Short answers, pulled from the story.
Haplogroup R1a (R-M420) is a human Y-chromosome DNA lineage defined by the M420 mutation. It is distributed across a large region of Eurasia, from Scandinavia and Central Europe to Central Asia, Siberia, and South Asia.
The genetic divergence of R1a from its parent haplogroup R1 is estimated to have occurred 15,000 to 25,000 years ago, coinciding with the Last Glacial Maximum. The oldest recovered R1a genome was dated to between 10,785 and 10,626 BCE, from Peschanitsa in Arkhangelsk, Russia.
Haplogroup R1a shows a strong correlation with Indo-European languages across Southern and Western Asia, Central and Eastern Europe, and Scandinavia. Three genetic studies published in 2015 supported the Kurgan theory that R1a expanded from the Pontic-Caspian steppes alongside Indo-European languages. The connection between R1a and Indo-European language spread was first noted by T. Zerjal and colleagues in 1999.
The highest frequency of R1a1a in Europe is found in Sorbs, a West Slavic ethnic group, at 63%, followed by Hungarians at 60%. In Central Asia, Tajiks of Tajikistan and Kyrgyz of Kyrgyzstan both tested at around 63-64%. Among certain Pashtun communities and the Yusufzai tribe, frequencies exceed 50-80%.
The origin of R1a is debated. A 2014 study of over 16,000 individuals found compelling evidence for the Middle East, possibly near present-day Iran, as the geographic origin. Some researchers from 2006 to 2010 proposed South Asian origins based on high STR diversity there, while more recent studies favor multiple waves of migration from a Central Asian source pool into the Indian subcontinent.
All seven Yamnaya samples examined in one major study belonged to the R1b-M269 subclade; no R1a1a was found. This absence has been described by archaeologist Barry Cunliffe as a major weakness in proposals that R1a has a Yamnaya origin, since R1a is prevalent in the Corded Ware culture that is thought to derive largely from Yamnaya ancestry.