From 5500 to 5000 BC, copper started to be used in the Balkans, and Eastern and Central Europe. However, the key factor could be the use of horses, which would increase mobility. From 5500 BC onwards, Eastern Europe was apparently infiltrated by people originating from beyond the Volga (Yamna culture), creating a plural complex known as Sredny Stog culture, which substituted the previous Dnieper-Donets culture, pushing the natives to migrate northwest to the Baltic and to Denmark, where they mixed with the natives (TRBK A and C). That may be correlated with the linguistic fact of the spread of Indo-European languages, according to the Kurgan hypothesis. Near the end of the period, another branch left many traces in the lower Danube area (culture of Cernavodă I) in what seems to be another invasion.