The ethnic group that came to be known as the Russians sprang from the East Slavs, one of the three groups into which the original Slavic people divided sometime before the seventh century A.D. The West Slavs eventually became differentiated as the Poles, Czechs, and Slovaks; the South Slavs divided into the Bulgarians, Croats, Serbs, and Slovenes. The East Slavic tribes settled along the Dnepr River in present-day Ukraine in the first centuries A.D. From that region, they then spread northward and eastward. In the ninth century, these tribes constituted the largest part of the population of Kievan Rus', the medieval state ruled by a Varangian dynasty from Scandinavia (see The East Slavs and the Varangians, ch. 1).
The East Slavs became more politically united in the tenth century when they adopted Christianity as the state religion of Kievan Rus'. Nevertheless, tribal and regional differences were exacerbated in subsequent centuries as the state expanded, bringing the East Slavs into contact with other ethnic groups on their borders. Thus, Baltic and Finno-Ugric tribes mixed with the East Slavs to the northwest and the northeast, respectively. By the time the state of Kievan Rus' began disintegrating into independent principalities in the twelfth century, the East Slavs had begun to evolve into three peoples with distinct linguistic and cultural characteristics: the Russians to the north and northeast of Kiev, the Belorussians to the northwest of Kiev, and the Ukrainians in the Kiev region and to its south and southwest. In the thirteenth century, the invasion of the Mongols brought the final collapse of Kievan Rus' as a political entity, accelerating differentiation and consolidation of the three ethnic groups (see The Golden Age of Kiev, ch. 1). Although the three groups remained related culturally, linguistically, and religiously, each of them also was influenced by different political, economic, religious, and social developments that further separated them.
Building a state of increasing vitality as the Mongol occupation weakened in the fourteenth century, the principality of Muscovy became the base from which the Russian cultural and political systems expanded under a series of strong rulers. By the end of the nineteenth century, Russians had settled the remote stretches of Siberia to the Pacific Ocean and colonized Central Asia and the Caucasus, becoming in the process the most numerous and ubiquitous of the Slavic peoples (see Ruling the Empire, ch. 1).