Short answers, pulled from the story.
The Pontic-Caspian steppe served as the crucible for a genetic merger around 5000 BC. Archaeogeneticists identified this initial formation in individuals from the Khvalynsk II cemetery and the Progress 2 archaeological site north of the Caucasus mountains.
Corded Ware culture members possessed approximately 75% Western Steppe Herder ancestry during their expansion period. This high level of ancestry accompanied their movement across Eurasia alongside Indo-European languages around 3000 BC.
The Sintashta culture emerged in the southern Urals between 2100 and 1800 BC as a result of back migration onto the steppe. Eastern Corded Ware people remigrated eastward to form this new group along with the Andronovo and Srubnaya cultures.
Later groups like the Sintashta and Andronovo cultures saw sharp increases in blue eye frequencies ranging from 30.8% to 43.5%. Blond hair also rose significantly among these populations to between 34.8% and 38.5% due to constant selection pressure over millennia.
Northern Europeans such as Irish people, Norwegians, Swedes, Belarusians, Lithuanians, Scots, and Icelanders show peaks in this ancestry component. Contemporary European populations model their ancestry as a mixture of Western Hunter-Gatherers, Early European Farmers, and Western Steppe Herders.