Proto-Indo-European society is the reconstructed culture of Proto-Indo-Europeans, the ancient speakers of the Proto-Indo-European language, ancestor of all modern Indo-European languages. Historical linguistics combined with archaeological and genetic evidence have provided the current basis for understanding the culture and its people. The most widely accepted theory suggests that the culture emerged on the Pontic-Caspian steppe after 5000 BCE, a period known as the Chalcolithic where smelted copper and stone tools were in use simultaneously. Proto-Indo-European speakers are considered to have been semi-nomadic, but domestication of cattle, first for ritual sacrifice and later for consumption, dairy production, and cereal cultivation emerged as the culture shifted from herding and hunter-gatherer to farming. The social hierarchy included an upper class of priests, warriors and tribe chiefs, and a lower class of commoners and slaves; patrilineality and patriarchy characteristics have been well-established. Trade, bolstered by access to wheeled wagons, connected Proto-Indo-European culture to others with archaeological and linguistic evidence supporting relations with Proto-Uralic peoples, Uruk, and Old European cultures.