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The domestication of the horse, which occurred roughly 5,500 years ago on the open grasslands of Central Asia, transformed human societies more profoundly than almost any other animal ever has. Before the horse was tamed, people moved themselves and their goods on foot or with the help of oxen. Oxen are powerful and can pull heavy loads, but they are slow and tire quickly over long journeys. The horse offered something genuinely new: a combination of speed and endurance that no domesticated animal had previously provided. A single rider could cover in one day a distance that would take a traveler on foot the better part of a week. Archaeologists have long debated exactly when and where horses were first ridden rather than merely hunted for meat. The question is difficult because riding, unlike butchery, leaves few obvious traces in the archaeological record. One important line of evidence comes from the teeth of ancient horses. When a horse is controlled with a bit—a bar placed in the animal's mouth and attached to reins—it habitually chews on the bit, and over time this produces a distinctive pattern of wear on certain teeth. Horse skeletons excavated from sites of the Botai culture, in what is now Kazakhstan, show precisely this kind of wear. To many researchers, the wear indicates that the Botai horses were bridled, and therefore ridden, more than five thousand years ago. A second line of evidence is less direct but equally suggestive. The settlements of horse-using peoples tend to spread across wide territories with remarkable speed. Their distinctive pottery, metal tools, and burial customs appear over enormous distances within a relatively short span of time. Such rapid cultural expansion is difficult to account for without some efficient means of transportation. The horse, many scholars argue, was the engine that drove this expansion, carrying not only people and trade goods but also languages, technologies, and beliefs across the vast open steppe. Some linguists have even connected the spread of an entire family of related languages to the movements of these early horse-riding cultures. Not every researcher is persuaded that riding came first. A minority view holds that horses were initially kept primarily for their milk and meat, and that riding developed only considerably later. These skeptics point out that the tooth-wear evidence, although suggestive, is not decisive: similar wear can occasionally result from a horse's natural chewing behavior rather than from a metal bit. They also note that the remains of corrals and large quantities of horse bones at Botai sites are equally consistent with herding animals for food. Riding, in their view, may have been a secondary development that left the more dramatic mark on history only much later. Whatever the precise sequence, the long-term consequences of the partnership between humans and horses are not in dispute. Once mounted horses became widespread, they reshaped warfare, allowing armies to strike swiftly and retreat before an enemy could respond. They revolutionized trade by linking distant communities that had previously had little contact. And they accelerated the movement of entire peoples across continents. For thousands of years, until the arrival of the railway and the automobile, the horse remained the fastest reliable means of overland travel available to human beings—a single domestication event whose effects rippled through nearly every later chapter of human history.