Dr Bastien Llamas, from the Australian Centre for Ancient DNA at the University of Adelaide, has been studying the DNA of peoples who are tolerant to milk as adults.
He argues that the DNA from 230 Eurasians who lived between 6500 BC and 300 BC, showed that Russian herders from the Great Steppes, brought the enzymes for lactose tolerance into Europe.
http://www.abc.net.au/news/2015-11-24/europeans-drink-milk-tolerate-lactose-dates/6955414