The review summarizes the faunal, molecular, and palaeogeographic data that make it possible to discuss on where and when the aquatic animals could have crossed the drainage divide between the Caspian Sea and Arctic Ocean basins. The freshwater faunas of Europe and northern Asia were largely similar in the Pliocene, while the Pliocene Siberian fauna went almost totally extinct during the Pleistocene glacial periods. In interglacial periods, cold-water freshwater organisms colonized the Volga River basin, most likely via dispersal events from both the Pechora and Ob river basins. Marine animals might have found their way into the Caspian Sea with saline waters arriving from the Arctic Ocean in the late Pliocene and early Pleistocene. There was probably no possibility to cross the drainage divide during the last glacial period. After the end of the period, freshwater organisms, including mostly thermophilic groups, entered the Onega, Northern Dvina, and Ob basins as the drainage divide has shifted to another location. Similar expansions occur now due to human-mediated introduction and dispersal events through the channels connecting the Volga and Northern Dvina rivers.