A small population size can "diminish the power of natural selection to remove slightly deleterious evolutionary changes," Green said.
The researchers estimate the Neanderthal population living in Europe 38,000 years ago never reached more than 10,000 at any one time.
This could have been a factor in their demise, Green said.
Homo neanderthalis first appeared in Europe about 300,000 years ago but mysteriously vanished about 35,000 years ago, shortly after the arrival of modern humans—Homo sapiens—in Europe.
"If there were only a few, small bands of Neanderthals, barely hanging on, then any change to their way of life could have been enough to drive them to extinction," Green said.
"One obvious change would have been the introduction of another large hominid—modern humans."
Stephen Schuster, a molecular biologist at Pennsylvania State University, said the new study should silence a lot of theories about interbreeding between Neanderthals and modern humans.
The study shows that "at least for the maternal lineage, there are no traceable genetic markers that suggest admixture of Neanderthals and modern humans," he said.
Schuster added that the researchers were exceptionally careful to isolate the Neanderthal DNA.
"Many more precautions were taken to ensure that no contamination with human DNA has flawed the analysis," he said, noting that researchers sequenced each letter about 35 times to be sure of their work.
"This was the weak point of previous reports," said Schuster, who was not involved with the study.
Thomas Gilbert, an ancient DNA expert at the University of Copenhagen in Denmark who also was not involved in the study, called the research a "step forward" and a taste of what might come when the Neanderthal nuclear DNA is finished.
The team's argument that the Neanderthal population was small 38,000 years ago is speculative, Green said, but "it's better than what we could have said before."
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