Minimal gene flow from western pastoralists in the Bronze Age eastern steppes

jeong-steppes-mongolia

Open access paper Bronze Age population dynamics and the rise of dairy pastoralism on the eastern Eurasian steppe, by Jeong et al. PNAS (2018).

Interesting excerpts (emphasis mine):

To understand the population history and context of dairy pastoralism in the eastern Eurasian steppe, we applied genomic and proteomic analyses to individuals buried in Late Bronze Age (LBA) burial mounds associated with the Deer Stone-Khirigsuur Complex (DSKC) in northern Mongolia. To date, DSKC sites contain the clearest and most direct evidence for animal pastoralism in the Eastern steppe before ca. 1200 BCE.

Most LBA Khövsgöls are projected on top of modern Tuvinians or Altaians, who reside in neighboring regions. In comparison with other ancient individuals, they are also close to but slightly displaced from temporally earlier Neolithic and Early Bronze Age (EBA) populations from the Shamanka II cemetry (Shamanka_EN and Shamanka_EBA, respectively) from the Lake Baikal region. However, when Native Americans are added to PC calculation, we observe that LBA Khövsgöls are displaced from modern neighbors toward Native Americans along PC2, occupying a space not overlapping with any contemporary population. Such an upward shift on PC2 is also observed in the ancient Baikal populations from the Neolithic to EBA and in the Bronze Age individuals from the Altai associated with Okunevo and Karasuk cultures.

pca-eurasians-karasuk-khovsgol
Image modified from the article. Karasuk cluster in green, closely related to sample ARS026 in red. Principal Component Analysis (PCA) of selected 2,077 contemporary Eurasians belonging to 149 groups. Contemporary individuals are plotted using three-letter abbreviations for operational group IDs. Group IDs color coded by geographic region. Ancient Khövsgöl individuals and other selected ancient groups are represented on the plot by filled shapes. Ancient individuals are projected onto the PC space using the “lsqproject: YES” option in the smartpca program to minimize the impact of high genotype missing rate.

(…) two individuals fall on the PC space markedly separated from the others: ARS017 is placed close to ancient and modern northeast Asians, such as early Neolithic individuals from the Devil’s Gate archaeological site (22) and present-day Nivhs from the Russian far east, while ARS026 falls midway between the main cluster and western Eurasians.

Upper Paleolithic Siberians from nearby Afontova Gora and Mal’ta archaeological sites (AG3 and MA-1, respectively) (25, 26) have the highest extra affinity with the main cluster compared with other groups, including the eastern outlier ARS017, the early Neolithic Shamanka_EN, and present-day Nganasans and Tuvinians (Z > 6.7 SE for AG3). Main cluster Khövsgöl individuals mostly belong to Siberian mitochondrial (A, B, C, D, and G) and Y (all Q1a but one N1c1a) haplogroups.

mongolia-botai-ehg-ane-cline
The genetic affinity of the Khövsgöl clusters measured by outgroup-f3 and -f4 statistics. (A) The top 20 populations sharing the highest amount of >genetic drift with the Khövsgöl main cluster measured by f3(Mbuti; Khövsgöl, X). (B) The top 15 populations with the most extra affinity with each of the three Khövsgöl clusters in contrast to Tuvinian (for the main cluster) or to the main cluster (for the two outliers), measured by f4(Mbuti, X; Tuvinian/Khövsgöl, Khövsgöl/ARS017/ARS026). Ancient and contemporary groups are marked by squares and circles, respectively. Darker shades represent a larger f4 statistic.

Previous studies show a close genetic relationship between WSH populations and ANE ancestry, as Yamnaya and Afanasievo are modeled as a roughly equal mixture of early Holocene Iranian/ Caucasus ancestry (IRC) and Mesolithic Eastern European hunter-gatherers, the latter of which derive a large fraction of their ancestry from ANE. It is therefore important to pinpoint the source of ANE-related ancestry in the Khövsgöl gene pool: that is, whether it derives from a pre-Bronze Age ANE population (such as the one represented by AG3) or from a Bronze Age WSH population that has both ANE and IRC ancestry.

