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R-M420 (R1a) is a common Y DNA haplogroup in many parts of Eurasia. One sub-clade (branch) of R-M420, R-M17 (R1a1a), is much more common than the others in all major geographical regions. R-M17, defined by the SNP mutation M17, (and sometimes alternatively defined as R-M198), is particularly common in a large region extending from South Asia and Southern Siberia to Central Europe and Scandinavia.
Essentially, the time frame and precise reason for distibution is still debated. Should be noted that statistical analysis on modern population samples are not sure fire ways of dating it. One can choose Caoloscence Theory or Zhivotovsky method for trying to date it in a given region and end up with two different cultural contexts and dates for its appearance.