The eventual expansion of the use of Slavic languages in the south and east, and Germanic languages in the west, reduced the geographic distribution of Baltic languages to a fraction of the area that they formerly covered. The Russian geneticist Oleg Balanovsky speculated that there is a predominance of the assimilated pre-Slavic substrate in the genetics of East and West Slavic populations, according to him the common genetic structure which contrasts East Slavs and Balts from other populations may suggest that the pre-Slavic substrate of the East Slavs consists most significantly of Baltic-speakers, which predated the Slavs in the cultures of the Eurasian steppe according to archaeological references he cites.