The Scythians: Who Were They?

It is widely accepted that animal domestication (starting ∼5,000 YBP), particularly of the horse, gave the inhabitants of the Central Asian steppes the opportunity to expand geographically in different directions [12]. Historical records and archaeological data indicate that the early populations’ movements across the Eurasian steppe involved Indo-European-speaking people, being most probably in the Altai Mountains the eastern boundary. By the time of the 3rd century BC, Turkic-speaking peoples from the Altai region began to migrate westwards, replacing Indo-European languages in parts of Central Asia [12], [13], [14], [70]. Today, the Altai region is home to numerous Turkic-speaking ethnic groups, with a mixed mtDNA gene pool between West and East Eurasia [71]. However, the origin of this genetic admixture was prior to westward Turkic migrations and may be traced back to the Iron Age in Central Asia, or even earlier in South Siberia [40], [72]. With the present study, we have aimed to unravel the early contacts between European and Asian people in the Altai region by analyzing ancient mtDNA in human remains from Bronze and Iron Age burials of Mongolian Altai.



The high rate of success of the genetic analyses performed in the present study, evidenced by the amplification of both nuclear and mitochondrial DNA, suggests that the DNA of the samples is well preserved. In accordance with other authors [18], this is the expected result in samples that originated in cold and arid environmental conditions such as those found at most of the Altai sites. The agreement between morphological and genetic sex diagnoses, the concordant results obtained using coding and non-coding regions of mtDNA, the high haplotype diversity in sequence motifs, the concordant results between different extracts of the same sample, the clone results and the independent replication of some samples in another ancient DNA laboratory, all guarantee the authenticity of the obtained results [27], [28], [29]. Moreover, in terms of phylogenetic consistency, the observation of 16 different and phylogenetically plausible haplotypes among 19 ancient individuals and the coherence of the phylogenetic networks build further points towards the authenticity of the results [73].



Concerning Bronze Age samples from the Mongolian Altai mountains analyzed in the present study, 100% of the mtDNA lineages (3 different lineages from 2 archaeological sites) belong to East Eurasian haplogroups, an opposite profile to that detected in the West side of the Altai [18], [20]. On the other hand, in the Iron Age samples of Mongolian Altai, the same proportion (50%) of East and Western Eurasian lineages were found, evidencing a perfect admixture between East and Western Eurasian lineages as in other Iron Age populations from central Asia and Siberia [10], [18], [19], [20], [40]. Combined with the previous studies performed so far in the Altai region, our results suggest that the Altai represented a boundary to gene flow up to the beginning of the Iron Age and that during the Scythian period of the Altai (5th to 3rd century BC) there were demographic events in the region that led to a population admixture in both sides of the Altai.



Half of the shared haplotypes between ancient populations from Central Asia and South Siberia represent lineages present in both pre-Iron Age and Iron Age populations and all of these lineages have a west Eurasian origin. Moreover, considering both shared and non-shared haplotypes, it seems that the number of West Eurasian lineages does not increase in the Iron Age. These results allow us to hypothesise that the substrate of mtDNA lineages is already present in pre-Iron Age populations of the central Asia and that in the Iron Age (Scythian period) a population expansion lead to the admixture of pre-existing lineages. Thus, the admixture profile observed in the region during the Iron Age would not derive from a migratory movement from west to east, as has been hypothesised, but would represent a local population expansion in different directions. This population expansion, however, would be probably be a consequence of the introduction of new technology by the adoption of a new culture, supporting the idea of cultural transmission against the demic diffusion during Scythian period.



The Pazyryk groups analysed so far appear to be genetically homogeneous and they did not present significant genetic differences to current Altaians. These results suggest that roots of the current genetic diversity and admixture of the Altai region in Central Asia could be traced back to the Iron Age.

González-Ruiz M, Santos C, Jordana X, Simón M, Lalueza-Fox C, Gigli E, et al. (2012) Tracing the Origin of the East-West Population Admixture in the Altai Region (Central Asia).

The genome data set generated in the study also reveals important lessons for the history of horse management, which started some 5,500 years ago, and animal domestication as a whole. By contrasting patterns of genetic variation in ancient and present-day horses, the authors found support for a significant demographic collapse during the last 2,300 years, which resulted in an important reduction of genetic diversity within horse domesticates. During the same time period, reproductive management has involved an increasingly reduced number of stallions, up to the point that, today, almost all domesticates virtually carry the same, or highly similar, Y-chromosome haplotype(s).

"Many Y-chromosome haplotypes co-existed within Scythian horse populations. The first three millennia of horse domestication thus preserved a large diversity of male lineages. It only vanished within the last 2,000 years," adds Dr. Cristina Gamba, post-doctoral researcher at the Centre for GeoGenetics at the time of the study, and co-leading author of the study.



The authors also found that the demographic collapse and loss of Y-chromosomal diversity observed within the last 2,300 years was mirrored by a significant accumulation of deleterious mutations in the genome of the horse. As these mutations reduce the fitness of their carriers, it shows that the last two millennia of horse management have negatively impacted the horse. However, early domestication stages, as represented by the Sintashta and Scythian genomes, did not have such an impact. This contrasts with the Cost-of-domestication hypothesis, which posits a negative impact starting from early domestication stages. In the case of horse domestication, it is likely that the demographic collapse within the last 2,000 years reduced the efficacy of negative selection to purge out deleterious mutations, which could then accumulate in the horse genome.

The domestication of the horse was a seminal event in human cultural evolution. Librado et al. obtained genome sequences from 14 horses from the Bronze and Iron Ages, about 2000 to 4000 years ago, soon after domestication. They identified variants determining coat color and genes selected during the domestication process. They could also see evidence of admixture with archaic horses and the demography of the domestication process, which included the accumulation of deleterious variants. The horse appears to have undergone a different type of domestication process than animals that were domesticated simply for food.

Abstract
The genomic changes underlying both early and late stages of horse domestication remain largely unknown. We examined the genomes of 14 early domestic horses from the Bronze and Iron Ages, dating to between ~4.1 and 2.3 thousand years before present. We find early domestication selection patterns supporting the neural crest hypothesis, which provides a unified developmental origin for common domestic traits. Within the past 2.3 thousand years, horses lost genetic diversity and archaic DNA tracts introgressed from a now-extinct lineage. They accumulated deleterious mutations later than expected under the cost-of-domestication hypothesis, probably because of breeding from limited numbers of stallions. We also reveal that Iron Age Scythian steppe nomads implemented breeding strategies involving no detectable inbreeding and selection for coat-color variation and robust forelimbs.

Science 28 Apr 2017:
Vol. 356, Issue 6336, pp. 442-445
DOI: 10.1126/science.aam5298