Ethnically the Slavs possess little unity, for they have mixed for centuries with other peoples, including Turko-Tatars, Finnic peoples, Germans, Mongols, Greeks, and Illyrian tribes. The Bulgarians are not of Slavic origin. The obscure beginnings of the Slavs have given rise to several theories, all of which include as a possible place of origin the area of the Polesie marshes in Galicia. The ancestors of the Slavs were Neolithic tribes who occupied this territory a few centuries before the Christian era, and the similarities of these Proto-Slavs to the Proto-Balts has led to a theory of a Proto-Baltic Slav period (see Balts). There was presumably no one Proto-Slavic language, but rather a blending of tribal dialects that emerged as differentiated Slavic languages; the Slavs' unifying medium is today chiefly that of language.