European Neanderthals were dying off and already on the verge of extinction even before the arrival of competing modern humans, Swedish researchers say.

Scientists said a study of ancient DNA indicates most Neanderthals in Europe died off as early as 50,000 years ago, leaving only a small group that re-colonized central and western Europe and survived for another 10,000 years before modern humans arrived on the scene.

"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 to us," Love Dalen at the Swedish Museum of Natural History in Stockholm said.

"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," he said.

DNA studies of fossils showed the genetic variation among later European Neanderthals was extremely limited during the last 10,000 years before they disappeared, researchers said.

"The amount of genetic variation in geologically older Neanderthals as well as in Asian Neanderthals was just as great as in modern humans as a species, whereas the variation among later European Neanderthals was not even as high as that of modern humans in Iceland," researcher Anders Gotherstrom of Uppsala University in Sweden said.

The research was published in the journal Molecular Biology and Evolution.