Up to now, the dispersal of modern humans outside of Africa is a highly debated topic both in terms of the number of major expansions and their timing. An international team of researchers retrieved DNA from 35 ancient hunter-gatherers spanning almost 30,000 years of European pre-history. They obtained unexpected insights into early modern human population dynamics, finding a single and rapid dispersal of all non-Africans around 50,000 years ago, not only across Asia but also into Europe. They furthermore uncovered a previously unknown population turnover at the end of the last Ice Age, a time of severe climatic instability around 14,500 years ago.
Genetic information about early modern humans, who lived for 35-40,000 years as hunter-gatherers in Europe, is scarce and their population structure and dynamics are almost unknown. Using molecular and bioinformatic techniques researchers from the Max Planck Institute (MPI) for the Science of Human History in Jena and Tübingen University in Germany, together with an international team of collaborators, were able to reconstruct complete maternally inherited mitochondrial DNA (mtDNA) genomes of 35 hunter-gatherers who lived in Italy, Germany, Belgium, France, Czech Republic and Romania between 35,000 and 7,000 years ago.
Analysis of these ancient mtDNAs unexpectedly revealed that three individuals from before the Last Glacial Maximum (LGM) – the coldest period in the last Ice Age – that were excavated in present-day Belgium and France belong to a type of mtDNA called haplogroup M. “I couldn’t believe it. The first time I got this result I thought it must be a mistake, because in contemporary Europeans haplogroup M is effectively absent, but is found at high frequency in modern Asians, Australians and Native American populations.” reports Cosimo Posth of Tübingen University, the lead author of the study. When the LGM began around 25,000 years ago, hunter-gatherer populations retreated to a number of proposed refugia in the south of Europe. The researchers suggest that haplogroup M may have been lost due to the reduction in population size, which later re-expanded across Europe when the climatic conditions turned more favourable for humans.
Making use of ancient radiocarbon-dated mtDNA as molecular calibration points, the authors were able to revise the mtDNA mutation rate, the pace at which DNA accumulates mutations over time, and precisely dated the origin of the two non-African mtDNA types, N and M, to around 50,000 years ago. “This date estimate supports a late and rapid dispersal of all non-African populations carrying both M and N haplogroups, not only across Asia but also into Europe,” explains Johannes Krause, director at the MPI for the Science of Human History.
Previously unknown major population shift
The new data provided still more exciting results. While large-scale population replacement events have already been shown during the Neolithic and Bronze Age, the team found evidence for a previously unknown major population shift in Europe around 14,500 years ago, at the beginning of the Late Glacial. “During this period of drastic warming, it looks like the European hunter-gatherers were largely replaced by a population from a different maternal source,” explains Adam Powell, a senior author of the study at the MPI in Jena. He adds, “Our modelling of ancient European hunter-gatherers demonstrates that their demography was likely far more complicated than previously assumed.”
Future analyses of ancient nuclear DNA of additional geographically and temporally distributed specimens will help to obtain a more comprehensive picture. The researchers hope to better characterize the genetic consequences of the retraction to LGM climatic refugia, as well as identify the source of the later incoming hunter-gatherer population.
Paper – Pleistocene mitochondrial genomes suggest a single major dispersal of non-Africans and a Late Glacial population turnover in Europe, Current Biology (open access).