R1a is thought to have been the dominant haplogroup among the northern and eastern Indo-European speakers who evolved into the Indo-Iranian, Mycenaean Greek, Macedonian, Thracian, Baltic and Slavic branches. The Proto-Indo-Europeans originated in the Yamna culture (3300-2500 BCE), in the Pontic-Caspian steppe between modern Ukraine and south-west Russia. Their expansion is linked to the domestication of horses in the Eurasian steppes, and the invention of the chariot (see R1b above).

The eastern part of the Pontic-Caspian steppes is strongly associated with the Indo-Iranian and Balto-Slavic branches of Indo-European languages. Based on archeological, linguistic and genetic data, it is possible to say that the pastoralist nomads who lived in the northern Russian steppes and forest-steppes 5,000 years ago carried predominantly R1a paternal lineages.

Nowadays, high frequencies of R1a are found in Poland (56% of the population), Ukraine (50 to 65%), European Russia (45 to 65%), Belarus (45%), Slovakia (40%), Latvia (40%), Lithuania (38%), the Czech Republic (34%), Hungary (32%), Croatia (29%), Norway (28%), Austria (26%), Sweden (24%), north-east Germany (23%) and Romania (22%).