By Paul Rincon
BBC News
Neanderthals were already on the verge of extinction in Europe by the time modern humans arrived on the scene, a study suggests.
DNA analysis suggests most Neanderthals in western Europe died out as early as 50,000 years ago – thousands of years before our own species appeared.
A small group of Neanderthals then recolonised parts of Europe, surviving for 10,000 years before vanishing.
The work is published in the journal Molecular Biology and Evolution.
An international team of researchers studied the variation, or diversity, in mitochondrial DNA extracted from the bones of 13 Neanderthals.
This type of genetic information is passed down on the maternal line; because cells contain multiple copies of the mitochondrial genome, this DNA is easier to extract from ancient remains than the DNA found in the nuclei of cells.
The fossil specimens came from Europe and Asia and span a time period ranging from 100,000 years ago to about 35,000 years ago.
The scientists found that west European fossils with ages older than 48,000 years, along with Neanderthal specimens from Asia, showed considerable genetic variation.
But specimens from western Europe younger than 48,000 years showed much less genetic diversity (variation in the older remains and the Asian Neanderthals was six-fold greater than in the western examples).
In their scientific paper, the scientists propose that some event – possibly changes in the climate – caused Neanderthal populations in the West to crash around 50,000 years ago. But populations may have survived in warmer southern refuges, allowing the later re-expansion.
Low genetic variation can make a species less resilient to changes in its environment, and place it at increased risk of extinction.
“The fact that Neanderthals in Europe were nearly extinct, but then recovered, and that all this took place long before they came into contact with modern humans, came as a complete surprise,” said lead author Love Dalen, from the Swedish Museum of Natural History in Stockholm.
“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.”