anyhow is the huge hoard of ancient Y-DNA R1b in the Iron Gates region (Lepenski Vir) before the Neolithic. This not only demonstrates, again, that this haplogroup is Paleoeuropean, at least in part, but, quite intriguingly makes earlier findings on modern data suggesting a possible origin or R1b-M269 in or near modern Serbia (Morelli 2010 and Myres 2010) at least somewhat plausible. However none of the Iron Gates R1b is described as R1b-M269 and in some cases it is excluded that it could be this sublineage. 

Thus the issue of the ultimate origins of this key lineage remains open, but let me underline that these Iron Gate individuals belonged to the WHG grouping, as did Villabruna (so far the oldest R1b carrier kown) and that they breach this way the assumed haplogroup homogeneity I2 conceived on merely Central and Northern European samples. Just as happened with mtDNA U haplogroup homogeneity when mtDNA H was detected by several independent studies of Iberian ancient DNA. It is normal to expect more diversity towards the south for several reasons but maybe the most critical of them is just average temperature, which makes the southern lands naturally more fertile (notably so for crops domesticated in the Middle East) and easier to inhabit. 

This trend was only broken in the Middle Ages when the heavy plough allowed the improved exploitation of deep Oceanic soils, being useless in the Mediterranean region of shallow soils however. It was only then when the center of European development moved from south to north, to Belgium specifically, where it remains till present day. So let's take Southern Europe a bit seriously, please.

Ancient genomes of SE Asia

Just a quick mention because I have such a long queue of stuff from Europe that I really have no time to look but very shallowly onto this study, which looks extremely interesting. Credit for the reference to Kristiina.

Hugh McColl, Fernando Racimo, Lasse Vinner, Fabrice Demeter et al., Ancient Genomics Reveals Four Prehistoric Migration Waves into Southeast Asia, BioRXiv (pre-pub) 2018. doi:10.1101/278374

Abstract

Two distinct population models have been put forward to explain present-day human diversity in Southeast Asia. The first model proposes long-term continuity (Regional Continuity model) while the other suggests two waves of dispersal (Two Layer model). Here, we use whole-genome capture in combination with shotgun sequencing to generate 25 ancient human genome sequences from mainland and island Southeast Asia, and directly test the two competing hypotheses. We find that early genomes from Hoabinhian hunter-gatherer contexts in Laos and Malaysia have genetic affinities with the Onge hunter-gatherers from the Andaman Islands, while Southeast Asian Neolithic farmers have a distinct East Asian genomic ancestry related to present-day Austroasiatic-speaking populations. We also identify two further migratory events, consistent with the expansion of speakers of Austronesian languages into Island Southeast Asia ca. 4 kya, and the expansion by East Asians into northern Vietnam ca. 2 kya. These findings support the Two Layer model for the early peopling of Southeast Asia and highlight the complexities of dispersal patterns from East Asia.


March 2, 2018

Two big issues with Olalde 2018 (Indoeuropean Bell Beaker speculation)

"Just for being published in Nature it does not mean it is necessarily wrong" (popular saying).

Iñigo Olalde et al., The Beaker Phenomenon And The Genomic Transformation Of Northwest Europe. doi:10.1101/135962 (pre-pub version, no way I'm spending 1/3 of my monthly income on this, in case you're willing to waste your money, it's been recently published in Nature)

Issue 1: All comparisons are made between Anatolia Neolithic or other Early Neolithic in some cases and Yamna or Corded Ware.  Late Neolithic is not used nor, critically, is Hunter-Gatherer populations or anything related. 

This means that everything will be much more Yamna-like than it should, just because Yamna are 50% HG, while Early Neolithic are very low in this component. 

This is very apparent in the PCA:



It's junk-in: junk-out.

Issue 2: there is a huge sampling gap precisely where ancient mtDNA (and modern Y-DNA) tells us that the origin of the modern West-Central European genetics should be: in France and nearby areas like the Basque Country, West Germany. Sure: they sample some Eastern French sites but see "issue 1" above.



Nuff said.

February 21, 2018

Caribbean Taino ancient DNA still alive in admixed populations

Taino Native Americans also had a very high genetic diversity, comparable to other continental large native populations such as Andeans or Amazonians, what speaks of high mobility in the Caribbean islands before European colonization.

The mitochondrial lineage B2 was sequenced, although it is today rare in the region.

Hannes Schroeder et al., Origins and genetic legacies of the Caribbean Taino. PNAS 2018. DOI:10.1073/pnas.1716839115

The Caribbean was one of the last parts of the Americas to be settled by humans, but how and when the islands were first occupied remains a matter of debate. Ancient DNA can help answering these questions, but the work has been hampered by poor DNA preservation. We report the genome sequence of a 1,000-year-old Lucayan Taino individual recovered from the site of Preacher’s Cave in the Bahamas. We sequenced her genome to 12.4-fold coverage and show that she is genetically most closely related to present-day Arawakan speakers from northern South America, suggesting that the ancestors of the Lucayans originated there. Further, we find no evidence for recent inbreeding or isolation in the ancient genome, suggesting that the Lucayans had a relatively large effective population size. Finally, we show that the native American components in some present-day Caribbean genomes are closely related to the ancient Taino, demonstrating an element of continuity between precontact populations and present-day Latino populations in the Caribbean.

