Haplogroup R1a, or haplogroup R-M420, is a human Y-chromosome DNA haplogroup which is distributed in a large region in Eurasia, extending from Scandinavia and Central Europe to Central Asia, southern Siberia and South Asia.
While one genetic study indicates that R1a originated 25,000 years ago, its subclade M417 (R1a1a1) diversified c. 5,800 years ago. The place of origin of the subclade plays a role in the debate about the origins of Proto-Indo-Europeans.
The SNP mutation R-M420 was discovered after R-M17 (R1a1a), which resulted in a reorganization of the lineage in particular establishing a new paragroup (designated R-M420*) for the relatively rare lineages which are not in the R-SRY10831.2 (R1a1) branch leading to R-M17.
The genetic divergence of R1a (M420) is estimated to have occurred 25,000 years ago, which is the time of the last glacial maximum. A 2014 study by Peter A. Underhill et al., using 16,244 individuals from over 126 populations from across Eurasia, concluded that there was "a compelling case for the Middle East, possibly near present-day Iran, as the geographic origin of hg R1a". The ancient DNA record has shown the first R1a during the Mesolithic in Eastern Hunter-Gatherers (from Eastern Europe), and the earliest case of R* among Upper Paleolithic Ancient North Eurasians, from which the Eastern Hunter-Gatherers predominantly derive their ancestry.
According to , the downstream R1a-M417 subclade diversified into Z282 and Z93 circa 5,800 years ago "in the vicinity of Iran and Eastern Turkey". Even though R1a occurs as a Y-chromosome haplogroup among various languages such as Slavic and Indo-Iranian, the question of the origins of R1a1a is relevant to the ongoing debate concerning the urheimat of the Proto-Indo-European people, and may also be relevant to the origins of the Indus Valley civilization. R1a shows a strong correlation with Indo-European languages of Southern and Western Asia, Central and Eastern Europe and to some extent Scandinavia being most prevalent in Eastern Europe, West Asia, and South Asia.