The Neanderthal genome included harmful mutations that made the hominids around 40% less reproductively fit than modern humans, according to estimates published in the latest issue of the journal Genetics. Non-African humans inherited some of this genetic burden when they interbred with Neanderthals, though much of it has been lost over time. The results suggest that these harmful gene variants continue to reduce the fitness of some populations today. The study also has implications for management of endangered species.

"Neanderthals are fascinating to geneticists because they provide an opportunity to study what happens when two groups of humans evolve independently for a long time--and then come back together," says study leader Kelley Harris, of Stanford University. "Our results suggest that inheriting Neanderthal DNA came at a cost."

Previous studies of DNA extracted from Neanderthal remains revealed that these Eurasian hominids were much more inbred and less genetically diverse than modern humans. For thousands of years, the Neanderthal population size remained small, and mating among close relatives seems to have been common.

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