The Neanderthal genome included harmful mutations that made the hominids around 40 percent less reproductively fit than modern humans, according to new estimates. 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.

Source: Inbred Neanderthals left humans a genetic burden: Non-African human populations today have marginally lower fitness thanks to Neanderthal inheritance — ScienceDaily