How an empire of steppe nomads coped with environmental stress


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.

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, posted by Spencer Wells.


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.

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).

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).

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.