The amount of WSH contribution remains small (e.g., 6.4 ± 1.0% from Sintashta). Assuming that the early Neolithic populations of the Khövsgöl region resembled those of the nearby Baikal region, we conclude that the Khövsgöl main cluster obtained ∼11% of their ancestry from an ANE source during the Neolithic period and a much smaller contribution of WSH ancestry (4–7%) beginning in the early Bronze Age.

khovsgol-shamanka-sintashta
Admixture modeling of Altai populations and the Khövsgöl main cluster using qpAdm. For the archaeological populations, (A) Shamanka_EBA and (B and C) Khövsgöl, each colored block represents the proportion of ancestry derived from a corresponding ancestry source in the legend. Error bars show 1 SE. (A) Shamanka_EBA is modeled as a mixture of Shamanka_EN and AG3. The Khövsgöl main cluster is modeled as (B) a two-way admixture of Shamanka_EBA+Sintashta and (C) a three-way admixture Shamanka_EN+AG3+Sintashta.

Apparently, then, the first individual with substantial WSH ancestry in the Khövsgöl population (ARS026, of haplogroup R1a-Z2123), directly dated to 1130–900 BC, is consistent with the first appearance of admixed forest-steppe-related populations like Karasuk (ca. 1200-800 BC) in the Altai. Interestingly, haplogroup N1a1a-M178 pops up (with mtDNA U5a2d1) among the earlier Khövsgöl samples.

I will repeat what I wrote recently here: Samoyedic arrived in the Altai with Karasuk and hg R1a-Z645 + Steppe_MLBA-like ancestry, admixed with Altai populations, clustering thus within an Ancient Altai cline. Only later did N1a1a subclades infiltrate Samoyedic (and Ugric) populations, bringing them closer to their modern Palaeo-Siberian cline. The shared mtDNA may support an ancestral EHG-“Siberian” cline, or else a more recent Afanasevo-related origin.

east-uralic-clines
Modified image from Jeong et al. (2018), supplementary materials. The first two PCs summarizing the genetic structure within 2,077 Eurasian individuals. The two PCs generally mirror geography. PC1 separates western and eastern Eurasian populations, with many inner Eurasians in the middle. PC2 separates eastern Eurasians along the north-south cline and also separates Europeans from West Asians. Ancient individuals (color-filled shapes), including two Botai individuals, are projected onto PCs calculated from present-day individuals. Read more.

Also interesting, Q1a2 subclades and ANE ancestry making its appearance everywhere among ancestral Eurasian peoples, as Chetan recently pointed out.

Related

The Iron Age expansion of Southern Siberian groups and ancestry with Scythians

iron_age-sarmatians

Maternal genetic features of the Iron Age Tagar population from Southern Siberia (1st millennium BC), by Pilipenko et al. (2018).

Interesting excerpts (emphasis mine):

The positions of non-Tagar Iron Age groups in the MDS plot were correlated with their geographic position within the Eurasian steppe belt and with frequencies of Western and Eastern Eurasian mtDNA lineages in their gene pools. Series from chronological Tagar stages (similar to the overall Tagar series) were located within the genetic variability (in terms of mtDNA) of Scythian World nomadic groups (Figs 5 and 6; S4 and S6 Tables). Specifically, the Early Tagar series was more similar to western nomads (North Pontic Scythians), while the Middle Tagar was more similar to the Southern Siberian populations of the Scythian period. The Late Tagar group (Tes`culture) belonging to the Early Xiongnu period had the “western-most” location on the MDS plot with the maximal genetic difference from Xiongnu and other eastern nomadic groups (but see Discussion concerning the low sample size for the Tes`series).

In a comparison of our Tagar series with modern populations in Eurasia, we detected similarity between the Tagar group and some modern Turkic-speaking populations (with the exception of the Indo-Iranian Tajik population) (Fig 7; S2 Table). Among the modern Turkic-speaking groups, populations from the western part of the Eurasian steppe belt, such as Bashkirs from the Volga-Ural region and Siberian Tatars from the West Siberian forest-steppe zone, were more similar to the Tagar group than modern Turkic-speaking populations of the Altay-Sayan mountain system (including the Khakassians from the Minusinsk basin) (Fig 7).

