Single dispersal of modern humans to Eurasia

In a typical ancient DNA study where the number of authors exceeds the number of specimens (actually, equals this time), Cosimo Posth and colleagues sequenced 35 pre-Neolithic modern humans from Europe.
By sequencing 35 mitochondrial DNA (mtDNA) genomes, Posth et al. tripled the currently available dataset of hunter-gatherers spanning in age from 35 ka (thousand years) to 7 ka, which covers most of time that hunther-gatherers were present in Europe.
The researchers combined their data with previously published ancient and modern mtDNAs and thanks to the improved sample size discovered that both mtDNA haplogroups (M and N) found in present-day non-Africans were also represented in the hunter-gatherers of Pleistocene Europe.
This finding is surprising because haplogroup M was previously observed in modern Asians, Australasians, and Native Americans, but not in Europeans. It was considered as evidence of two separate dispersal events outside of Africa (Maca-Meyer et al. 2001).
The fact that both haplogroups existed in different parts of the world supports an alternative scenario of a single dispersal of modern humans to Eurasia. Samples with known radiocarbon or stratigraphic age were used to calculate the timing of the dispersal and the most recent common ancestor of haplogroups M and N was dated to time period between 44 and 55 ka. Together with the archeological evidence of modern human presence in Europe about 45 ka (Benazzi et al. 2011), data suggest that hunter-gatherers spread from Africa to Eurasia in a single, rapid event.

The haplogroup M disappeared from Europe during the Last Glacial Maximum (LGM, 25 to 19.5 ka), which indicates a genetic bottleneck caused by the expanding ice sheets. Interestingly, the climatic changes at the end of Pleistocene had also another effect on modern human population structure. According to the mtDNA, there was a population replacement about 14.5 ka when temperatures increased for a short time period.
References:
Posth et al. (2016) Pleistocene mitochondrial genomes suggest a single major dispersal of non-Africans and a Late Glacial turnover in Europe. Current Biology, published online on February 4, 2016. DOI: http://dx.doi.org/10.1016/j.cub.2016.01.037
Benazzi et al. (2011) Early dispersal of modern humans in Europe and implications for Neanderthal behaviour. Nature 479, 525-528. DOI:10.1038/nature10617
Maca-Meyer et al. (2001) Major genomic mitochondrial lineages delineate early human expansions. BMC Genetics 2. DOI: 10.1186/1471-2156-2-13