Key to the Indo-European language question is India. Indian cattle were domesticated from a different wild species to the European cattle but there are cultural similarities in ancient Hindu writing that suggest that pastoralists spread both east and west taking the milk mutation with them.
Or did they? African pastoralists have lactose tolerance but their mutation is slightly different, so this is a case of convergent evolution. But now in a major genetic study, Professor Mark Thomas and his team have shown that the genetic background of Indians with the lactose tolerance mutation is identical to that of Europeans. This is an enormous boost to the idea that pastoralism and the Indo-European language originated in Turkey or somewhere north of the Black Sea and then spread both East and West.
There has been much argument over whether farming spread by means of cultural transmission or whether it was taken by physical migration of peoples. The Indian study is powerful evidence and it is further backed up by a paper by another team involving Thomas showing that within Europe it was southern populations that took farming to the north rather than merely showing the northerners how to do it. It does rather look as if that single base pair mutation shaped the destiny of about half the human race.
Romero et al, ‘Herders of Indian and European Cattle Share Their Predominant Allelle for Lactase Persistence’, Mol. Biol. Evol. 2012, 29 (1), pp. 249-260.
Skoglund et al, ‘Origins and Genetic Legacy of Neolithic Farmers and Hunter-Gatherers in Europe’, Science, 2012, 336 (No 6080), pp. 466-9.