According to authors:
(quote)
The elevated frequency and high diversity of I2a1b-M423 lineages among different SEE populations shows a genetic signature of their common paternal history over a long period of time. The PCA plot clearly shows a SEE cluster comprised of Bosnian and Herzegovinian, Serbian and Croatian samples based predominantly on the I2a1b-M423 component. As shown in the I2a1b-M423 spatial gradient map, a clear cline of this clade is evident inside Europe, spreading from the area of Southern Croatia and Bosnia and Herzegovina mostly northward and eastward. Although the initial STR-based time estimate for this clade gave support for a Upper Paleolithic origin (Pericˇic´ et al., 2005), new studies based on whole Y chromosome sequencing suggest a somewhat younger age of this clade, between 5 and 7.5 kya (Batini et al., 2015; Karmin et al., 2015). In addition, the high haplotype diversity of this lineage in Croatia reveals its significant expansion only after the adoption of agriculture by Mesolithic hunter-gatherers in SEE (Battaglia et al., 2009).
More evidence of its autochthonous European origin also came recently from European aDNA studies. Lazaridis et al. (2014) sequenced nine ancient genomes (with age estimates of cca 8 kya) and analyzed the complete NRY sequence of five male individuals (one from Luxemburg and four from Sweden). Their results showed all five of them belonged to the I haplogroup. The authors warned that, at present, the limited number of ancient samples for which Y chromosome data are available makes it difficult to assess how statistically surprising it is that this NRY haplogroup occurs in all five of the ancient Mesolithic males but in only a quarter of present-day males from that geographic area. They appear to argue that the haplogroup I today is found in a wider European area at a 6 J. SARAC ET AL. American Journal of Human Biology much lower frequency than it occurred around 8 kya. This haplogroup has also been found and described in a Scandinavian Neolithic hunter-gatherer from Sweden (Skoglund et al., 2014), as well as in Neolithic remains from southern France and northern Spain (Lacan et al., 2011a). The fact that a significant portion of investigated Mesolithic males belonged to haplogroup I suggests that this paternal lineage might represent a major pre-Neolithic European clade, and the results obtained in this study support this hypothesis.
(end of quote)