The author of the article linked in post #2 above asks, "What conferred advantage to these post Neolithic men? What was that advantage?"
Here is something from the book, Gaelic and Gaelicised Ireland during the Middle Ages
, by Kenneth Nicholls, that could help answer that question. It was first posted by Paul Duffy in the L21 subforum here. Although it deals with the top-down expansion in medieval Ireland, I think the same principle could easily (and probably accurately) be applied to the rapid expansion of R1b in Europe 4k-5k years ago.
One of the most important phenomena in a clan-based society is that of expansion from the top downwards. The seventeenth-century Irish scholar and genealogist Dualtagh Mac Firbisigh remarked that 'as the sons and families of the rulers multiplied, so their subjects and followers were squeezed out and withered away'; and this phenomenon, the expansion of the ruling or dominant stocks at the expense of the remainder, is a normal feature in societies of this type. It has been observed of the modern Basotho of South Africa that 'there is a constant displacement of commoners by royals [i.e. members of the royal clan] and of collateral royals by the direct descendants of the ruling prince', and this could have been said, without adaptation, of any important Gaelic or Gaelicized lordship of late medieval Ireland. In Fermanagh, for example, the kingship of the Maguires began only with the accession of Donn Mór in 1282 and the ramification of the family - with the exception of one or two small and territorially unimportant septs - began with the sons of the same man. The spread of his descendants can be seen by the genealogical tract called Geinelaighe Fhearmanach; by 1607 they must have been in the possession of at least three-quarters of the total soil of Fermanagh, having displaced or reduced the clans which had previously held it. The rate at which an Irish clan could multiply itself must not be underestimated. Turlough an fhíona O'Donnell, lord of Tirconnell (d. 1423) had eighteen sons (by ten different women) and fifty-nine grandsons in the male line. Mulmora O'Reilly, the lord of East Brefny, who died in 1566, had at least fifty-eight O'Reilly grandsons. Philip Maguire, lord of Fermanagh (d. 1395) had twenty sons by eight mothers, and we know of at least fifty grandsons. Oliver Burke of Tirawley (two of whose sons became Lower Mac William although he himself had never held that position) left at least thirty-eight grandsons in the male line. Irish law drew no distinction in matters of inheritance between the legitimate and the illegitimate and permitted the affiliation of children by their mother's declaration (see Chapter 4), and the general sexual permissiveness of medieval Irish society must have allowed a rate of multiplication approaching that which is permitted by the polygyny practised in, for instance, the clan societies of southern Africa already cited.