A study of ancient horse genomes reveals the genetic changes that contributed to making the animals tame, strong, and rideable by humans thousands of years ago. The domestication of horses, which occurred at least 4,500 years ago, had a transformative effect on the evolution of human society, altering mobility, farming, and warfare. Across much of the world, horses served as a primary mode of human transportation until the rise of the combustion engine in the late 19th century. However, despite considerable advances in the genomic sequencing of ancient horses, the full suite of biological traits that reshaped the species during domestication, including those that facilitated human equestrianism, remains unknown. Here, Xuexue Liu and colleagues present an extensive horse genome time series analysis of 266 trait-associated genetic markers and show how human selection shaped horse biology over time. According to Liu et al., early domestication favored behavioral traits. Traits like coat color expanded only modestly at first. From the Iron Age onward, breeding practices increasingly emphasized larger body size and tameness. Especially pivotal was the intensive selection at the GSDMC locus, which showed one of the strongest signals of positive selection during the rise of the modern domesticated horse lineage ~4,200 years ago. The GSDMC gene, tied to skeletal conformation in horses and spinal anatomy, motor coordination, and strength in other mammals, likely enabled horses to meet the demands of large-scale mobility and warfare, fundamentally reshaping human societies. “Although the precise circumstances and the cultural identity of the people responsible for this early, intensive breeding remain a mystery, they must have had the necessary ingenuity, technology, and foresight,” writes Laurent Frantz in a related Perspective. “What is certain is that these first riders kick-started a revolution that changed the world, demonstrating how the immense currents of history can turn on the smallest of biological changes.”