Fig. 2.
Taino demography. Total estimated length of genomic ROH for the Taino and the Clovis genome (13) and selected Native American and Siberian genomes (15, 31, 32) in a series of length categories. ROH distributions for modern individuals have been condensed into population-level silhouettes (SI Appendix, section 14).

February 14, 2018

Dystruct versus Admixture

Not really able yet to discern if this is an alternative way ahead for autosomal archaeogenetics or just another dead end. But it does seem interesting enough to mention here in any case.

It may be very important in the deciphering of the so-called "ANE" ghostly genetic influence.

Tyler A. Joseph & Itsik Pe'er. Inference of population structure from ancient DNA. bioRXiv 2018 (pre-pub). DOI:10.1101/261131

Methods for inferring population structure from genetic information traditionally assume samples are contemporary. Yet, the increasing availability of ancient DNA sequences begs revision of this paradigm. We present Dystruct (Dynamic Structure), a framework and toolbox for inference of shared ancestry from data that include ancient DNA. By explicitly modeling population history and genetic drift as a time-series, Dystruct more accurately and realistically discovers shared ancestry from ancient and contemporary samples. Formally, we use a normal approximation of drift, which allows a novel, efficient algorithm for optimizing model parameters using stochastic variational inference. We show that Dystruct outperforms the state of the art when individuals are sampled over time, as is common in ancient DNA datasets. We further demonstrate the utility of our method on a dataset of 92 ancient samples alongside 1941 modern ones genotyped at 222755 loci. Our model tends to present modern samples as the mixtures of ancestral populations they really are, rather than the artifactual converse of presenting ancestral samples as mixtures of contemporary groups.


Still digesting this one but I do find very intriguing that they claim that Dystruct has much less time-entropy than ADMIXTURE (i.e. the relation between ancient and modern populations seems to be better identified) and that, using this method they get that the Samara (proto-Indoeuropean) population becomes much more clearly related to Kostenki-14 (a Gravettian hunter-gatherer from the Don area) and that the Paleo-Siberian "ANE" individuals form then their own distinct cluster with very limited impact in Europe (but much larger in parts of Asia (not labeled: South Asia?). This Kostenki-Samara "orange" component keeps influencing Western Indoeuropeans (Corded Ware, Unetice) but at markedly decreasing frequencies of "purity". 

However the first admixture of Corded Ware is not with earlier farmers (mostly "green") but with some sort of late "hunter-gatherer" population ("brown" or "maroon" component. Only after the backlash of Bell Beaker, which in Central Europe appears as a mix of Neolithic peoples, Indoeuropeans and maybe even more of that mysterious extra HG element, we see some "return of the farmers", which clearly persists in Unetice.

In general, modern Europeans are (fig.5a, not shown here) quite "greener" than Unetice and some populations (I'm guessing Sardinians and Basques, no labels provided) have zero "orange" (IE) component, which ranges (my visual estimate) between 9% and  27% otherwise.

Fig.5-b (click to expand): Ancestry estimates for 92 ancient samples. The three leftmost samples are the Pleistocene hunter-gatherers. In Dystruct, late Neolithic samples and beyond present as a mixture of hunter-gatherers, Yamnaya steppe herders,and early Neolithic samples, matching supported historical migrations of steppe herders into Eastern and Western Europe.

February 13, 2018

Scandinavian hunter-gatherers had double west-east origins

SHG origins' mystery solved? What about possible Norwegian "EHG-like" genetic influences into Atlantic Europe?


Torsten Günther, Helena Malmström, Emma Svensson, Ayça Omrak et al. Genomics of Mesolithic Scandinavia reveal colonization routes and high-latitude adaptation. bioRXiv 2017 (pre-pub). DOI:10.1101/164400


Scandinavia was one of the last geographic areas in Europe to become habitable for humans after the last glaciation. However, the origin(s) of the first colonizers and their migration routes remain unclear. We sequenced the genomes, up to 57x coverage, of seven hunter-gatherers excavated across Scandinavia and dated to 9,500-6,000 years before present. Surprisingly, among the Scandinavian Mesolithic individuals, the genetic data display an east-west genetic gradient that opposes the pattern seen in other parts of Mesolithic Europe. This result suggests that Scandinavia was initially colonized following two different routes: one from the south, the other from the northeast. The latter followed the ice-free Norwegian north Atlantic coast, along which novel and advanced pressure-blade stone-tool techniques may have spread. These two groups met and mixed in Scandinavia, creating a genetically diverse population, which shows patterns of genetic adaptation to high latitude environments. These adaptations include high frequencies of low pigmentation variants and a gene-region associated with physical performance, which shows strong continuity into modern-day northern Europeans.


