The book explores the origins of Indo-European languages (now spoken by three billion people) in the context of the domestication of the horse and invention of the wheel. The relevant archaeological evidence for the early origins and spread of the Indo-European languages is examined, giving support to a version of the Kurgan hypothesis. A key insight is that early expansions of the area in which Indo-European was spoken were often due to "recruitment" (originally to a way of life in which intensive use of horses allowed herd animals to be pastured in areas of the Ukrainian / South Russian steppe outside of river valleys), rather than due only to military invasions. According to Anthony's researches, the earliest effective domestication of horses occurred in approximately the same period and geographical area where the Indo-European languages started to spread. The splitting off of the major branches of Indo-European (except perhaps Greek) can be correlated with archaeological cultures showing steppe influences, in a way that makes sense chronologically and geographically in light of linguistic reconstructions.