on pack animals. Another climate change that occurred after around 3000 led to a more favourable environment allowing for grassland productivity. Yamnaya new pastoral economy then experienced a third wave of rapid demographic expansion, that time towards Central and Northern Europe. Migrations of Usatovo people towards southeastern Poland, crossing through the Old European Tripolye culture from around 3300 BCE, followed by Yamnya migrations towards the Pannonian Basin between 3100 and 2800, are interpreted by some scholars as movements of pre-Italic, pre-Celtic and pre-Germanic speakers. The Proto-Indo-European language probably ceased to be spoken after 2500 as its various dialects had already evolved into non-mutually intelligible languages that began to spread across most of western Eurasia during the third wave of Indo-European migrations (3300–1500). Indo-Iranian languages were introduced to Central Asia, present-day Iran, and South Asia after 2000 BCE. Social structure Class structure It is generally agreed that Proto-Indo-European society was hierarchical, with some form of social ranking and various degrees of social status. It is unlikely however that they had a rigidly stratified structure, or castes such as are found in historical India. There was a general distinction between free persons and slaves, typically prisoners of war or debtors unable to repay a debt. The free part of society was composed of an elite class of priests, kings and warriors, along with the commoners, with each tribe following a chief (*) sponsoring feasts and ceremonies, and immortalized in praise poetry. The presence of kurgan graves prominently decorated with dress, body ornaments and weaponry, along with well-attested roots for concepts such as "wealth" (*), "to be in need" (*) or "servant" (*, "one who moves about on both sides"; and *, "one standing below"), indicate that a hierarchy of wealth and poverty was recognized. Some graves, larger than the average and necessitating a considerable number of people to be built, likewise suggest a higher status given to some individuals. These prestigious funerals were not necessarily reserved to the wealthiest person. Smiths in particular were given sumptuous graves, possibly due to the association of smithery with magic during the early Bronze Age. In general, such graves were mostly occupied by males in the eastern Don-Volga steppes, while they were more egalitarian in the western Dnieper-Donets region. Kinship Linguistics has allowed for the reliable reconstruction of a large number of words relating to kinship relations. These all agree in exhibiting a patriarchal, patrilocal and