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

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... 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. demic spread | microsatellites | demography T he origin of horse domestication has been studied intensively for decades, yet the key question of whether horse domestication originated in a small number of geographically defined areas or whether numerous wild populations were domesticated independently remains unanswered. An increasing body of archaeological evidence points to an origin of horse domestication in the steppes of modern-day Ukraine and Kazakhstan (1) (see ref. 2 for review). However, there is as yet no corroborating molecular evidence for a geographically restricted origin of horse domestication anywhere in the Eurasian steppes, the main distribution area of wild horses at the time of domestication. A related question concerns the spread of horse domestication across the Eurasian steppes: Did the spread of horse domestication involve actual movement of herds ("demic spread"), as appears to have been the case in most other domestic animal species (3)? Or was it primarily the knowledge of horse domestication techniques that spread, thus enabling pastoralist societies throughout the steppes to domesticate locally available wild stock? Whereas a demic spread of small herds of domestic horses out of a single, geographically restricted area has been put forward as one possible explanation for the observed low Y chromosome diversity in modern horses (4), the multiple-origins scenario is commonly invoked to account for the large number of female lineages in the domestic horse gene pool (2, 5, 6). In this paper, we use a spatially and demographically explicit model, parameterized with autosomal genotype data from >300 horses, sampled in 12 localities distributed throughout northern Eurasia (Fig. 1A) , to investigate the origin and spread of horse domestication. The model presented here allows us to distinguish between horse domestication in a single geographic area vs. multiple geographic areas, to pinpoint the geographic origin of domesticated horses in the former case, and to determine the relative roles of demic spread and recruitment of local wild stock in the spread of horse domestication. Results The wild progenitor of domestic horses, Equus ferus, is extinct; we therefore used a stepping-stone dynamic that allowed us to simultaneously reconstruct both the population genetic structure of E. ferus and that of its domestic descendants (see Fig. 1B for a diagrammatic representation of the model). We considered three population origins of E. ferus-western, central, and eastern Eurasia (Fig. 1C )-and combined each of the three wild horse origins with four putative origins of horse domestication (Fig. 1D) , yielding a total of 12 combined scenarios. Depending on the choice of parameters, the model can cover a wide range of possibilities, spanning from populations having undergone range expansions to populations at migration-drift equilibrium. In the latter case, all three putative origins of E. ferus in Eurasia would be equally well supported. In this paper, we define domestication as a process whereby founder populations of domestic stock are established de novo and distinguish it from introgression as a process of restocking already domesticated herds with wild individuals. For each of the 12 combined scenarios, we investigated a variety of population dynamics in both wild and domestic horse populations, as well as the full spectrum of possible modes of spread of domestication, ranging from a purely demic spread without wild horse introgression to numerous local domestications without population movements. In the latter case, all four putative