For millennia, horses shaped human destiny, powering travel, farming, warfare, and empire. But exactly how wild steppe animals became the rideable companions that carried warriors and kings has long been a mystery. Now, an unprecedented ancient DNA study points to a single genetic turning point that transformed both horse and human history.⁠ ⁠ Researchers analyzing hundreds of horse genomes discovered that around 4,200 years ago, a rare mutation in a gene called GSDMC spread like wildfire across Eurasia. This variant reshaped vertebrae, boosted coordination, and strengthened limbs, making horses far more suited for riding. Within just a few centuries, the frequency of this mutation leapt from 1 percent to nearly universal, an evolutionary surge faster than even the human gene for lactose tolerance.⁠ ⁠ Earlier domestication attempts, such as those by the Botaï people in Kazakhstan 5,500 years ago, faltered. Their horses eventually returned to the wild. But when breeders on the Pontic-Ca...