Revision as of 13:58, 15 September 2017 by Admin
Genetic admixture refers to the analysis of the gene flow between populations that had previously been relatively isolated from one another. Since isolated populations develop linguistic differences relatively quickly, linguistic changes might be expected in a newly hybridised population[Jobling et al. 2014]. However, pidgin languages are quite rare, and often one language – usually that of the successful migrants – becomes the superstrate, and another the substrate.
On the other hand, language and culture are unlike a genome in several different ways. While it is possible to obtain admixture percentage of any ancestral population, ancestral language reconstruction and its identification with cultures needs the intervention of careful anthropological investigation. For admixture results to be meaningful, studied loci have to be correctly averaged (and samples should be as complete as possible); genetic drift and selection since admixture have to be taken into account (e.g. distant populations might show a higher differentiation from the original territory); ancestral populations have to be correctly identified, including their number and precise alleles[Jobling et al. 2014]. Ancient DNA is best collected with the goal of testing specific hypotheses.
Some linguists have used the biological foundations of phylogenetics to extrapolate questionable methods to linguistics, and have thus obtained questionable results[Gray and Atkinson 2003]. Similarly, scientists are using the available statistical means to study genetic admixture in modern human populations, extrapolating admixture mapping methods to scarce ancient human samples, and deriving simplistic, far-fetched conclusions. This paper demonstrates the need to include wide anthropological investigation of the historical context of the samples studied, including linguistics, archaeology, and cultural anthropology, as well as careful investigation of haplogroups, to obtain plausible explanations for the complex data obtained in human biology.
It has been proposed that migrating Yamna pastoralists into already expanding Corded Ware groups[Wencel 2015] might have created the necessary environment for the spread of Indo-European languages. Previous mainstream models for Indo-European expansion, based on the “kurgan hypothesis”[Gimbutas 1977] associated the spread of Pre-Germanic (adopted on the Dniester) and Pre-Balto-Slavic (adopted on the middle Dnieper) to the expansion of Corded Ware cultures[Anthony 2007]. Given the lack of direct cultural connections between Yamna and the Corded Ware culture, this spread was explained in terms of either an incorporation of languages through centuries of interaction into Funnel Beaker cultures, or through the emulation of the language of Indo-European chiefs by Corded Ware cultures (beginning ca. 2700-2600 BC) for politico-religious reasons[Anthony 2007].
The admixture of Yamna aDNA samples found elevated (up to 76%) in Corded Ware samples has been said to support the migration of Yamna populations into Corded Ware groups, while the lower percentage found in Bell Beaker and Únětice groups (50-70%) has been explained as a subsequent, less profound displacement process triggered by western and central European groups[Haak et al. 2015][Allentoft et al. 2015][Mathieson et al. 2015]. It has also been found that samples from Globular Amphorae culture do not show evidence of steppe ancestry[Mathieson et al. 2017].
These limited results, apparently challenging archaeological interpretations previously considered established, are propagating quickly within the field of Indo-European studies. David W. Anthony has recently supported the appearance of the Corded Ware culture through the contacts of Yamna immigrants with indigenous people of the Globular Amphorae culture in southern Poland[Anthony and Brown 2017], based on their previously known contacts and early dating. Similarly, Kristian Kristiansen has supported the dominance of Corded Ware in central Europe south and north of the Carpathians, asserting that their pottery was apparently shared later by the Bell Beaker culture[Kristiansen et al. 2017].
Many concerns have been raised about obtaining simplistic conclusions based on genetic results[Heyd 2017].
Modified file from recent papers on ancient samples from Eastern European, Southeastern European, Western European, and Bell Beaker cultures: Left: ADMIXTURE clustering analysis with k=8 showing ancient individuals. E/M/MLN, Early/Middle/Middle Late Neolithic; W/E/S/CHG, Western/Eastern/Scandinavian/Caucasus hunter-gatherers[Olalde et al. 2017]. Center: 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 (orange), EHG (red) and WHG (blue). Dates indicate approximate range of individuals in each population[Mathieson et al. 2017]. Right: Ancestral components in ancient individuals estimated by ADMIXTURE (k=11)[Mittnik et al. 2017]. Original images under a CC-BY-NC 4.0 International license.
