By sequencing 523 ancient humans, we show that the primary source of ancestry in modern South Asians is a prehistoric genetic gradient between people related to early hunter-gatherers of Iran and Southeast Asia. After the Indus Valley Civilization’s decline, its people mixed with individuals in the southeast to form one of the two main ancestral populations of South Asia, whose direct descendants live in southern India. Simultaneously, they mixed with descendants of Steppe pastoralists who, starting around 4000 years ago, spread via Central Asia to form the other main ancestral population. The Steppe ancestry in South Asia has the same profile as that in Bronze Age Eastern Europe, tracking a movement of people that affected both regions and that likely spread the distinctive features shared between Indo-Iranian and Balto-Slavic languages.

The natural environment and prehistoric human activity in the Holocene floodplains of the Low Volga River and in the southern Urals are important research objects in geomorphology, soil science and archaeology. The alternating sequences of soil-alluvium sequences represent a sedimentary archive with chrono-stratigraphic records of human land use, sediment accumulation and soil formation. The central floodplain of the Derkul River (western Kazakhstan) was studied using the multiproxy approach to investigate the soil-alluvium sequence dating from 8000 years ago until the present and containing a buried Stagnic Fluvic Phaeozem. Alluvial deposition began with stream sedimentation in the early Holocene, followed by a prolonged period of soil formation under low water conditions (7.5–5.7 ka cal year BP). Humans started habitation the floodplain in 6.6–5.7 ka cal year BP. Increased atmospheric precipitation in 5.7–3.4 ka cal year BP accelerated alluvial sedimentation. Soil formation followed the synsedimentation model. Conditions for the stationary land use by humans in the floodplain were less optimal. In 3.4–2.1 ka cal year BP, alluvial sedimentation was less pronounced, and solonetz carbonated soils were formed, reflecting increased climate aridity and continentality. Humans returned to the floodplain area, but in 2.1–1.9 ka cal year BP, the flooding frequency increased, and in 1.9 ka cal year BP, the surface of the floodplain passes to function in a high floodplain. Thus, synsedimentation formation resumed, with colluvium discharge from the adjacent hills being the main source of material input.