The investigators found Neanderthals carried more copies of mutations that would alter the amino acid makeup of proteins than modern humans possess. This suggests that Neanderthal populations across Eurasia were likely small and isolated.
"Neanderthals seem to have been few in numbers either over a long time or for some periods," Pääbo said. "There is also an indication that they have been subdivided in populations that had little contact with each other."
The fact that Neanderthals carried more copies of potentially detrimental mutations did not necessarily contribute to their extinction, said lead study author Sergi Castellano, at the Max Planck Institute for Evolutionary Anthropology. "No claim should be made that this is related to their extinction," Castellano told Live Science.
The researchers also found skeleton genes changed more than expected within the Neanderthal lineage.
"For example, genes that affect the curvature of the spine have changed in Neanderthals," Pääbo said. "This fits with how their skeletons have changed quite drastically during their evolution."
On the other hand, genes involved with pigmentation and behavior changed more in the modern human lineage.
"We do not yet know if and how these very mutations affect behavior," Pääbo said. "Clearly, it will be interesting to study more Neanderthals so that one can begin to reconstruct their history in more detail."
The scientists detailed their findings online today (April 21) in the journal Proceedings of the National Academy of Sciences.
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