Behind a paywall, but an interesting theory. "In Europe and Asia about 5,000 to 4,200 years ago, people were probably domesticating horses. (Genetics show) they came from short horses that roamed North American grasslands and crossed over the Bering land bridge..."
AnimalsAbout 4,200 years ago, horseback riding allowed people to travel farther and faster than ever before, spurring migration throughout Europe and Asia.
https://www.nationalgeographic.com/animals/article/do-we-finally-know-where-horses-evolved[https://www.nationalgeographic.com/animals/article/do-we-finally-know-where-horses-evolved?cmpid=org%3Dngp%3A%3Amc%3Dcrm-email%3A%3Asrc%3Dngp%3A%3Acmp%3Deditorial%3A%3Aadd%3DAnimals_20211021%3A%3Arid%3D%24%7BProfile.CustomerKey%7D&loggedin=true]