tagar-archaeology
Location of Tagar archaeological sites from which samples for this study were obtained. Burial grounds: 1—Novaya Chernaya-1; 2—Podgornoe Ozero, Barsuchiha-1, Barsuchiha-6, Barsuchiha-7; 3—Perevozinskiy; 4—Ulug-Kyuzyur, Kichik-Kyuzyur, Sovetskaya Khakassiya; 5—Tepsey-3, Tepsey-8, Tepsey-9; 6—Dolgiy Kurgan. https://doi.org/10.1371/journal.pone.0204062.g001

Mitochondrial DNA diversity and genetic relationships of the Tagar population

Our results are not inconsistent with the assumption of a probable role of gene flow due to the migration from Western Eurasia to the Minusinsk basin in the Bronze Age in the formation of the genetic composition of the Tagar population. Particularly, we detected many mtDNA lineages/clusters with probable West Eurasian origin that were dominant in modern populations of different parts of Europe, Caucasus, and the Near East (such as K and HV6) in our Tagar series based on a phylogeographic analysis.

We detected relatively low genetic distances between our Tagar population and two Bronze Age populations from the Minusinsk basin—the Okunevo culture population (pre-Andronovo Bronze Age) and Andronovo culture population, followed by Afanasievo population from the Minusinsk Basin and Middle Bronze Age population from the Mongolian Altai Mountains (the region adjacent to the Minusinsk basin) (Figs 3 and 6; S3 and S5 Tables). Among West Eurasian part of our Tagar series we also observed haplogroups/sub-haplogroups and haplotypes shared with Early and Middle Bronze Age populations from Minusinsk Basin and western part of Eurasian steppe belt (Fig 4; S5 Table). Thus, our results suggested a potentially significant role of the genetic components, introduced by migrants from Western Eurasia during the Bronze Age, in the formation of the genetic composition of the Tagar population. It is necessary to note the relatively small size of available mtDNA samples from the Bronze Age populations of Minusinsk basin; accordingly, additional mtDNA data for these populations are required to further confirm our inference.

tagar-mtdna-tree
Phylogenetic tree of mtDNA lineages from the Tagar population. Color coding of the Tagar stages: orange—the Early Tagar stage; blue—the Middle Tagar Stage; green—the Late Tagar stage. Color of haplogroup labels: yellow—for Western Eurasian haplogroups; red—for Eastern Eurasian haplogroups. https://doi.org/10.1371/journal.pone.0204062.g002

Another substantial part of the mtDNA pool of the Tagar and other eastern populations of the Scythian World is typical of populations in Southern Siberia and adjacent regions of Central Asia (autochthonous Central Asian mtDNA clusters). Most of these components belong to the East Eurasian cluster of mtDNA haplogroups. Moreover, the role of each of these components in the formation of the genetic composition of subsequent (to the present) populations in South Siberia and Central Asia could be very different. In this regard, cluster C4a2a (and its subcluster C4a2a1), and haplogroup A8 are of particular interest.

Genetic features of successive Tagar groups

We compared successive Tagar groups (Early, Middle, and Late Tagar) with each other and with other Iron Age nomadic populations to evaluate changes in the mtDNA pool structure. Despite the genetic similarity between the Early and Middle Tagar series and Scythian World nomadic groups (Figs 5 and 6; S4 and S6 Tables), there were some peculiarities. For example, the Early Tagar series was more similar to North Pontic Classic Scythians, while the Middle Tagar samples were more similar to the Southern Siberian populations of the Scythian period (i.e., completely synchronous populations of regions neighboring the Minusinsk basin, such as the Pazyryk population from the Altay Mountains and Aldy-Bel population from Tuva).

We observed differences in the mtDNA pool structure between the Early and the Middle chronological stages of the Tagar culture population, as evidenced by the change in the ratio of Western to Eastern Eurasian mtDNA components. The contribution of Eastern Eurasian lineages increased from about one-third (34.8%) in the Early Tagar group to almost one-half (45.8%) in the Middle Tagar group.

