As these events occurred, humans in Europe first experienced a “bottleneck” when their numbers decreased during the last Glacial Maximum roughly 25,000 to 19,500 years ago, says the new research, published Thursday in Current Biology. But later, as a warm period kicked in around 14,500 years ago, it looks like this population was replaced or overwhelmed by a genetically somewhat different one, which may have been able to migrate into the area, thanks to retreating ice.

The paper calls this “surprising evidence of a major population turnover in Europe around 14,500 years ago,” at the same time when there was also “a period of climatic instability.”

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“I think it’s a very interesting correlation, and I don’t think this correlation is just by chance, especially for those hunter-gatherer cultures who are largely dependent on the environment,” said Johannes Krause, one of the study authors and a researcher at the Institute for Archaeological Sciences at the University of Tübingen in Germany. The research was conducted by Krause and no less than 34 other authors.

The study, based on 55 early modern human genomes from the period between 35,000 and 7,000 years ago, examined mitochondrial DNA — DNA that is contained within cellular structures called mitochondria rather than in the nucleus where most of our genetic material is located, and is passed down from mothers to their offspring. Examining this DNA, scientists have found slight mitochondrial differences between Asians, Australians and Native Americans on the one hand, and Europeans on the other. European mitochondrial DNA today shows what is called the “N” lineage, or “haplogroup,” but not what is called an “M” lineage. By contrast, Asian, Australasian and Native Americans show both.

The M lineage, the research explains, is “today found predominantly in Asia, Australasia, and the Americas, although it is almost absent in extant populations with European ancestry.” Because of this difference, it has been argued in the past that humans did not all leave Africa at the same time some 50,000 years ago — rather, they left in waves that were, genetically, somewhat different.

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But the new research suggests that genetic information characteristic of the M lineage was “unexpectedly” present in Europe — present-day France and Belgium — 35,000 and 28,000 years ago, although it is not today. That not only suggests a single human migration out of Africa; it also raises the question — what happened to these people who were once in Europe but did not persist there?

The study suggests that as the Last Glacial Maximum set in between 25,000 and 19,500 years ago, ice sheets grew to a vast extent and covered much of the continent, as well as much of present-day North America. Populations of humans in Europe in this time were not only driven southward by the ice sheets but may also have been separated into isolated areas by them, what the researchers call “refugia.”

“We had a large glacier coming down from Scandinavia that pushed out people from central Europe,” Krause said.

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At the same time, the populations decreased in size — what the researchers call a “genetic bottleneck that may have been influenced by climatic events” — and may have lost some of their genetic diversity, including the mitochondrial DNA characteristic of the M lineage. “What we found was that there was more genetic diversity before the Last Glacial Maximum,” Krause said. “During the Glacial Maximum, diversity got lost.”

But then came a period called the Bølling-Allerod “interstadial” (interstadials are relatively warmer periods during ice ages), beginning about 14,500 years ago. Ice sheets retreated, and indeed, this is when humans crossed the land bridge from Asia to North America.

Something similarly dramatic happened in Europe, the new research suggests. A new flux of humans, perhaps able to move to new territory as ice sheets retreated, appears to have come into Europe and effected a “replacement” of the older one. These people did show the “N” lineage characteristic of Europeans today.

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“The people who survive the ice age, they change again,” Krause said. “There is a turnover.”

It isn’t known, Krause said, where this mysterious new group came from. Nor is it known whether this means the two groups fought, interbred, or simply that the incoming group was much larger and, in effect, subsumed the smaller one. “We do not know what happened about 14.5 thousand years ago. That is just too little data that we had,” he said. “Whether it was a traumatic event that caused those new people coming in, like a pandemic or sudden climatic change, or was the population just bigger, and they absorbed the smaller population.”

But it seems likely that warmer conditions helped the event happen. At the same time, Krause noted, forests were regrowing across Europe around the same time, and there were extinctions of major land mammals (or megafauna), probably due in part to their hunting by humans.

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Later on came another cold snap — the Younger Dryas, beginning around 12,900 years ago and lasting another millennium. Then, finally, starting around 11,700 years ago, we moved into the current Holocene period, a warmer period of relative climatic stability that has fostered the growth of civilization.

These climate snaps — warm, then cold again — are said by many scientists to involve shifts in the circulation of the Atlantic Ocean, whose “overturning” circulation is said to have started and stopped many times in Earth’s past. When it stops, less warm water is carried northward in the Northern Hemisphere, which triggers cooling in Europe and, some research suggests, potentially across the globe.

It is important to note that while the new research does suggest significant mitochondrial DNA changes in human populations during these climate swings over 10,000 years ago, that does not mean that the world’s humans became substantially different from one another genetically. We’re all still the same species.

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“We’re focusing on minute differences in maternal ancestry here, definitely not huge differences,” said Jennifer Raff, a professor of anthropology at the University of Kansas who was not involved in the research, by email. However, these slight differences are permitting increasingly powerful studies of the history of human populations.

The new research only used mitochondrial DNA — and on this basis, it will be “interesting to see what the nuclear genome shows,” said Eske Willerslev, a professor at the Centre for GeoGenetics at the Natural History Museum of Denmark, who was not involved in the study. Studies of this broader body of genetic material “has the potential power to reveal complex population histories,” Willerslev said by email.

“This finding highlights how ancient DNA research is transforming our understanding of human history and evolution,” added the University of Kansas’s Raff by email. She continued: