The History of Latvia began around 9000 BC with the end of the last glacial period in northern Europe. Ancient Baltic peoples appeared during the second millennium BC, and four distinct tribal realms in Latvia's territories were identifiable towards the end of the first millennium AD. Latvia's principal river, the Daugava River, was at the head of an important mainland route from the Baltic region through Russia into southern Europe and the Middle East that was used by the Vikings and later Nordic and German traders.