tagar-mtdna-fst
Results of multidimensional scaling based on matrix of Slatkin population differentiation (FST) according to frequencies of mtDNA haplogroup in Tagar populations and modern populations of Eurasia. Populations: Tagar (red pentagon) (this study); Mongolian-speaking populations: Khamnigans (Buryat Republic, Russia) [43]; Barghuts (Inner Mongolia, China) [44]; Buryats (Buryat Republic, Southern Siberia, Russia) [43]; Mongols (Mongolia) [45]. Turkic-speaking populations: Tuvinians (Tuva Republic, Russia) [43]; Tofalars (Irkutsk region, Russia) [46]; Altai-Kizhi ((Altai Republic, Russia) [43, 47]; Telenghits (Altai Republic, Russia) [43,47]; Tubalars (Altai Republic) [48]; Shors (Kemerovo region, Russia) [43, 47]; Khakassians (Khakassian Rupublic, Russia) [43, 46]; Altaian Kazakhs (Altai Republic) [49]; Kazakhs (Kazakhstan, Uzbekistan) [50, 51]; Kirghiz (Kyrgyzstan) [50, 51]; Uighurs (Kazakhstan and Xinjiang) [50, 52]; Siberian Tatars (Tyumen and Omsk regions, Russia) [53]; Tatars (Volga-Ural rigion, Russia) [54]; Bashkirs (Volga-Ural region, Russia) [55]; Uzbeks (Uzbekistan) [51, 56]; Turkmens (Turkmenistan) [51, 56]; Nogays [57]; Turkeys [58]; other populations: Evenks [43, 46]; Ulchi [59]; Koreans (South Korea) [43]; Han Chinese [60]; Zhuang (Guangxi, China) [61]; Tadjiks (Tadjikistan) [43, 51]; Iranians [60]; Russians [62]. https://doi.org/10.1371/journal.pone.0204062.g007

At the level of mtDNA haplogroups, we detected a decrease in the diversity of phylogenetic clusters during the transition from the Early Tagar to the Middle Tagar. This decline in diversity equally affected the West Eurasian and East Eurasian components of the Tagar mtDNA pool. It should be noted that this decrease can be partially explained by the smaller number of Middle Tagar than Early Tagar samples. Under a simple binomial approximation the mtDNA clusters, observed at frequencies of 6.3% and 11.7%, could be lost by chance in our Early (N = 46) and Middle (N = 24) Tagar samples, respectively. However, the simultaneous lack of several such clusters, with a total frequency in the gene pool of the Early group of 34.8%, is unlikely.

The observed reduction in the genetic distance between the Middle Tagar population and other Scythian-like populations of Southern Siberia(Fig 5; S4 Table), in our opinion, is primarily associated with an increase in the role of East Eurasian mtDNA lineages in the gene pool (up to nearly half of the gene pool) and a substantial increase in the joint frequency of haplogroups C and D (from 8.7% in the Early Tagar series to 37.5% in the Middle Tagar series). These features are characteristic of many ancient and modern populations of Southern Siberia and adjacent regions of Central Asia, including the Pazyryk population of the Altai Mountains. We did not obtain strong evidence for an intensification of genetic contact between the population of the Minusinsk basin and the Altai Mountains in the Middle Tagar period compared with the Early Tagar period. Although, several archaeologists have found evidence for the intensification of contact at the level of material culture, namely, a cultural influence of the population of the Altai Mountains (represented by the Pazyryk population) on the population of the Minusinsk basin (the Saragash Tagar group) [6, 71, 72].

Another important issue is the change in the genetic structure of the Tagar population during the transition from the Middle (Saragash) to the Late (Tes`) stage. The Late Tagar stage refers to the Xiongnu period. Many archaeologists suggest that the formation of the Tes`stage involved the direct cultural influence of the Xiongnu and/or related groups of nomads from more eastern regions of Central Asia [71, 73]. Some archaeologists have even suggested renaming the Tes`stage in the Tes`culture [71], emphasizing the role of new eastern cultural elements. If this influence also existed at the genetic level, then we would expect to observe new genetic elements in the Tes`gene pool, particularly those of East Eurasian origin.

