Neanderthals Faced a Genetic Crisis During the Ice Age, Setting the Stage for Their Extinction 

In their final years, Europe’s declining Neanderthal population was stripped of its genetic diversity; the last members of this dying species were descended from a single lineage, which would eventually become extinct around 40,000 years ago.

A new study published in the Proceedings of the National Academy of Sciences has filled a major gap in the genetic history of Europe’s Neanderthals, revealing how ancient hominids became increasingly homogeneous over time. The harsh climatic conditions of the last ice age hit the Neanderthals hard; their diminished genetic diversity would never fully recover, driving them toward extinction.

Learn more: Humans outlived Neanderthals, likely due to differences in anatomy and social skills

Gaps in Neanderthal knowledge

During the Middle to Late Pleistocene, Neanderthals dispersed throughout Eurasia. The first indications of their presence in Europe come from the archaeological site of Sima de Los Huesos in Spain; Archaeologists have found that the hominids that lived here around 430,000 years ago possessed several characteristics seen in later Neanderthals – although these hominids are often described as “pre-Neanderthals.”

Archaeological sites across Europe indicate that Neanderthals were well established on the continent by 400,000 years ago. They are believed to live another 360,000 years, but many gaps in their genetic history remain unknown.

“We have evidence that Neanderthals inhabited Europe continuously from 400,000 to 40,000 years ago. However, we only have fragmentary details about their population history,” study author Cosimo Posth, a professor at the University of Tübingen, said in a statement. “So far, we know very little about the evolutionary developments that preceded their extinction.”

Refuge during the ice age

Neanderthals in Europe did not always live comfortably, especially during the last ice age. The new study reveals that Neanderthals were pushed to a corner of southwestern Europe 75,000 years ago, seeking refuge as a devastating ice age threatened populations across the continent.

To get a better idea of ​​the history of the Neanderthal population, the researchers sequenced the mitochondrial DNA of tooth and bone samples from 10 new Neanderthal individuals from six archaeological sites in Belgium, France, Germany and Serbia. They combined the results with 49 previously published Neanderthal mitochondrial DNA samples and tied it all together by looking at established data on the presence of Neanderthals in Europe.

“This allowed us to combine the two data sources and reconstruct the demographic history of Neanderthals in terms of space and time,” said Jesper Borre Pedersen, co-author of the study and postdoctoral researcher at the University of Tübingen.

Based on this analysis, they determined that between 130,000 and 80,000 years ago, Neanderthals ranged from the Iberian Peninsula in the west to the Black Sea in the east. But soon after, 80,000 years ago, their range was reduced considerably. Archaeological sites occupied by Neanderthals are beginning to be concentrated in the southwest of France.

Lack of genetic diversity

The researchers focused on the later Neanderthals, those who lived around 60,000 to 40,000 years ago. Their analysis of mitochondrial DNA showed that the Neanderthal group that settled in southwest France to endure the Ice Age eventually spread across Europe. This new population began to reoccupy the areas previously left behind, but the problem was that all its members belonged to the same lineage.

Researchers found that Neanderthals experienced a sharp decline in population size between 45,000 and 42,000 years ago. Their numbers may have declined so quickly because of their low genetic diversity; this was the result of a population bottleneck during the Ice Age, which wiped out many Neanderthal groups, each with their own genetic identity.

“Genetically speaking, the later Neanderthals were a very homogeneous group,” Posth said. “It could therefore be that low genetic diversity – and perhaps also the subsequent isolation of small groups – contributed to the disappearance of Neanderthals.”

Learn more: Neanderthals roamed Eurasia before modern humans

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