(Shutterstock)The complex story of ancient human migration just got murkier.

Researchers discovered a new population shift in Europe at the end of the last Ice Age, around 15,000 years ago, that dispelled the resident humans of northern Europe with a new group that may have survived the Ice Age in a pocket of warmer weather to the south, according to researchers.

The team analyzed 55 complete mitochondrial genomes from hunter-gatherers overall, dating as far back as 35,000 years ago, during the Pleistocene period, and from as recently as 7,000 years ago from the Holocene period.

The researchers extracted new usable DNA from bones and teeth of 35 ancient humans living throughout Europe over a span of 35,000 years. Their work added those 35 new genomes, nearly tripling the number of mtDNA genomes available for pre-Neolithic European humans, according to the study.

“The genetic diversity of the first modern humans who spread into Europe during the Late Pleistocene and the impact of subsequent climatic events on their demography are largely unknown,” wrote the authors of the study.

Non-African humans living today fall into two haplogroups—or single lines of decent usually dating back thousands of years. Haplogroup M and haplogroup N. The M group is common in modern Asian, Australasian, and Native American people—and mostly absent in modern Europeans.

The new DNA research showed three ancient individuals, living in modern-day France and Belgium, shared the M haplogroup.

“We uncovered a completely unknown chapter of human history: a major population turnover at the end of the last Ice Age,” Johannes Krause, one of the authors of the study, told media outlets.

There are two leading models of how non-Africans dispersed throughout Eurasia and Australasia. The first theory is a single mass exodus, while the second theory alleges multiple waves of migration. This work seems to support the single mass exodus theory.

“Little is known about the genetic makeup of the first European hunter-gatherers,” according to the study. Researchers are now trying to construct a more complete picture of Europe at this time to determine what caused the population bottleneck, and hopefully prove which theory is correct.

But like much of human history, there might not be a single answer.

The study can be found in the Current Biology journal from Feb. 4, 2016.