Neolithic expansions: how the European foragers were assimilated

... So, in short: U experienced after the end of the last Ice Age, as European hunters found new room to expand in space, and economic opportunities to expand in population size. By 10,000 yBP they had maximized their population size. Subsequently, incoming Neolithic H-bearers started expanding as they arrived in Europe and started growing in numbers and filling up the landscape. At some point, (c. 4ky BP) all the hunter-gatherers had been assimilated, and the composite gene pool consisted of the descendants of mainly the Neolithic farmers and a minority of assimilated hunter-gatherers. Since H and U were no longer tied to different subsistence strategies, they underwent expansion in similar fashion.
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PLoS ONE 7(3): e32473. doi:10.1371/journal.pone.0032473
Complete Mitochondrial Genomes Reveal Neolithic Expansion into Europe
Qiaomei Fu, Pavao Rudan, Svante Pääbo, Johannes Krause

Neolithic expansions: how the European foragers were assimilated