Yamna ancestry: CHG before, during, and after Chalcolithic migrations
Samples from the Pontic-Caspian steppe – from which ‘steppe’ or ‘Yamna’ ancestry has been defined as a precise combination of EHG and CHG ancestry – are scarce, and the most recent ones mostly from one eastern region (Kalmykia). Because of that, east Yamna was considered the best-known proximate source for the incoming gene flow in Corded Ware samples. The exact source could have been another, yet unsampled, group of people closely related to them[Kristiansen et al. 2017], and a western or earlier (pre-Yamna) steppe population has been suggested as the potential missing link in the chain of transmission of steppe ancestry[Haak et al. 2015]. A similar western Yamna ancestry is also found in a sample from Vučedol, which probably also descended directly from early western Yamna migrants.
This so-called ‘steppe’ ancestry from eastern Yamna samples has been found in Corded Ware, Afanasevo, Andronovo, and Srubna cultures, and even a late individual of Bronze Age Bulgaria from Merichleri, ca. 1690 BC. All of them show higher ‘steppe’ ancestry than some samples clearly identified as from the Yamna culture in Ukraine and in the Balkans, more than a thousand years earlier [Mathieson et al. 2017].
Samples from central Balkans show in fact a relative increase in steppe ancestry later, during the Bronze Age – unlike west Europe and the southern Balkans, where ancient Indo-European languages were most likely spoken by that time. Furthermore, admixture analyses of modern populations show more steppe proportion in modern north-eastern European populations (including peoples probably speaking Finno-Ugric languages since the Neolithic) than in western European peoples that are known to have spoken Indo-European languages for millennia.
Most Corded Ware samples are late, almost coincident with the Bell Beaker expansion. No samples have been published from potentially controversial areas – like the Contact Zone, north-eastern Europe and western Yamna – during the most relevant periods. Old samples (closer to admixture events) tend to show a higher range of variation, and could inform better of the real impact of migrations, while younger ones – depending on non-random mating processes, influenced by geographic structure or socioeconomic factors – may falsely show a relatively homogenous high or low ancestral contribution[Jobling et al. 2014].
[Lazaridis et al. 2017], and Scythian and Sarmatian[Unterländer et al. 2017] samples. PC2 vs. PC1. The graphic has been arranged so that ancestries and samples are located in geographically friendly axes similar to north-south (Y), east-west(X). Symbols are used, in a simplified manner, in accordance with symbols for Y-DNA haplogroups used in the maps. Labels have been used for simplification. Areas are drawn surrounding Yamna/Poltavka, Corded Ware (including samples from Estonia, Battle Axe, and Poltavka outlier), and succeeding Sintashta and Potapovka cultures, as well as Bell Beaker. Corded Ware sample I0104, from Erperstedt, has also been labelled.PCA analysis of free datasets including Minoans and Mycenaeans
The oldest sample from Erperstedt (labelled I0104), a second-degree relative to the Erperstedt family[Monroy Kuhn, Jakobsson, and Günther 2017], has been found to cluster the closest to steppe samples, closer than any other Corded Ware sample, previous or posterior, or samples from eastern Corded Ware-derived cultures Sintashta, or Potapovka[Haak et al. 2015]. This, connected with the exogamy prevalent among Corded Ware peoples, and the nomadic nature of its culture, precludes a proper interpretation of the ancestry found in the family.
In fact, PCA analysis reveals that early Corded Ware samples, like those from Estonia and Latvia, as well as the Poltavka outlier – in contrast e.g. with Poltavka samples – cluster closer to EHG and central European populations rather than to Yamna. Late Corded Ware samples, as well as Sintashta and Potapovka samples, also cluster closely to central European samples, in contrast with Poltavka and Afanasevo individuals, suggesting an east-central European genesis of the Corded Ware culture.