Fig. 1:
Mesolithic samples and their genetic affinities – (A) Map of the Mesolithic European samples used in this study. The pie charts show the model-based [16,17] estimates of genetic ancestry for each SHG individual. The map also displays the ice sheet covering Scandinavia 10,000 BP (most credible (solid line) and maximum extend (dashed line) following [10]). Newly sequenced sites are shown in bold and italics, SF11 is excluded from this map due to its low coverage (0.1x). Additional European EHG and WHG individuals used in this study derive from sites outside this map (...)

Intriguingly, Swedish Epipaleolithic peoples (SHG) have the light skin variant in the gene SLC45A2, variant that is now uniformly spread through all Europe and accounts for 15% of the skin color variance in a key Cape Verde study and that was so far attributed solely (or almost solely) to Neolithic farmers (among which it was fixated and who had indeed a very large genetic impact in the European subcontinent). They also have the blue eyes allele, as Western Hunter-Gatherers did. However, if we are to follow, Günther's explanations in this video, the variance of looks in Epipaleolithic Scandinavia was greater than in present day. 


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PS- Legend says that when my namesake Fray Luis de León returned to his classroom after five years imprisoned by the Inquisition, he began his class with these words: "As we were saying yesterday..."

Expect less lengthy articles because I really want to use this blog as open notebook, and not spend so much time following news and dissecting them thoroughly, so my style may become a bit more telegraphic. 

I also don't know for how long I will be able to continue blogging, as my personal economic situation is bad and worsening and you never know when police may come and arrest you for saying "fuck the king" or whatever other quite reasonable opinion. We live very troubled times and I'm personally quite bad at winning.

Also my apologies in advance if I fail to quickly approve comments. I usually check my email once per day or so but sometimes I just forget. Please, be patient if that happens: I only censor fascism, racism, sexism and homophobia (and those individuals who have managed to really get on my nerves, and they know who they are). Sadly I have to keep pre-moderating all comments, else the lonely troll will get away with his abuses.

January 9, 2017

Extensive ancient Iberian mtDNA analysis

A very interesting thesis on Iberian ancient mitochondrial DNA is available (found via Bell Beaker Blogger):

Christina Roth, Once upon a time in the West : paleogenetic analyses on Mesolithic to Early Bronze Age individuals from the Iberian Peninsula. University of Mainz (thesis), 2016. Freely availableLINK

As all theses, it is very extensive, and I can only make some comments here (space and time are always too limited, you know), hoping to grasp the most interesting aspects of it.

First of all Roth finds that Iberian hunter-gatherers (HGW in the paper) were not quite like Central European ones (HGC), at least not in the mtDNA. Instead, in this aspect the Central European hunter-gatherers were closer to Eastern European ones (HGE), as well as to the Pitted Ware late subneolithic population of Gotland.



Iberia-only mtDNA analysis


This will be a recurrent issue along the analysis she makes. But let's focus on Iberia by the moment. Notice that she does not just use her own data but also from many other sources, including some unpublished materials, this makes a bit difficult to follow all the details, so I feel I can only focus on the statistical analysis she makes and which is the core of the thesis.

Annotated by me, because I found the abbreviations quite hard to follow

The general overview is confirmed, with minor variations, in the cluster analysis (which I also took the liberty of annotating, but take my notes for what they are: mere scribbling on the margin in an attempt to better understand the data, nothing else):



Particular care not to take the arrows of my notes too seriously: they are just conceptual, a very loose sequencing of the available data for a very tentative visualization of it. If it helps you, cool, else ignore please.

In any case, it seems clear cluster 2 is more influenced by hunter-gatherer genetics and that, with the data available in this study, we reach the end of the timeline within it in the following regions: Upper Ebro and Basque Country (data up to Late Neolithic but see my own 2013 compilation for a longer period), Southern Iberia (up to Chalcolithic) Southeast and Northeast, this one after originally being in the "more Neolithic" cluster 1 (in fact the author does argue for Catalonia as main "gate" for Neolithic genetics into Iberia on light of the available data, which does not include another important "gate", the SE one, for lack of data for the early period).

Inversely, cluster 1 is more influenced by "farmer" genetics, first detected in the Northeast (Catalonia and nearby areas of Aragon) but then also affecting the Northern Plateau and the West (Central Portugal). This last is very important because it is here where a key civilizational hub, the oldest known civilization of the Atlantic shores, emerged in the Chalcolithic, playing a key role in the wider Megalithic and Bell Beaker phenomena. I have at times speculated that it might be the origin of "modernity" in Western European genetic pools but on light of this data I have to recant, the origin should be elsewhere, probably in/around what is now France (and therefore it's not likely to be directly related to Bell Beaker, except maybe in the islands, but rather to Michelsberg/SOM, Artenacian and such -- always in wait of more data, just a cautious hypothesis). 

Finally the farmer-leaning cluster reaches the Southern Plateau, near Madrid, what suggests a N→S move of the then-forming Cogotas I herder culture, rather than the opposite. 

