A summary of some of the pertinent research is found in today's New York Times:
http://www.nytimes.com/2015/06/16/science/dna-deciphers-roots-of-modern-europeans.html?hpw&rref=science&action=click&pgtype=Homepage&module=well-region®ion=bottom-well&WT.nav=bottom-well&_r=0
Matthew Kapstein
Directeur d'études,
Ecole Pratique des Hautes Etudes
Numata Visiting Pro
fessor of Buddhist Studies,
The University of Chicago