The features of the earliest climate aridization and the onset of the arid open landscapes in the Transbaikalian region are shown by the early Early Pleistocene mammalian faunas of the Itantsinian Complex, and by geological data. The climate deterioration intensified through the Middle and Late Pleistocene. The uplift of mountains surrounding Lake Baikal is considered to be the main cause for the deep aridization of the Western Transbaikalia, because they became the major orographic barrier preventing humid West Atlantic cyclones from reaching the Transbaikalian area. Comparative analyses of successive Early, Middle and Late Pleistocene faunas have shown that they differ significantly in species composition and in stages of the evolution of taxa belonging to different lineages. However, the dominant species of faunas demonstrate similar evolutionary trends, such as the development of complex-shaped teeth structure due to adaptation to feeding on coarse grass and to inhabit arid landscapes.