Europe wasn't a very hospitable place fifteen millennia ago. The westernmost landmass of the Eurasian continent had endured a long ice age, with glaciers stretching across northern Europe and into the region we now call Germany. But suddenly, about 14,500 years ago, things started to warm up quickly. The glaciers melted so fast around the globe that they caused sea levels to rise 52 feet in just 500 years. Meanwhile, the environment was in chaos, with wildlife trying vainly to adjust to the rapid fluctuations in temperature. Humans weren't immune to the changes, either.

A new, comprehensive analysis of ancient European DNA published today in Current Biology magazine by an international group of researchers reveals that this period also witnessed a dramatic shift in the human populations of Europe. Bloodlines of hunter-gatherers that had flourished for thousands of years disappeared, replaced with a new group of hunter-gatherers of unknown origin.

Researchers discovered this catastrophic population meltdown by sequencing the mitochondrial DNA of 35 people who lived throughout Europe between 35 and 7 thousand years ago. Mitochondrial DNA is a tiny amount of genetic material that's inherited virtually unchanged via the maternal line, and thus it serves as a good proxy for relatedness over time. Two people from the same maternal stock share almost the same mitochondrial DNA, even if separated by thousands of years, because this kind of DNA evolves very slowly.

It's long been known that two such related groups, called M clade and N clade, poured out of Africa and across the Eurasian continent about 55 thousand years ago. Some of these people wandered so far that they even made it to Australia, eventually. And yet something rather odd happened to the people of Europe. Only members of the N clade survived into the present day, while Asia, Australia, and the Americas are full of the offspring of both N and M. Until the new study in Current Biology, scientists believed that the most likely explanation was that roughly 45 thousand years ago, Europe was colonized solely by the N clade, while both clades settled elsewhere around the world.

But thanks to sequencing the mitochondrial DNA in those 35 ancient people, the researchers uncovered something previously unknown. There were, in fact, people from the M clade alive in Europe as recently as 25 thousand years ago. But something happened to wipe them out during the cold, dry glacial maximum that gripped the world between 25 and 14.5 thousand years ago.

There are obvious reasons why Europeans might have suffered a population bottleneck during the ice age, or the Last Glacial Maximum. Food was scarce, and once-fecund habitats became unlivable. Groups that once roamed the wide-open fields of Europe retreated into small refuges, separated by walls of ice or frozen drought wastelends created when glaciation locks up atmospheric water. The researchers believe that the M clade, whose members were found far to the north, may have slowly died out during that period. After the glaciers retreated, the survivors were replaced by a new N-related population from elsewhere on the continent.

Write the researchers:

The potential impact of climatic events on the demography, and thus the genetic diversity of early Europeans, has previously been difficult to quantify, but it likely had consequences for the relative components of ancient ancestry in modern-day populations. Our demographic modeling reveals a dynamic history of hunter-gatherers, including a previously unknown major population shift during the Late Glacial interstadial (the BøllingAllerød, 14.5 ka). Under our best-fitting model, the small initial founder population of Europe slowly grows up until 25 ka and survives with smaller size in LGM [Last Glacial Maximum] climatic refugia (25–19.5 ka) before re-expanding as the ice sheets retract. Although this model supports population continuity from pre- to post-LGM, the genetic bottleneck is consistent with the apparent loss of hg M in the post-LGM. Globally, the early warming phases of the Late Glacial are strongly associated with substantial demographic changes, including extinctions of several megafaunal species and the first expansion of modern humans into the Americas. In European hunter-gatherers, our model best explains this period of upheaval as a replacement of the post-LGM maternal population by one from another source.

Essentially, an entire genetic line in Europe was wiped out by climate change. You might say that today's European population still bears the scars of an ancient ice age in its mitochondrial DNA.

Current Biology, 2016. DOI: 10.1016/j.cub.2016.01.037.