What we do see is not inconsistent with modern Iberian genetics anyhow: while Central Europe seem to have seen an increase of "hunter-gatherer" genetics along time, in Iberia the main tendency is the opposite: an increase of "farmer" genetics and further dilution of the aboriginal genetic pool. There is however at least one clear exception in the Northeast and that is also consistent with modern Iberian genetics. Some regions (south, southeast and the totally unknown northwest) have only fragmentary sequences here, so unclear, although the final tendency, up to the Bronze Age, is to cluster 2b (i.e. mixed but rather tending to "hunter-gatherer" genetics).

To finish with the Iberia-only bloc, notice that these are the lineages found among early Iberian farmers by region (in color those haplogroups associated to the arrival of Neolithic per the available data):



Notice how the pool in the Northern region is quite modern-like, not yet exactly as it is locally today but it would pass quite unnoticed in a map of Europe.


Iberia and the rest of Europe


What about the pan-European context (with the usual huge blanks in France, Britain, etc.)? Quite interesting as well:

Annotated by Maju on fig. 23 (click to enlarge)



As mentioned before, the Iberian hunter gatherers (HGW) appear clearly distinct from their Central and Eastern cousins in the mtDNA analysis. And with all this Iberian dataset it becomes apparent that there seem to be two "zones of admixture": one for Iberians and another for Central Europeans, the difference being on which aborigines they mix with. 

Notice that it is not possible to differentiate here between local Central European and intrusive Eastern European admixture, as both aboriginal hunter-gatherer populations appear closely related in all analyses (maybe an artifact of the sampling strategy or maybe actually relevant, can't say). 

It also caught my eye that a German site (Blätterhöhle, Westfalia, famed because farmer and hunter-gatherers living side by side were located there some years ago), clusters intensely with Iberian hunter-gatherers and related populations. I have to research more on this matter (which I had ignored so far) but I suspect it may be very relevant, because we could get an even longer chain of early "modern" mtDNA pools, adding this site to Paternabidea (Navarre) and Gurgy (Burgundy), spanning a long stretch of Western Europe, an area quite neglected by archaeogenetics so far, it must be said.

It is also worth mentioning that UC (which I believe stands for Unetice Culture) pulls the "Central European zone of admixture" in the PCA downwards, with a polarity of its own, a polarity that should probably be considered as specifically Indoeuropean. 

The cluster analysis confirms much of what I just said above, not annotated for a change:



We see very clearly here a larger cluster more influenced by "farmer" genetics and a smaller one that includes all pre-Neolithic aborigines, plus two populations already post-Neolithic but clearly identifiable as at least largely aboriginal (PWC and BLA), plus a subcluster of Neolithic Iberians, from the North (NSE and EVN) and the West (CPE). 

The author notices that: Bernburg (BEC) and Funnel Beaker (FBC) samples (...) show almost no significant differences to any Iberian group, except to the Early Neolithic of Northern Spain (NSE) and Chalcolithic of East Spain (ESC). Hard to interpret but worth noticing, no doubt.

And there is a lot more in the thesis but I can only review so much, so take a look and tell me and the World if I'm missing something of relevance or you feel I'm misinterpreting something or whatever.

January 1, 2017

Ancient aboriginal DNA from El Hierro (Canary Islands)

The island of El Hierro (lit. The Iron) is one of the westernmost of the Canary Islands. I have never visited but it seems to be very beautiful, far enough from the Sahara to enjoy a warm yet humid climate. It wasn't far enough to remain uninhabited however and now we get to know something more about its original dwellers, generally known as Guanches (although technically this name only applied to the inhabitants of La Palma originally).

Alejandra C. Ordóñez et al., Genetic studies on the prehispanic population buried in Punta Azul cave (El Hierro, Canary Islands). Journal of Archaeological Science 2016. Pay per viewLINK [doi:10.1016/j.jas.2016.11.004]

Abstract

The aim of this study was to establish the genetic studies of the population from one of the most important known aboriginal funerary spaces of the island of El Hierro (Canary Islands), the Punta Azul cave, which harbors remains of 127 individuals. Sixty-one adult tibiae were examined, 32 left and 29 right. Radiocarbon dating yields an antiquity of 1015–1210 AD. We have obtained an overall success rate of 88.5% for the molecular sexing, and of 90.16% for the uniparental markers. Short tandem repeats (STR) profiles were also possible for 45.9% of the samples. This performance is a consequence of the good conservation of the bones in their archaeological context. The mtDNA composition of the sample is characterized by the complete fixation of the H1-16260 lineage. These results can be explained by a mixture of consecutive founding events, a bottleneck episode at the beginning of the colonization and/or as a consequence of genetic drift. Paternal lineages were also affected by these processes but in a less acute way. These differences lead us to propose social behaviors as an explanation for this difference. The maternal transmission of the lineages, mentioned in ethnohistorical sources of the Archipelago, could be an explanation. These results could be in agreement with endogamous practices, but the autosomal STR results indicate a relative high diversity. These results have allowed us to characterize the Punta Azul cave population and see the way in which geographical isolation, the process of adaptation and specific social behaviors affected the aboriginal population of the Island.