Siberian ancestry

Just a reminder of the recent session in ISBA 8 on expanding Scythians (and also Mongolians and Turks) spreading Siberian ancestry, usually (wrongly) identified as “Uralic-Yeniseian” based on modern populations (similar to how steppe ancestry is wrongly identified as “Indo-European”), see the following graphic including the Tagar population:

siberian-genetic-component-chronology
Very important observation with implication of population turnover is that pre-Turkic Inner Eurasian populations’ Siberian ancestry appears predominantly “Uralic-Yeniseian” in contrast to later dominance of “Tungusic-Mongolic” sort (which does sporadically occur earlier). Alexander M. Kim

And also the poster by Alexander M. Kim et al. Yeniseian hypotheses in light of genome-wide ancient DNA from historical Siberia:

The relevance of ancient DNA data to debates in historical linguistics is an emphatic strand in much recent work on the archaeogenetics of Eurasia, where the discussion has focused heavily on Indo-European (Haak et al. 2015; Narasimhan et al. 2018; de Barros Damgaard et al. 2018a,b). We present new genome-wide ancient DNA data from a historical Siberian individual in relation to Yeniseian, an isolated language “microfamily” (Vajda 2014) that nonetheless sits at the center of numerous controversial proposals in historical linguistics and cultural interaction. Yeniseian’s sole surviving representative is Ket, a critically endangered language fluently spoken by only a few dozen individuals near the Middle Yenisei River of Central Siberia.

In strong contrast to the present-day picture, river names and argued substrate influences and loanwords in languages outside the current range of Yeniseian, as well as direct records from the Russian colonial period, indicate that speakers of extinct Yeniseian languages had a formerly much broader presence in the taiga of Central Siberia as well as further south in the mountainous Altai-Sayan region – and perhaps even further afield in Inner Asia (Vajda 2010; Gorbachov 2017; Blažek 2016). The consilience of these proposals with genetic data is not straightforward (Flegontov et al. 2015, 2017) and faces a major obstacle in the lack of genetic information from verifiable speakers of Yeniseian languages other than the Kets, who have had complex ongoing interactions with speakers of non-Yeniseian languages such as the Samoyedic Selkups. We attempt to remedy this with new historical Siberian aDNA data, orienting our search for common denominators and systematic difference in a broader landscape of concordance, discordance, and uncertainty at the interface of diachronic linguistics and genetics.

Related

Origins of equine dentistry in Mongolia in the early first millennium BC

New paper (behind paywall) Origins of equine dentistry, by Taylor et al. PNAS (2018).

Interesting excerpts (emphasis mine):

The practice of horse dentistry by contemporary nomadic peoples in Mongolia, coupled with the centrality of horse transport to Mongolian life, both now and in antiquity, raises the possibility that dental care played an important role in the development of nomadic life and domestic horse use in the past. To investigate, we conducted a detailed archaeozoological study of horse remains from tombs and ritual horse inhumations across the Mongolian Steppe, assessing evidence for anthropogenic dental modifications and comparing our findings with broader patterns in horse use and nomadic material culture.

We conducted a detailed study of archaeological horse collections spanning the past 3,200 y, including those from the Late Bronze Age DSK complex (ca. 1200–700 BCE, n = 70), Early Iron Age Slab Burial culture (ca. 700–300 BCE, n = 4), Pazyryk culture (ca. 600–200 BCE, n = 2), Late Iron Age Xiongnu Empire (ca. 200 BCE–200 CE, n = 3), Early Middle Ages post-Xiongnu period (ca. 100–550 CE, n = 3), and Turkic Khaganate (ca. 600–800 CE, n = 3).

horse-riding-mongolia
A (top): Contemporary Mongolian herder engaged in horseback riding, using left-handed rein position causing asymmetric pressures to the horse’s skull. Photo by Orsoo Bayarsaikhan. B(center) contemporary Mongolian horse skulls, showing asymmetric and skewed thinning to the nasal bones caused by bridle pressure. C(bottom) Asymmetric deformation to the cranial bones of a Deer Stone-Khirigsuur horse (left), alongside an early Middle Ages horse with a similar feature (right). Modified from Taylor and Tuvshinjargal (2018).

Discussion

This Late Bronze Age dental modification counts among the earliest documented instances of equine veterinary care, and the oldest known evidence for horse dentistry. At first glance, the detailed historical record of early equine veterinary care in places such as China, Greece, Rome, and Syria, which spans the late second millennium BCE through the early centuries CE (11, 15, 16), might imply that equine dentistry emerged in the sedentary civilizations of the Old World. However, the earliest textual references describe only nonsurgical medicinal treatments and make few mentions of oral health (11). Recent archaeological discoveries suggest that human care of domestic animals was practiced by hunter-gatherers as far back as the Paleolithic (46), and that pastoralists may have occasionally practiced surgical procedures on domestic animals as early as the Neolithic in Europe (47). The evidence presented here indicates that horse dentistry was developed by nomadic pastoralists living on the steppes of Mongolia and northeast Asia during the Late Bronze Age, concurrent with the local adoption of the metal bit and many centuries before the first mention of dental practices in historical accounts from sedentary Old World civilizations.

