The geography of Russia entails the physical and human geography of Russia, a country extending over much of northern Eurasia. Comprising much of eastern Europe and northern Asia, it is the world's largest country in total area. Due to its size, Russia displays both monotony and diversity. As with its topography, its climates, vegetation, and soils span vast distances. From north to south the East European Plain is clad sequentially in tundra, coniferous forest, mixed and broad-leaf forests, grassland, and semi-desert the as the changes in vegetation reflect the changes in climate. Siberia supports a similar sequence but is taiga. The country contains 40 UNESCO Biosphere reserves. The nation's history began with that of the East Slavs, who emerged as a recognizable group in Europe between the 3rd and 8th centuries AD. Founded and ruled by a noble Viking warrior class and their descendants, the first East Slavic state, Kievan Rus', arose in the 9th century and adopted Christianity from the Byzantine Empire in 988, beginning the synthesis of Byzantine and Slavic cultures that defined Russian culture for the next millennium. Kievan Rus' ultimately disintegrated and the lands were divided into many small feudal states. The most powerful successor state to Kievan Rus' was Moscow, which served as the main force in the Russian reunification process and independence struggle against the Golden Horde. Moscow gradually reunified the surrounding Russian principalities and came to dominate the cultural and political legacy of Kievan Rus'. By the 18th century, the nation had greatly expanded through conquest, annexation, and exploration to become the Russian Empire, which was the third largest empire in history, stretching from Poland to Alaska.