The northern Pontic area – from where many Yamna migrants seem to have expanded west along the Danube – had been a zone of interaction with peoples from the upper Danube and the Eastern Baltic for millennia – and could thus cluster closer genetically to peoples from Carpathian cultures than the eastern Pontic-Caspian steppe. Individuals from the Balkans at Varna I (ca. 4630 BC), Smyadovo (ca. 4500 BC), and outliers from Ukraine Eneolithic (ca. 3500 BC) and Trypillian culture (ca. 3325 BC) clearly show such steppe ancestry before the main Chalcolithic expansion[Mathieson et al. 2017][Haak et al. 2015]. Yamna migrants from the eastern zone (whose samples are used to define steppe ancestry) had migrated westward to the north Pontic area and beyond along the Danube at least twice: first in the formation process of the early Khvalynsk and Sredni Stog cultures, and later during the formation of the Yamna culture. On the other hand, late Sredni Stog regions seem to have adopted a different culture than the developing Yamna to the east, potentially suggesting a different ethnolinguistic nature. This most likely created a more mixed Balkan and east-central European genetic pool among qualitatively different cultures (probably speaking different languages, with expanding clans dominated by different lineages), which questions the validity of certain conclusions about the origin of the Corded Ware admixture.
Modified from Mathieson et al (2017). Left: «Individuals projected onto axes defined by the principal components of 799 present-day West Eurasians (not shown in thisplot for clarity, but shown in Extended Data Figure 1). Projected points include selected published individuals (faded colored circles, labeled) and newly reported individuals (other symbols; outliers shown by additional black circles). Colored polygons indicate the individuals that had cluster memberships fixed at 100% for the supervised admixture analysis [on the right]». Right: «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 (orange), EHG (red) and WHG (blue)». Dates indicate approximate range of individuals in each population[Mathieson et al. 2017]. Original image under a CC-BY-NC 4.0 International license.
Scattered samples from different periods (by millennia) from the Forest Zone and steppe already showed certain common clusters before the Neolithic and Chalcolithic expansions in global ancestry profiles, in the first articles published. More recently, Estonian samples have shown a genetic component associated with CHG coinciding with the spread of R1a-Z645, which rules out Corded Ware and Yamna as the only origin of this component[Saag et al. 2017]. A female from Zvejnieki, dated ca. 2885 BC and classified as from Latvian Neolithic – Corded Ware culture[Jones et al. 2017], has been found to cluster quite closely with eastern Yamna samples[Mathieson et al. 2017], in spite of its ancestral component’s supposed origin further south, and necessarily including some generations of admixture with the local population. This is coincident with the ‘southern drift’ observed in samples from western Yamna in the PCA, supporting thus a common drift of forest-steppe and steppe populations of the whole northern Pontic-Caspian region.
It is known that the genetic isolation of Eurasian hunter-gatherers came to an end with the arrival of farming and pastoralism. This is seen in the evolution of Middle Eastern ancestries during the Neolithic and Chalcolithic[Lazaridis et al. 2016], and it is becoming clearer too with the genetic flow seen in eastern Europe during the Neolithic and Chalcolithic. Even though samples are scarce and distant, Neolithic individuals from Comb Ware (Zvejnieki), Late Khvalynsk (Samara), and Old Europe (Varna I, Smyadovo) cultures show a clear pattern towards lesser inter-group genetic distances, clearly seen in the appearance of CHG in Admixture, and in their convergence in PCA analysis[Mathieson et al. 2017].
Two female samples from Bohemia were misidentified as Bell Beaker[Allentoft et al. 2015], when they were in fact three millennia younger, from Czech Slavs[Mathieson et al. 2017]. PCA or Admixture did not (and cannot) show differences with Bell Beaker or Balkan samples, since parental populations need to be available, or else archaeological context is needed to define demographic models and potential ancestral populations, to ascertain their actual link to the so-called steppe ancestry. In fact, there is a clear north-south cline of steppe ancestry in modern populations, peaking in the Forest Zone, which mimics to some extent its geographic distribution after the Corded Ware and Yamna expansions[Haak et al. 2015], and thus also potentially to some extent a previous situation[Klejn et al. 2017].