Our results reveal a fundamental link between equine dentistry and the emergence of horsemanship in the steppes of Eurasia. At the turn of the first millennium BCE, militarized, horse-mounted peoples reshaped the social and economic landscape of many areas of the Eurasian continent. Conflagrations with equestrian peoples, such as those between the Persian Empire and the Pontic “Scythians,” plagued alluvial civilizations from the Near East to India and China, while large-scale movements of people linked East and West in never-before-seen ways (48). The archaeological and historical records indicate that the earliest horseback riding was accomplished without stirrups or saddles, and probably using only bitless or organic-mouthpiece bridles (49, 50). The bronze snaffle bit, and the improved control it provided, was a key technological development that enabled the use of horseback riding for more stressful and difficult activities, such as long-distance transportation and warfare (32). We argue that these technological improvements in horse control were preceded and sustained by innovations in veterinary dentistry by nomadic peoples living in the continental interior. By increasing herd survival and mitigating behavioral and health issues caused by horse equipment, innovations in equine dentistry improved the reliability of horseback riding for ancient nomads, enabling horses to be used for nonpastoral activities like warfare, high-speed riding, and distance travel.

damage-tooth-horse
Damage to the retained wolf tooth in a 4-5 year old mummified horse, dating to the 2-4th centuries CE from the site of Urd Ulaan-Uneet in western Mongolia

Conclusion

Archaeozoological data from Mongolian horses indicate that the nomadic practice of equine dentistry dates back more than 3,000 y to the DSK complex, a Late Bronze Age culture associated with the first mounted horseback riding and mobile pastoralism in eastern Eurasia. Attempted removal of deciduous incisors through sawing of the exterior suggests experimentation with dental extraction, but not the removal of wolf teeth. The appearance of extracted first premolars in the first millennium BCE coincides with the arrival of metal bits in the archaeological record and oral trauma linked with metal bit use, suggesting that innovations in dental practice were an adaptation to the mechanical changes in horse equipment. These bronze and metal bits provided greater control over the horse, facilitating the development of military uses for the horse, but also introduced new dental problems with the first premolar. Our results indicate that, coincident with the earliest evidence for metal bit use, wolf tooth extraction was practiced in Mongolia by ca. 750 BCE and continued through the early Middle Ages. These results push back the earliest dates for equine dentistry by more than a millennium and suggest that nomadic peoples developed key innovations in veterinary care that enabled more sophisticated horse control, ultimately changing the structure of communication, exchange, and military power in ancient Eurasia.

Related

How an empire of steppe nomads coped with environmental stress

uyghur-empire

Recent paper (behind paywall), Environmental Stress and Steppe Nomads: Rethinking the History of the Uyghur Empire (744–840) with Paleoclimate Data, by Di Cosmo et al. JINH (2018) XLVIII(4):439-463.

Abstract (emphasis mine):

Newly available paleoclimate data and a re-evaluation of the historical and archaeological evidence regarding the Uyghur Empire (744–840)—one of several nomadic empires to emerge on the Inner Asian steppe—suggests that the assumption of a direct causal link between drought and the stability of nomadic societies is not always justified. The fact that a severe drought lasting nearly seven decades did not cause the Uyghur Empire to collapse, to wage war, or to disintegrate gives rise to speculations about which of its characteristics enabled it to withstand unfavorable climatic conditions and environmental change. More broadly, it raises questions about the complex suite of strategies and responses that may have been available to steppe societies in the face of environmental stress.

