If you've been following Ancient DNA lately, you know that Allentoft et al came out with results of eastern Bronze Age populations. The data confirm an expansion out of the steppes and likely give credence to a two or three wave Indo European expansion.
Paul Heggarty gives an excellent analysis here, perhaps the best on the web.
This post will focus on the study's implications for I-M26. It is possible that M26 was linked perhaps to the spread of Cardium Pottery, which spread from Italy rapidly: 2,000 km from the gulf of Genoa to the
estuary of the Mondego in probably no more than 100–200 years. This
suggests a seafaring expansion by planting colonies along the coast.