And now the interesting stuff, the findings:



As should be expected, the remote and rather small western island, which is the most direct threat to the security of the USA and the Western World in general because of the danger its volcano may slide into the Ocean and cause a huge tsunami of devastating consequences, something that no nuclear arsenal can do anything against, shows clear indication of very strong genetic drift relative to its relatives of the larger islands, caused either by founder effects and/or endogamous drift. Otherwise it is within the general patterns for the pre-colonial islanders. 

We see a lot of likely mtDNA H and certain Y-DNA R1b1a2. The Canary Islands were settled c. 1000 BCE (11,000 HE) by people arrived from what is now Morocco and the Western Sahara. They seem to retain a somewhat archaic genetic pool, relatively rich in Europe-related genetic elements, not so abundant at all today in Northwest Africa anymore, and by this I mean of course very especially Y-DNA haplogroups I and R1b. While we can still find some R1b in NW Africa, haplogroup I is almost impossible to find nowadays, yet it was present in the Canary Islands prior to European conquest and nearly disappeared afterwards (so it's definitely not a colonial input, accidentally misidentified, not at all). 

See also: Leherensuge: Ancient Guanche Y-DNA.

Iberomaurusian ancient mtDNA

This is an issue that has lingered for a long time in the online anthropology circles. Once upon a time (2005) there was online a presentation in which a good deal of mitochondrial DNA (HVS-I) sequences from the key Iberomaurusian (or Oranian) culture site of Taforalt (North Morocco) were (not too formally) published. Eventually this presentation became almost impossible to find... however now Rym Kefi (the original author if I remember correctly) and colleagues have got back to resurrect that important data set from oblivion, plus sequences from Afalou cave as well.

Rym Kefi et al., On the origin of Iberomaurusians: new data based on ancient mitochondrial DNA and phylogenetic analysis of Afalou and Taforalt populations. Forsenic Sciences Research 2016. Freely available at the time & space of writing thisLINK [doi:10.1080/24701394.2016.1258406]

Abstract

The Western North African population was characterized by the presence of Iberomaurusian civilization at the Epiplaeolithic period (around 20,000 years before present (YBP) to 10,000 YBP). The origin of this population is still not clear: they may come from Europe, Near East, sub-Saharan Africa or they could have evolved in situ in North Africa. With the aim to contribute to a better knowledge of the settlement of North Africa we analysed the mitochondrial DNA extracted from Iberomaurusian skeletons exhumed from the archaeological site of Afalou (AFA) (15,000–11,000 YBP) in Algeria and from the archaeological site of Taforalt (TAF) (23,000–10,800 YBP) in Morocco. Then, we carried out a phylogenetic analysis relating these Iberomaurusians to 61 current Mediterranean populations.

The genetic structure of TAF and AFA specimens contains only North African and Eurasian maternal lineages. These finding demonstrate the presence of these haplotypes in North Africa from at least 20,000 YBP. The very low contribution of a Sub-Saharan African haplotype in the Iberomaurusian samples is confirmed. We also highlighted the existence of genetic flows between Southern and Northern coast of the Mediterranean.

The bulk of the data is this (tables 1 and 2):



Notice that all sequences are "old style", i.e. HVS-I only, and that's why haplogroup assessment is not always 100% certain, with particular emphasis on the "CRS" sequences, which have been proven in some ancient cases to belong not to the modern normal (H1, rarely H2 o other H) but to a very rare modern haplotype within U*, AFAIK only preserved in Asturias but somewhat common in the Magdalenian of South Germany. 

In any case, that they produce 100% safe H1, H6, H14, etc. and, even more surprisingly, J and T2 variants, should make people think about the hidden Paleolithic mtDNA diversity in the wider Mediterranean area. And by this I do not just mean North Africa but also all Southern Europe, including many areas that can't be considered part of the Mediterranean basin, such as most of the crucial Franco-Cantabrian Region, the most densely populated province, by far, of post-LGM Upper Paleolithic Europe, source of several cultural waves (including the Solutrean one, almost certainly at the origin of Iberomaurusian), yet outrageously neglected by research.

Table 5 and other materials in the paper also deal with which modern populations appear closest to the Iberomaurusian ancient mtDNA pool and these are:
  1. Tuscans (0.00090)
  2. Catalans (0.00134)
  3. Galicians (0.00223)
  4. Sicilians (0.00377)
  5. El Alia (0.00699)
  6. Valencian (0.00787)
  7. Matmata (0.00788)
  8. Slougia (0.00831)
  9. Jerba Berbers (0.00934)
Figures in brackets are FST distances, the smaller the closer the match, statistically speaking. Populations in cursive are from Northwest Africa.

That is I'd say quite surprising because we tend to think of those near matches as quite impacted by Neolithic and post-Neolithic inflows, at least judging on other recent research. It seems that the debate on the origin of modern Europeans and North Africans refuses to come to a close, as different aspects of the evidence available may be somewhat contradictory. What do you make up of all this?

