Western Europe may have undergone a population crisis during a cold stretch of the Neanderthal era, a new study of mitochondrial DNA sequences suggests.

"The fact that Neanderthals in Western Europe were nearly extinct, but then recovered long before they came into contact with modern humans came as a complete surprise to us," study co-author Love Dalén, associate professor at the Swedish Museum of Natural History in Stockholm, told Phys.org.

"This indicates that the Neanderthals may have been more sensitive to the dramatic climate changes that took place in the last Ice Age than was previously thought."

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By analyzing the amount of genetic variation in the DNA from 13 Neanderthals, the scientists pieced together the puzzle of a demographic history. The DNA of Neanderthals from more than 50,000 years ago showed a high degree of genetic variation, in contrast to the DNA of those from less than 50,000 years ago. That group showed much less genetic variation.

Until now, the Neanderthal population was assumed to be stable until modern humans began showing up. Extinction was avoided when Neanderthals from surrounding areas repopulated the region. But the new research suggests that Neanderthals may have been more susceptible to cold than previously thought.

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"At the very least, this tells us that without the aid of material culture or technology, there is a limit to our biological adaptation," co-author Rolf Quam of the University of Binghamton said.

"It may very well have been the case that the European Neanderthal populations were already demographically stressed when modern humans showed up on the scene."

Photo: Skeleton and restoration model of Neanderthal from an exhibit in the the National Museum of Nature and Science, Tokyo, Japan.