The migration of Pontic-Caspian steppe into Neolithic/Bronze Age Central Europeans has been argued to be strongly male-biased[Poznik et al. 2016], with a study suggesting up to 14 migrating males for every migrating female[Goldberg, Günther, et al. 2017], but different in the rates regarding Corded Ware, Bell Beaker, and Únětice. The results of the latter study have been disputed[Lazaridis and Reich 2017], and this in turn contested by the original authors based on the impact of small, low-coverage ancient samples in admixture analyses[Goldberg, Gunther, et al. 2017]. This questions the accuracy of predictions made based on certain samples and methods used.
Ascertaining differences in demographic changes is especially important in light of an apparently mostly peaceful Yamna migration along the Danube[Heyd 2012], contrasting with the potentially violent and strong patrilocality shown by peoples of the Corded Ware cultures[Kristiansen et al. 2017]. Quite relevant for the effect of the invading population is population density prior to the invasions, and the actual increase in population estimated after such population expansions, having been greater in south-east Europe[Müller 2013]. That influences greatly the genetic drift observed, and must be taken into account to make inferences about the actual origin and influence of the population involved in those migrations.
Shortcomings of statistical methods used for the analysis of ancestral populations are usually not evident for the layman. They may affect any theory developed based solely on these methods.
Principal component analysis (PCA) is a variable-reduction technique, similar to exploratory factor analysis. It reduces a larger set of variables into a smaller set of artificial variables, called principal components (PC), which account for most of the variance in the original variable. This method assumes that there is a linear relationship between variables, that there is sampling adequacy (a precise number of cases is difficult to evaluate, but it is to be assumed that scarce, damaged samples of ancient DNA preclude an ideal sample size). Variables also need to have adequate correlations in order for them to be reduced to a smaller number of components, and there should be no significant outliers.
PCA of ancient DNA samples show usually a large number of principal components, of which the most common ones selected for graphic analysis (PC1 and PC2) can usually explain (have a combined eigenvalue of) no more than 5-10% of the total variance, depending on the samples selected. This is in line with the prediction that most eigenvalues of the theoretical covariance will be ‘small’, nearly equal, but is in contrast with the expectation that a few eigenvalues will be ‘large’, reflecting past demographic events[Patterson, Price, and Reich 2006]. Theoretically, though, under a small number of ancestral populations, with small divergence among them, just two significant eigenvalues will exist, and selecting the two main significant axes of variation captures the most relevant information. On the other hand, ancestral populations – like certain African populations – may show dozens of significant axes, whose meaning is unclear.
PCA analysis of dataset including Minoans and Mycenaeans[Lazaridis et al. 2017], and Scythian and Sarmatian samples[Unterländer et al. 2017]. Plots of different pairs of consecutive PCs, with symbols and corresponding samples. PC1 (3.756%) vs. PC2 (2.654%), PC3 (2.165%) vs. PC4 (2.146%), PC5 (2.129%) vs. PC6 (2.118%), and PC7 (2.114%) vs. PC8 (2.106%).
Analysis with STRUCTURE[Pritchard, Stephens, and Donnelly 2000] / ADMIXTURE[Alexander, Novembre, and Lange 2009] is also mainly reported in genetic papers, as PCA, following graphical patterns. It is important to take into account that mixed ancestry in an individual can result – apart from genetic admixture of two isolated populations, which is the object of the study – from shared ancestry (coinheritance of more than one ancestry from the same parental source, incomplete differentiation), and also from assimilation.
The number of ancestral populations selected is based on cross-validation error estimation and graphical analysis. It uses, therefore, a combination of numerical and graphical methods in a similar way to the factor extraction in PCA, but less formally explored.
In a recent study, evidence supported 21 ancestries to delineate genetic structure of present-day human populations[Baker, Rotimi, and Shriner 2017], although this is debatable. It is unclear whether this ‘ideal’ number would be greater or lesser for ancestral, more isolated populations, and the lack of proper sampling precludes a proper selection of K. The usual small number of inferred ancestral components, selected to show ancestries in a simplified manner (K=2-6), may thus be too simplistic, although a K = ~8-11 appears to be a good range of components for the analysis of modern populations.
