is itself a varied cultural horizon, spanning the entire Pontic–Caspian steppe from the mid-4th to the 3rd millennium. Usatovo culture (late 4th millennium) Stages of culture and expansion Gimbutas's original suggestion identifies four successive stages of the Kurgan culture: Kurgan I, Dnieper/Volga region, earlier half of the 4th millennium BC. Apparently evolving from cultures of the Volga basin, subgroups include the Samara and Seroglazovo cultures. Kurgan II–III, latter half of the 4th millennium BC. Stone circles, anthropomorphic stone stelae of deities. Includes the Sredny Stog culture and the Maykop culture of the northern Caucasus. Kurgan IV or Pit Grave (Yamnaya) culture, first half of the 3rd millennium BC, encompassing the entire steppe region from the Ural to Romania. In other publications she proposes three successive "waves" of expansion: Wave 1, predating Kurgan I, expansion from the lower Volga to the Dnieper, leading to coexistence of Kurgan I and the Cucuteni–Trypillia culture. Repercussions of the migrations extend as far as the Balkans and along the Danube to the Vinča culture in Serbia and Lengyel culture in Hungary. Wave 2, mid 4th millennium BC, originating in the Maykop culture and resulting in advances of "kurganized" hybrid cultures into northern Europe around 3000 BC (Globular Amphora culture, Baden culture, and ultimately Corded Ware culture). According to Gimbutas this corresponds to the first intrusion of Indo-European languages into western and northern Europe. Wave 3, 3000–2800 BC, expansion of the Pit Grave culture beyond the steppes, with the appearance of the characteristic pit graves as far as modern Romania, Bulgaria, eastern Hungary and Georgia, coincident with the end of the Cucuteni–Trypillia culture and Trialeti culture in Georgia (c. 2750 BC). Timeline 4500–4000: Early PIE. Sredny Stog, Dnieper–Donets and Samara cultures, domestication of the horse (Wave 1). 4000–3500: The Pit Grave culture (a.k.a. Yamnaya culture), the prototypical kurgan builders, emerges in the steppe, and the Maykop culture in the northern Caucasus. Indo-Hittite models postulate the separation of Proto-Anatolian before this time. 3500–3000: Middle PIE. The Pit Grave culture is at its peak, representing the classical reconstructed Proto-Indo-European society with stone idols, predominantly practicing animal husbandry in permanent settlements protected by hillforts, subsisting on agriculture, and fishing along rivers. Contact of the Pit Grave culture with late Neolithic Europe cultures results in the "kurganized" Globular Amphora and Baden cultures (Wave 2). The Maykop culture shows the earliest evidence of the beginning Bronze Age, and Bronze weapons and