See also:


UPDATE: single marker inferred lineages (not always the haplogroup is reported correctly):

Down in the comments' section, Capra questions with good sense the certainty of inferred haplogroups. The most clear one is TafV27, claimed to be H6a1a8, which cannot actually be that (would need another HVS-I marker defining H6, which is missing), however this means that it must be HV0 and possibly V, a lineage that appears (per the aDNA literature) in the European Neolithic, already quite towards the West, out of nowhere, and that "nowhere" should not be West Asia, where it has never been sequenced, unlike K. 

I'm therefore reviewing here all the single-marker inferred haplogroups as carefully as possible, please double-check them and report if I seem to be committing any error:
  • AF22B - 16126C (reported as JT or H14b1) - JT(xJ,xT) or H14b1 indeed, notice that JT* was probably also one of the lineages described (also by the HVS-I method) for Nerja cave (Solutrean, same time frame), just across Alboran Sea (cf. Fernández-Domínguez 2005). Whether is one or the other or even something else, it's almost certain that the lineage was shared across the sea between Europe and Africa, what just makes total sense for this culture.
  • Taf55-IB - 16239T (reported as H1) - I actually get specific H1 variants (H1bf'bg'bh'ch specifically) or H17c. H sublineages in any case, unless it is an extinct or unreported R* lineage, most unlikely.
  • TafV27 - 16298C (reported as H6a1a8) - must be HV0, possibly even V, a distributed Euro-African lineage with three hotspots: Kabylia, Catalonia and Lappland. Per Caramelli 2003 and successor studies, this lineage was already present in Italy (Pagilicci cave) in the Gravettian era.
  • TafVIII - 16223T (reported as U4a2b) - It cannot be U4 at all because it lacks the HVS-I mutation 16356C. In fact the reverse 16223C mutation defines macro-haplogroup R, and I can't find any other matches within R, so it must be L3(xR), maybe N(xR) but maybe M or other L3. The possibilities are many, for example: N1, N2, N9, N11, etc. within N, a lot of possibilities within M, and also several within L3e'i'k'x(xL3x). The only thing we should be certain here is that it is not R and also not upstream of L3 (that's how good as it gets with HVS-I methodology, really). 
  • TafXXV3 - 16126C (reported as H14b1) - as with AF22B, it can be JT(xJ,xT) or H14b1 and again I must emphasize that there is an extremely good chance that this matrilineage had relatives in Iberia (Nerja) within the Solutrean culture.

In case you want to re-check, the best resource is of course PhyloTree.



IMPORTANT CAVEAT: the above update was done assuming they had fully sequenced the HVS-I, which is not that long, but it seems that in many cases (second column of the tables above) they have only sequenced up to the 16317 locus, what makes prediction of haplogroup even harder. That would explain their H6a1a8 prediction, although there's no way they can exclude the much more common HV0 (H6 is unheard of in North Africa). Thanks again to Capra for pointing that out.

October 27, 2016

Mitochondrial DNA from post-Neolithic Santimamiñe (Basque Country)

Four human remains dated to the Bronze Age were sequenced for mitochondrial DNA in Santimamiñe cave (Kortezubi, Biscay, Basque Country), along with single instances from the Neolithic, Chalcolithic and Roman period.

J.C. López Quintana et al., NUEVOS DATOS SOBRE LA SECUENCIA DE USO SEPULCRAL DE LA CUEVA DE SANTIMAMIÑE (KORTEZUBI, BIZKAIA). Arqueología y Prehistoria del Interior Peninsular (ARPI), 2016. Freely accessible (PDF) → LINK [no DOI]


The mtDNA study is not "brand new" but a synthesis of a previous doctoral thesis and advance publications:

Un primer avance de este estudio fue publicado en la monografía de las campañas de 2004 a 2006 de Santimamiñe (Cardoso et al. 2011), incluyendo el conjunto completo en la Tesis Doctoral de L. Palencia Madrid (Palencia 2015).

So we are talking of relatively old data, that has partly remained within the (sometimes absurdly greedy and anti-social) academic circles until now. The relative antiquity of the DNA study is important when assessing it, because genetic analysis is evolving very fast and, in most cases in the rather closed and under-budgeted Spanish universitary circles, they tend to do things "the old way", so we are almost certainly dealing here with HVS-I sequencing, something that is not explicit in the paper (I'm searching for Leire Palencia's thesis to make sure but no luck until now). 

If I am correct in this (and I should be), then we must understand that it is impossible in many cases to determine the exact haplogroup in the crucial R0 upper tier haplogroup, which includes HV and the extremely common H. Lacking the original HVS-I sequences by the moment, I can't but take the authors labels at face value but I must warn here that where it reads "R0" it is almost certainly H (HV0 or V are easy to recognize with this method, as is R0a) and where it reads "H1" it is probably H1 but not 100% certain. 

For more details see the relevant PhyloTree page, where the HVS-I markers are the last bloc in blue, beginning always with the sequence "16" (the other markers in blue of lower numerical value are HVS-II, more rarely used, and the ones in black are the coding region markers, which are in this case fundamental for proper assignment).