Interesting excerpts:

The heart of the matter is whether the apparent capacity to withstand adverse climatic conditions was due to a set of cultural and/or political choices, unrelated to climate, or to a modification of their nomadic ways toward a greater reliance on agriculture and trade. In support of the first alternative, the Uyghur Empire’s initial military rise and expansion (c. 744 to 780) may have been aided by conditions of high moisture that would have made the grasslands especially productive. The alliance with China, the conversion to Manichaeism, and the growing incorporation of Sogdian elites in their empire that took place during this period may have aided the Uyghurs, serendipitously, to withstand the climatic crisis. In the long run, however, some areas of society most likely suffered adverse effects, which might explain the internal splits and the final defeat at the hands of the Kirgiz. These developments owe as much to the sedentary habits of the Uyghur elites and their ineffective leadership as to the erosion of the pastoral resources that constituted the main support of a nomadic army.

uyghur-climate
The findings are based on scPDSI reconstructions at 0.5-degree grid derived from tree
ring records across Asia.

The evidence from tree-ring data points to environmental conditions that alter the historical narrative of the rise and fall of the Uyghur Empire. Although several studies have focused on how climatic extremes affected agricultural societies, the fate of complex nomadic societies did not necessarily follow the same path. Steppe empires, at least from the first Türk Empire (established in the sixth century C.E.), and possibly earlier, were based on expansive geographical networks, diversified economies, and a degree of co-dependency with merchant classes. Even though these elements varied greatly between empires, they probably reacted to climatic variability in ways radically different from the ways in which agricultural and urban polities did. Once we account for socio-economic complexity, the case of the Uyghur Empire belies the general assumption that nomadic peoples are more vulnerable to climatic stress. The severe and protracted drought did not trigger migration, pillaging, or conquest. Sudden shocks, however, could have had a devastating effect on an economy already weakened.

The catastrophic event of the winter of 839/40, which appears to be a dzud (a weather condition of extreme cold and heavy snowfall that causes high animal mortality) was likely the coup de grâce to an already compromised pastoral production. Contemporary studies of the dzud emphasize that the degree of calamity of the winter disaster is closely related to the drought conditions that preceded it. Thus, the collapse of the Uyghur Empire, generally understood as a rapidly evolving political crisis compounded by a catastrophic weather event, is also connected to multidecadal climatic change, dependency mechanisms, and geopolitical constraints.

I found it interesting mainly because of the potential application of this kind of studies to other previous steppe societies.

Discovered via news on Sapiens.org, posted by Spencer Wells.

Related:

Decline of genetic diversity in ancient domestic stallions in Europe

Open access research article Decline of genetic diversity in ancient domestic stallions in Europe, by Wutke et al., Science (2018), 4(4):eaap9691.

Abstract (emphasis mine):

Present-day domestic horses are immensely diverse in their maternally inherited mitochondrial DNA, yet they show very little variation on their paternally inherited Y chromosome. Although it has recently been shown that Y chromosomal diversity in domestic horses was higher at least until the Iron Age, when and why this diversity disappeared remain controversial questions. We genotyped 16 recently discovered Y chromosomal single-nucleotide polymorphisms in 96 ancient Eurasian stallions spanning the early domestication stages (Copper and Bronze Age) to the Middle Ages. Using this Y chromosomal time series, which covers nearly the entire history of horse domestication, we reveal how Y chromosomal diversity changed over time. Our results also show that the lack of multiple stallion lineages in the extant domestic population is caused by neither a founder effect nor random demographic effects but instead is the result of artificial selection—initially during the Iron Age by nomadic people from the Eurasian steppes and later during the Roman period. Moreover, the modern domestic haplotype probably derived from another, already advantageous, haplotype, most likely after the beginning of the domestication. In line with recent findings indicating that the Przewalski and domestic horse lineages remained connected by gene flow after they diverged about 45,000 years ago, we present evidence for Y chromosomal introgression of Przewalski horses into the gene pool of European domestic horses at least until medieval times.

horses-y-chromosome-evolution
The frequencies of Y chromosome haplotypes started to change during the Late Bronze Age (1600–900 BCE).
Inferred temporal trajectories of haplotype frequencies. Each haplotype is displayed by a different color. The shaded area represents the 95% highest-density region. The trajectories were constructed taking the median values across frequencies from the simulations of the Bayesian posterior sample. The small chart represents the stacked frequencies; the amplitude of each colored area is proportional to the median haplotype frequencies (normalized) at a given time. The x and y axes of the small chart match those in the large one. Ka, thousands of years.

