The very term "Samogitians" is a Latinized form of the ancient Lithuanian name for the region's lowlanders, who dwelt in Central Lithuania's lowlands. The original subethnic Samogitia, i.e. Central Lithuania's flat burial grounds culture, was formed as early as the 5th–6th centuries. Before that, it was inhabited by southern Semigallians and southern Curonians. The western part of historical Žemaitija became ethnically Lithuanian between the 13th and 16th centuries. The primal eastern boundary of historical Samogitia was the Šventoji River (a tributary of the Neris River) since the end of the 13th century (at about that time, the Lithuanian ruler Vytenis had expanded the territory of his domain in Aukštaitija along the Nevėžis River at the expense of Žemaitija).