A theory which seems plausible is that which assumes them to have been a heterogenous collection of Mongol, Tungus, Turki and perhaps even Finnish hordes under a Mongol military caste, though the Mongolo-Tungus element probably predominated.
In order to study the different eastern kinsfolk of the Finns, Sjogren (1792-1855) extended his journeys to North Russia, and Castren to West and East Siberia (Nordische Reisen and Forschungen), and collected the materials which permitted himself and Schiefner to publish grammatical works relative to the Finnish, Lappish, Zyrian, Tcheremiss, Ostiak, Samoyede, Tungus, Buryat, Karagas, Yenisei-Ostiak and Kott languages.