NEW YORK – An international team of researchers has pinpointed the Western Eurasian steppes as the homeland of modern domestic horses and has found that human use of horses for equestrian purposes coincided with strong selection for critical locomotor and behavioral adaptations in the horse GSDMC and ZFPM1 genes.
In a paper published on Wednesday in Nature, the researchers said they mapped the population changes accompanying domestication from 273 ancient horse genomes gathered at all suspected domestication centers, including Iberia, Anatolia, and the steppes of Western Eurasia and Central Asia. This analysis revealed that modern domestic horses ultimately replaced almost all other local populations as they expanded rapidly across Eurasia from about 2000 BC. This was synchronous with equestrian material culture, including Sintashta spoke-wheeled chariots.
The researchers said their findings also ran counter to the commonly held association between horseback riding and the massive expansion of Yamnaya steppe pastoralists into Europe around 3000 BC, which drove the spread of Indo-European languages.
"This contrasts with the scenario in Asia where Indo-Iranian languages, chariots, and horses spread together, following the early second millennium BC Sintashta culture," the authors wrote.
The new genomic analyses identified four horse lineage clusters: one from the Late Pleistocene to the late fourth millennium BC in northeastern Siberia; one from the Late Pleistocene in Romania, Belgium, France, Britain, and the region from Spain to Scandinavia and Hungary, Czechia, and Poland during the sixth-to-third millennium BC; one from the earliest known domestic horses from Botai and Przewalski's horses (a type of horse from Mongolia), which extended to the Altai and Southern Urals during the fifth-to-third millennium BC; and one that comprised modern domestic horses clustered within a group that became geographically widespread and prominent following about 2200 BC and during the second millennium BC.
This cluster appeared genetically close to horses that lived in the Western Eurasia steppes but not further west than the Romanian lower Danube, south of the Carpathians, before and during the third millennium BC, the researchers said.
Further genetic and admixture analyses eliminated the possibility that ancestors of modern domestic horses came from further west than the western lower Volga-Don region and the Dnieper steppes. Furthermore, patterns of spatial autocorrelations in the genetic data indicated the Western Eurasia steppes as the most likely geographic location of modern horse ancestors.
Subsequent genomic analyses uncovered a high turnover of the horse population, in which past breeders produced large stocks of modern domestic horses to supply increasing demands for horse-based mobility from around 2200 BC, the researchers said. Notably, the genetic profile of these horses was ubiquitous among animals buried in Sintashta kurgans together with the earliest spoke-wheeled chariots around 2000 to 1800 BC.
Because human-induced domestic horse dispersal may have involved selection of phenotypic characteristics linked to horseback riding and chariotry, the researchers then screened their data for genetic variants that are overrepresented in modern domestic horses from the late third millennium BC. They found one outstanding locus immediately upstream of the GSDMC gene — in humans, there are variants in GSDMC that are associated with chronic back pain and lumbar spinal stenosis.
The second most differentiated locus extended over approximately 16 Mb on chromosome 3, with the ZFPM1 gene being closest to the selection peak. ZFPM1 is essential for the development of dorsal raphe serotonergic neurons involved in mood regulation and aggressive behavior, and inactivation of the gene in mice causes anxiety disorders and contextual fear memory.
"Combined, early selection at GSDMC and ZFPM1 suggests shifting use toward horses that were more docile, more resilient to stress, and involved in new locomotor exercise, including endurance running, weight bearing and/or warfare," the authors wrote.
Overall, they said, the study resolved longstanding debates about the origins and spread of domestic horses, pinpointing their ancestry to the Western Eurasia steppes in the late fourth and early third millennia BC, and showing no evidence that they facilitated the expansion of the human genetic steppe ancestry into Europe as previously thought.
"Instead of horse-mounted warfare, declining populations during the European late Neolithic may thus have opened up an opportunity for a westward expansion of steppe pastoralists," the authors concluded. "The results thus open up new research avenues into the historical developments of these different societal trajectories."