The mtDNA haplogroups (as reported) are:

  • Neolithic:
    • U5a2a (S2011-M2, c. 5100 BCE)
  • Chalcolithic:
    •  T2b (S-1, c. 2000 BCE)
  • Bronze Age:
    • U5b (S2011-M1 c. 1700 BCE) 
    • H1 (S2011-M4, c. 1700 BCE)
    • R0 (S2011-M6, c. 1500 BCE)
    • U3a (S2011-M3 c. 1300 BCE)
  • Roman period: 
    • R0 (S2011-M5, c. 300 CE)


Interpretation attempts


It's difficult to extract conclusions from them but they should be compared with other sequences from the area, for which I recommend my 2013 synthesis. In general, treat "R0" as meaning "H", even if I chose to use a different color (magenta instead of red) for exactitude. 

In order to aid that analysis, I reproduce here my 2013 graphic:


We cannot compare the single Neolithic and Roman Era individuals but we can compare the Satimamiñe Chalcolithic+Bronze group of five sequences with the peripheral Chalcolithic large dataset of De La Rúa:

  1. R*+H (very similar):
    1. Peripheral "Basque" Chalcolithic: ~40%
    2. Santimamiñe Chalcolithic+Bronze: 40% 
    3. Santimamiñe Bronze only: 50%
  2. U(xK) (very different):
    1. Peripheral "Basque" Chalcolithic: ~15%
    2. Santimamiñe Chalcolithic+Bronze: 40%
    3. Santimamiñe Bronze only: 50%
  3. Other lineages (all them of certain Neolithic immigrant origin, very different too):
    1. Peripheral "Basque" Chalcolithic: ~45%
    2. Santimamiñe Chalcolithic+Bronze: 20%
    3. Santimamiñe Bronze only: 0%

However one of the U(xK) lineages in Santimamiñe is U3, which is also quite certain to be of Neolithic immigrant origin, and one is an important figure when n=5 so we can also see it this way:
  1. Paleolithic lineages:
    1. Peripheral "Basque" Chalcolithic: ~55%
    2. Santimamiñe Chalcolithic+Bronze: 60%
    3. Santimamiñe Bronze only: 75%
  2. Neolithic lineages:
    1. Peripheral "Basque" Chalcolithic:  ~45%
    2. Santimamiñe Chalcolithic+Bronze: 40%
    3. Santimamiñe Bronze only: 25%

The comparison of #1 with #2 is much more similar. This could be important, because Santimamiñe is not anymore a "peripheral" site, as are those from De La Rúa's dataset, but a rather central one with a extremely long and uninterrupted Paleolithic sequence, dating to Neanderthal-made Chatelperronian culture. It is still a single site with a small number of samples but it does provide a counterpoint that, in one approach could produce similar results. 

But, surprisingly, when we consider a distinct Bronze Age category, comparing not anymore with #2 but with #3 everything changes, suggesting a totally different interpretation of the available dataset, in which, the "Chalcolithic interlude" (if real at all, more data is needed) would be reversed quickly with the onset of the Bronze Age. 

I am sorry but I cannot lean for either interpretation: the data is just not extensive enough to allow for conclusions. I am tempted to support the continuity hypothesis, allowing only for lesser changes to happen, and keep the Chalcolithic dataset under a big question mark, but the question mark is admittedly a bit smaller now: something in terms demographic may have happened in the Chalcolithic period and may have been reversed in the Bronze Age. But "may" is not "for sure", we need more data points.

Feel free to discuss in good mood, as always.

Thanks for the heads up to Jean Lohizun (again).

October 21, 2016

Wisent co-existed with true bison already in the Paleolithic

A fascinating story this one indeed: the European bison or wisent has some ancestry related to the cow, evident in its mitochondrial DNA. This was already known but what wasn't known is that this distinct "hybrid" species of bison dated to the Upper Paleolithic. Thanks to the excellent records of anonymous prehistorical biologists who recorded them in Southwestern European rock art with great detail and naturalism, modern researchers have realized that the wisent, with its bovid heritage, existed already in the Upper Paleolithic. Ancient DNA recovery has now confirmed the artist's impression.

Julien Soubrier et al. Early cave art and ancient DNA record the origin of European bison. Nature communications, 2016. Open access → LINK [doi:10.1038/ncomms13158]

Abstract

The two living species of bison (European and American) are among the few terrestrial megafauna to have survived the late Pleistocene extinctions. Despite the extensive bovid fossil record in Eurasia, the evolutionary history of the European bison (or wisent, Bison bonasus) before the Holocene (< 11.7 thousand years ago (kya)) remains a mystery. We use complete ancient mitochondrial genomes and genome-wide nuclear DNA surveys to reveal that the wisent is the product of hybridization between the extinct steppe bison (Bison priscus) and ancestors of modern cattle (aurochs, Bos primigenius) before 120 kya, and contains up to 10% aurochs genomic ancestry. Although undetected within the fossil record, ancestors of the wisent have alternated ecological dominance with steppe bison in association with major environmental shifts since at least 55 kya. Early cave artists recorded distinct morphological forms consistent with these replacement events, around the Last Glacial Maximum (LGM, ∼21–18 kya).


