The belief systems of these Slavic communities had many affinities with those of neighbouring linguistic populations, such as the Balts, Thracians and Indo-Iranians. Vyacheslav Ivanov and Vladimir Toporov studied the origin of ancient Slavic themes in the common substratum represented by the reconstructed purported Proto-Indo-European religion and what Georges Dumézil defined as the "trifunctional hypothesis". Marija Gimbutas, instead, found Slavic religion to be a clear result of the overlap of supposed Indo-European patriarchism and pre-Indo-European matrifocal beliefs, such proposed duality being a common theme in her studies about so-called "Old Europe" and the Kurgan hypothesis. Boris Rybakov emphasised the continuity and complexification of Slavic religion through the centuries.