Scientists have uncovered new information on the genetic composition of early modern humans, who lived alongside the Neanderthals during the ice age after arriving in Europe around 45,000 years ago.

Early modern humans co-existed with Neanderthals for millennia before the latter went extinct. Researchers have analyzed DNA from bones of 51 prehistoric humans that lived 45,000 to 7,000 years ago, finding evidence of waves of migration driven by advancing and retreating glaciers, writes Lonnie Shekhtman for The Christian Science Monitor.

Study Shows Ice Age Migration Using Genomes

Ice Age migration influenced by retreating glaciers

The team of international scientists also discovered evidence of population turnover. Scandinavia and northern Europe was covered by glaciers during the last ice age, which peaked around 25,000-19,000 years ago.

“The demographic history of early European populations was much more dynamic than previously thought,” said Cosimo Posth, an archaeogenetics PhD student at the University of Tübingen in Germany, reported New Scientist.

Posth and his fellow researchers published the results of their findings on May 2 in the journal Nature. They report that all of the specimens younger than 37,000 years old from Europe and western Asia come from a single population. The team used genetic analysis to discover the pattern.

Study provides new knowledge of early European genomes

The paper reveals that modern day Europeans also share genes with that population. However it doesn’t look like people that lived more than 37,000 years ago contributed to today’s European gene pool.

The study shows that the older prehistoric humans had largely left Europe by 33,000 years ago, before reappearing around 19,000 years ago. This is because the ice sheet that covered northern Europe during the ice age retreated and populations that were previously confined to the Iberian peninsula started to spread throughout the rest of the continent.

Warmer conditions after the Ice Age, around 14,000 years ago, coincided with another population movement. Early humans that lived in what is now Turkey and Greece moved north and west into Europe and displaced the first group.

“What we see is a population history that is no less complicated than that in the last 7,000 years,” said David Reich, a paper co-author and geneticist from Harvard Medical School, “with multiple episodes of population replacement and immigration on a vast and dramatic scale, at a time when the climate was changing dramatically.”

Neanderthal DNA less common in modern humans

According to the study the proportion of Neanderthal DNA in Europeans from 45,000 was around 3-6% compared to around 2% in modern day Europeans. The scientists believe that natural selection is responsible for this reduction.

“Neanderthal DNA is slightly toxic to modern humans,” said Dr. Reich in a statement.

Previous research into prehistoric European genomes relied on data from just four of our human ancestors. As a result it was not possible to accurately say how populations migrated or evolved.

“Trying to represent this vast period of European history with just four samples is like trying to summarize a movie with four still images,” Reich said. “With 51 samples, everything changes; we can follow the narrative arc; we get a vivid sense of the dynamic changes over time,” he said.

The researchers have provided a massive boost for our knowledge of early European populations and the way that they moved around the continent. It is interesting to see how these movements were influenced by the retreat of ice sheets, which made areas of northern Europe more hospitable to our ancestors.

The full findings of the study can be found in the journal Nature.