The depictions of both types of bison are rather distinct but it seems nobody had noticed the difference until now, as the researchers explain in this article.

Fig. 1 - (a) Reproduction from Lascaux cave (France), from the Solutrean or early Magdalenian period (20,000 kya—picture adapted from ref. 53). (b) Reproduction from the Pergouset cave (France), from the Magdalenian period (<17,000 kya—picture adapted from ref. 54).

The ancient wisents sequenced now carry a distinct mtDNA haplogroup, called "clade X", which is sister to that of modern wisents (all descending from just 12 survivors). This wisent macro-haplogroup forms a clade with that of bovine cattle (cows of all sorts, both taurine and indicine) but they are joined only at the root, suggesting that the hybridization event that created the wisents as distinct species is very old, just a bit more recent than the divergence of cow and bison.

Fig. 2 - (a) Phylogenetic tree inferred from bovine mitochondrial control region sequences, showing the new clade of bison individuals. The positions of the newly sequenced individuals are marked in red for CladeX. (b) Bovine phylogeny estimated from whole-mitochondrial genome sequences, showing strong support for the grouping of wisent and CladeX with cattle (cow) and zebu. For both trees (a,b) numbers above branches represent the posterior probabilities from Bayesian inference, numbers below branches represent approximate likelihood ratio test support values from maximum-likelihood analysis and scale bars represent nucleotide substitutions per site from the Bayesian analysis. (c) Maximum-clade-credibility tree of CladeX and wisent estimated using Bayesian analysis and calibrated with radiocarbon dates associated with the sequenced bones. Dates of samples older than 50 kyr were estimated in the phylogenetic reconstruction. (d) Map showing all sampling locations, using the same colour code (red for CladeX, orange for wisent and blue for steppe bison).

So it is not random auroch hybridization but a very specific and very ancient episode of admixture between the ancestors of bisons and cows.

The two species appear to have distinct ecological niches:

The detailed records of the southern Ural sites allow the timing of the population replacements between steppe bison and wisent to be correlated with major palaeoenvironmental shifts, revealing that the wisent was associated with colder, more tundra-like landscapes and absence of a warm summer.

This pattern seems to correspond with the periods in which the two species are portrayed in rock art, as two of the researchers explain in this video (third part):




Post-statement: I must say that, on second thought, I'm not really convinced by the claim of wisent corresponding to colder periods. In fig. 1 above it is apparent that it is the steppe bison which corresponds to the last glacial maximum (LGM) in Southwestern Europe and not the wisent, which only shows up after the end of this coldest period. 

I wonder if the researchers are explaining themselves well enough on this aspect or if it is a case of wishful thinking, maybe caused by different conditions in SW Europe (where the rock art is) and the Southern Urals (where most of the archaeogenetic and paleontological data comes from). 

At the very least, judging on fig. 1, it would be the steppe bison the one corresponding with the coldest spell and the wisent the one corresponding to more temperate conditions. Can someone explain me what is going on here?

August 21, 2016

Paleolithic European mtDNA lineage U5b2c1 in Carthaginian man

Quickies

Elizabeth A. Matisoo-Smith et al. A European Mitochondrial Haplotype Identified in Ancient Phoenician Remains from Carthage, North Africa. PLoS ONE 2016. Open accessLINK [doi:10.1371/journal.pone.0155046]

Abstract

While Phoenician culture and trade networks had a significant impact on Western civilizations, we know little about the Phoenicians themselves. In 1994, a Punic burial crypt was discovered on Byrsa Hill, near the entry to the National Museum of Carthage in Tunisia. Inside this crypt were the remains of a young man along with a range of burial goods, all dating to the late 6th century BCE. Here we describe the complete mitochondrial genome recovered from the Young Man of Byrsa and identify that he carried a rare European haplogroup, likely linking his maternal ancestry to Phoenician influenced locations somewhere on the North Mediterranean coast, the islands of the Mediterranean or the Iberian Peninsula. This result not only provides the first direct ancient DNA evidence of a Phoenician individual but the earliest evidence of a European mitochondrial haplogroup, U5b2c1, in North Africa.

The lineage is the same one as La Braña 1, an Epipaleolithic man buried in a cave at the mountains NE of León. Its presence on a Carthaginian from the 6th century BCE almost certainly indicates that he had native Iberian maternal ancestry, that his family had arrived to Carthage from Gadir (modern Cádiz) or some of the other Phoenician colonies of Andalusia. The location of his burial at the acropolis and the wealth of the burial goods indicate that he belonged to the highest social elite of the still incipient Carthaginian empire. He has been nicknamed "Ariche" (the loved one) and his face reconstructed as you can see in this blog.

Thanks to Jamel of Lapurdi for the reference an a nice related discussion.