Ancient genomes suggest the eastern Pontic-Caspian steppe as the source of western Iron Age nomads

[Elderberry]

Science Advances  03 Oct 2018

Abstract

For millennia, the Pontic-Caspian steppe was a connector between the Eurasian steppe and Europe. In this scene, multidirectional and sequential movements of different populations may have occurred, including those of the Eurasian steppe nomads. We sequenced 35 genomes (low to medium coverage) of Bronze Age individuals (Srubnaya-Alakulskaya) and Iron Age nomads (Cimmerians, Scythians, and Sarmatians) that represent four distinct cultural entities corresponding to the chronological sequence of cultural complexes in the region. Our results suggest that, despite genetic links among these peoples, no group can be considered a direct ancestor of the subsequent group. The nomadic populations were heterogeneous and carried genetic affinities with populations from several other regions including the Far East and the southern Urals. We found evidence of a stable shared genetic signature, making the eastern Pontic-Caspian steppe a likely source of western nomadic groups.

INTRODUCTION

The Pontic-Caspian steppe (PCS), stretching from the southern Urals to the western North Pontic lands, was the stage of various demographic changes in the past, and several of those remain unknown. During the Bronze and Iron Age, the area was inhabited by a succession of nomadic populations that had significant impact on the cultural development of both Asia and Europe (1, 2). Possibly the best known of these groups is the Yamnaya. Recent genomic studies have revealed cross-continental Early Bronze Age migrations (~3000 BCE) of the nomadic people associated with the Yamnaya horizon (3, 4). The migration introduced the Caucasus genetic component to the genetic landscape of Europe. In Central Europe, Yamnaya ancestry first appeared among people from the Corded Ware complex and has since been found in many subsequent ancient and present-day populations. However, the Pontic-Caspian steppe was critical not only for Early Bronze Age Yamnaya migrations but also because of succeeding movements and population transformations that took place in the developed classical stage of the Late Bronze and Iron Ages between 1800 BCE and 400 CE. This period covered the development of the Srubnaya and Alakulskaya Cultures (~1800–1200 BCE), associated with small settlement sites distributed from the Urals to the Dnieper valley (1). From around 1000 BCE, pre-Scythian nomadic populations started to appear in the western Pontic-Caspian steppe including the Cimmerians known from historical sources (5). Despite regional variation and local peculiarities, the Cimmerians were not associated with any uniform type of archaeological material culture (6). In the seventh century BCE, they were succeeded by the Scythians, who plausibly pushed the Cimmerians into Asia Minor (7). Between 700 and 300 BCE, the Scythians, representing mobile pastoral nomads of a new militaristic type (1), dominated the Pontic-Kazakh steppe, occupying an area from the Altai to the Carpathian Mountains. Their decline began around 300 BCE and was caused by intensifying hostile relations with the Macedonians in the West and the invasion of the Sarmatians from the East. The Sarmatians and the Scythians are thought to have coexisted for a few centuries, but eventually, the former group prevailed (2), resulting in the Scythian downfall. The Sarmatians are believed to comprise a number of groups of similar nomadic background (8), and they became the politically most influential force within the eastern fringes of the Roman Empire at the time. Their decline (~400 CE) was associated with the attack of the Goths and the subsequent invasion of the Huns (8).

The genomic structure of the Bronze and Iron Age (1800 BCE–400 CE) populations in the Pontic-Caspian steppe has not been fully resolved. While earlier genomic studies have suggested close links between the Srubnaya and the central European Late Neolithic and Bronze Age populations (9), our knowledge of the genetic origins of the Cimmerians is limited. Genetic analyses of maternal lineages of Scythians suggest a mixed origin and an east-west admixture gradient across the Eurasian steppe (10–12). The genomics of two early Scythian Aldy-Bel individuals (13) showed genetic affinities to eastern populations of Central Asia (12). However, population interactions and the origin of Scythians of the Pontic-Caspian steppe remain poorly understood. Similarly, little is known about the origins and genetic affinities of the Sarmatians. Genomic studies suggest that the latter group may have been genetically similar to the eastern Yamnaya and Poltavka Bronze Age groups (12). To investigate the demographic dynamics in the Pontic-Caspian steppe, we generated and analyzed genomes of the Late Bronze and Iron Age individuals from the region (Fig. 1, A and B).

RESULTS

We produced genome-wide sequence data with genome coverage between 0.01× and 2.9× per individual for 35 Bronze Age and Iron Age individuals from the Pontic-Caspian steppe from four chronologically sequential cultural groups, which comprise Srubnaya-Alakulskaya individuals (n = 13), Cimmerians (n = 3), Scythians (n = 14), and Sarmatians (n = 5), with radiocarbon dates between ca. 1900 BCE and 400 CE (Fig. 1, A and B; tables S1 to S3; and fig. S1, A and B). All DNA libraries displayed damage patterns typical of ancient DNA (fig. S2) (14). To ensure data integrity, we calculated mitochondrial DNA (mtDNA)–based contamination levels using distribution of private polymorphisms in mtDNA (15) and a Bayesian likelihood method (16). The former yielded point estimates of contamination between 0 and 10% [95% confidence intervals (CIs) between 0 and 17%], and the latter method revealed that all individuals carried sequences with >89% probability of being authentic (table S4). Thus, we included all sequenced individuals in the downstream analyses.

More: http://advances.sciencemag.org/content/4/10/eaat4457