Reconstructing the origin and spread of horse domestication in the Eurasian steppe

Abstract

Despite decades of research across multiple disciplines, the early history of horse domestication remains poorly understood. On the basis of current evidence from archaeology, mitochondrial DNA, and Y-chromosomal sequencing, a number of different domestication scenarios have been proposed, ranging from the spread of domestic horses out of a restricted primary area of domestication to the domestication of numerous distinct wild horse populations. In this paper, we reconstruct both the population genetic structure of the extinct wild progenitor of domestic horses, Equus ferus, and the origin and spread of horse domestication in the Eurasian steppes by fitting a spatially explicit stepping-stone model to genotype data from >300 horses sampled across northern Eurasia. We find strong evidence for an expansion of E. ferus out of eastern Eurasia about 160 kya, likely reflecting the colonization of Eurasia by this species. Our best-fitting scenario further suggests that horse domestication originated in the western part of the Eurasian steppe and that domestic herds were repeatedly restocked with local wild horses as they spread out of this area. By showing that horse domestication was initiated in the western Eurasian steppe and that the spread of domestic herds across Eurasia involved extensive introgression from the wild, the scenario of horse domestication proposed here unites evidence from archaeology, mitochondrial DNA, and Y-chromosomal DNA.

Fig. 1.

Geographic distribution of sampling locations (country and administrative unit) and the model. (A) LTA, Lithuania, Vilnius; UKZ, Ukraine, Zakarpattia; UKL, Ukraine, Lviv; KAL, Russia, Kalmykia; GGA, Georgia, Samegrelo; KSW, Kazakhstan, Mangystau; KAR, Kazakhstan, Kyzylorda; KKA, Kazakhstan, Karagandy; KYR, Kyrgyzstan, Naryn; ALT, Russia, Altai Republic; CHA, China, Xinjiang; MGL, Mongolia, Övörkhangai. (B) Schematic representation of the linear stepping-stone model used in this study (see Materials and Methods for a detailed description of the parameters). Note that for illustrative purposes only five demes are shown. (C) Location of the three putative origins of E. ferus. (D) Locations of the four putative domestication origins.

Fig. 2.

Bayes factors for 12 scenarios describing the origin of the wild progenitor of domestic horses, Equus ferus, and the origin of horse domestication in Eurasia. Bayes factors are relative to the best-supported scenario, which posits an origin of E. ferus in eastern Eurasia and a domestication origin in western central Eurasia. The scenarios are grouped by domestication origin and colored by the origin of E. ferus.

Fig. 3.

Posterior distributions of demographic parameters for wild and domestic populations estimated through ABC analysis. (A) Timing (t) of the expansion of E. ferus out of eastern Eurasia. (B) Effective founder population sizes of the wild horse (cK, red line), the first domestic population (c_d0K_d, blue dashed line), and subsequent domestic populations (c_dK_d, blue solid line). (C) Carrying capacity (effective population sizes) of the ancestral (K₀, dashed red line), wild (K, solid red line), and domestic populations (K_d, blue line). (D) Growth rate, r, of wild horse populations. (E) Migration rates in wild (mK, red line), and domestic populations (m_dK_d, blue line). (F) The proportion of domestic (q) and wild (1 − q) horses in domestic horse founder populations as a function of their effective size, c_dK_d.