«Cross-validation error as a function of the number of ancestral components K. The red symbol indicates the minimum cross-validation error, which occurs at K=21»[Baker, Rotimi, and Shriner 2017].
Therefore, the selection and naming of a population as ‘ancestral’ to another is indeed conventional, and can lead to error when its nature as approximate source or proxy among poorly investigated populations is not fully understood. With new results, the naming of certain ancestral populations may become obsolete, as more ancestral proxy populations are discovered.
Formal tests to investigate whether mixture occurred, and to infer proportions and dates of mixture are relatively new, and include the three-population test, D-statistics, F4-ratio estimation, admixture graph fitting, and rolloff, included – among other tools – in the free software package ADMIXTOOLS[Patterson et al. 2012]. They are robust tools based on statistical methods, but each method is dependent on certain assumptions. So, for example, an estimation of mixing proportions in a three-population test, when phylogeny for the populations studied is incorrect, leaves such proportions without useful meaning. Even discussing mixing from an ancestral population – like Corded Ware from Yamna –, when an intermediate admixing event occurs, hardly makes sense.
Cognitive bias, conflicts of interest, contextual bias
In the academic community prestige, access to grants, and even jobs depend on getting articles published in journals of high impact factor. These journals prefer short articles, mainly based on mathematical methods (preferably with reference to improvements in such methods), groundbreaking conclusions, and self-important titles, with a tendency to “culture-historicism”. Pressure to publish means also pressure to gather, analyse and interpret the data. However, knowledge and expertise in gathering genetic data from archaeological remains does not mean expertise in statistics and computer science. And statistical knowledge does not qualify one to infer conclusions based on results, either, unless one has some previous knowledge of anthropological subjects involved. Otherwise, a researcher concerned with fieldwork and statistical methods is exposed during the interpretation of results to the risks of circular reasoning and confirmation bias, by searching only for anthropological information that fits their results. In this sense, a clear trend can be observed whereby wide-ranging conclusions in genetic papers tend to become outdated in very short periods, as new samples become available.
SNP investigation offers a simple view of one’s own paternal line, that a thousand years (or ca. 30 generations) ago would represent a 1,000,000,000th of one’s own genealogical tree; four or five thousand years ago, its contribution to a personal ethnolinguistic definition is almost non-existent. This, together with the perceived complexity (and lack of familiarity with) intricately linked anthropological disciplines, has made ancestry investigation quite popular among amateur geneticists, who can easily play with published open source software programs, due to their accessibility. However, the correct use of these programs needs much more than just knowing how to apply certain commands to some data. The quest for one’s own personal and national “ethnic proportion”, often as part of pre-existing simplistic ethnolinguistic beliefs and socio-political agendas, is also being promoted by commercial genetic testing companies to sell their products, in what would certainly be a reason for Kosinna’s smile today.
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- Iosif Lazaridis (Twitter 29/9/2017): “the Yamnaya are the best proximate source for the new ancestry that first appears with the Corded Ware in central Europe, as it has the right mix of both ANE (related to Native Americans, MA1, and EHG), but also Armenian/Caucasus/Iran-like southern component of ancestry”.
- The individual from Merichleri, of R1a1a1-M417 lineage might hint, in fact, to an ancient connection of the area with the second Corded Ware horizon.
- See Figure 3 in Haak et al. (2015), p. 23
- See e.g. Extended Data Figure 2 in Haak et al. (2015), Extended Data Table 2 in Mathieson et al. (2015), Figure 2 in Jones et al. (2017).
- This is a guesstimate based on the limited experience of the author with free datasets..
- Iosif Lazaridis (Twitter 3/9/2017), criticises the choice of K=21 as a “minimum”, as well as the concept of “mixed ancestry” meaning the possession of >1 of K=21 components in an admixture analysis over ~19k SNPs.