Interesting excerpts:

The first record of the modern domestic Y chromosome haplotype stems from two Bronze Age samples of similar age. Notably, both samples were found in two distantly located regions: present-day Slovakia (2000–1600 BCE, dated by archaeological context) and western Siberia (14C-dated: 1609–1436 cal. BCE). Although a very recent study proposes an oriental origin of this haplotype (14), we cannot determine the geographical origin of Y-HT-1 with certainty, because this haplotype has not been found thus far in predomestic or wild stallions. There are two possible scenarios: (i) Y-HT-1 emerged within the domestic population by mutation and (ii) Y-HT-1 was already present in wild horses and entered the domestic population either at the beginning of domestication (but initially restricted to Asian horses) or later by introgression (from wild Y-HT-1 carrying studs during the Iron Age). Crosses between domestic animals and their wild counterparts have been observed in several domestic species (15–18); thus, the simplest explanation would be that we missed Y-HT-1 in older samples because of limited geographical sampling. However, the estimated haplotype age is contemporary (Fig. 4) with the assumed starting point of horse domestication ~4000–3500 BCE (19), rendering it likely that Y-HT-1 originated within the domestic horse gene pool. Still, we cannot rule out definitively that it appeared before domestication.

Independent of its geographical origin, Y-HT-1 progressively replaced all other haplotypes—except for one additional lineage that is restricted to Yakutian horses (11). Considering our data, this trend in paternal diversity toward dominance of the modern lineage appears to start in the Bronze Age and becomes even more pronounced during the Iron Age. The Bronze Age was a time of large-scale human migrations across Eurasia (20–22), movements that were undoubtedly facilitated by the spread of horses as a means of transport and warfare. At that time, the western Eurasian steppes were inhabited by highly mobile cultures that largely relied on horses (20, 21, 23, 24). The genetic admixture of northern and central European humans with Caucasians/eastern Europeans did correlate with the spread of the Yamnaya culture from the Pontic-Caspian steppe (25), an area that has repeatedly been suggested as the center of horse domestication (19, 26, 27). Given the importance of domestic horses, it appears that deliberate selection/rejection of certain stallions by these people might have contributed to the loss of paternal diversity. The spread of humans out of this region might also have resulted in the spread of Y-HT-1 from Asia to Europe. This scenario also agrees with recent findings that the low male diversity of extant horses is not caused by recruiting only a limited number of stallions during early domestication (13).

horses-y-chromosome-map
Decline of paternal diversity began in Asia.
Maps displaying age, locality, and haplotype (different colors) of each successfully genotyped sample.

The presence of the Y chromosome haplotype carried by present-day Przewalski horses (Y-HT-2) in early domestic stallions and a European wild horse (Pie05; table S2) could be the result of introgression of Przewalski stallions. Although the original distribution of the Przewalski horse is unknown, it was probably much larger than that of the relict population in Mongolia that produced modern Przewalski horses and might even have extended into Central Europe. However, it is also possible that either Przewalski horses were among the initially domesticated horses or that Y-HT-2 occurred both in Przewalski horses and in those wild horses that are the ancestors of domestic horses, based on autosomal DNA data (30). Regardless of how Y-HT-2 entered the domestic gene pool, it was eventually lost, as were all haplotypes except Y-HT-1. In our sample set, Y-HT-2 was undetectable as early as the third time bin. However, it is possible that Y-HT-2 may have been present during this time period, but with a frequency below 0.11 (with 95% probability). The inferred time trajectories for Y-HT-2 frequencies suggest that it could nevertheless have persisted at very low frequencies until the Middle Ages (Fig. 3). On the basis of these simulations, this finding could be interpreted as a relic of this haplotype’s formerly higher frequency in the domestic horse gene pool. It is also possible that the presence of this haplotype could be the result of mating a wild stallion with a domestic mare, a frequently reported breeding practice when wild horses were still widely distributed. However, a significant contribution of the Przewalski horse to the gene pool of modern domestic horses has been almost ruled out by recent genomic studies (13, 31, 32).

horses-y-chromosome-selection
Stallion lineages through time.
Temporal haplotype network of the four detected Y chromosome haplotypes. Age of the samples indicated by multiple layers separated by color; vertical lines connecting the haplotypes of consecutive layers/ages represent which haplotype was transferred into a later/younger period. Numbers constitute the respective number of individuals showing this particular haplotype for that period. Prz, Przewalski; Dom, domestic.

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