A leading geneticist has stated that neanderthals would have mated with modern man, even though it wasn't clear whether or not such couplings would have been capable of producing offspring.
"Modern man and Neanderthals had sex across the species barrier, according to leading geneticist Professor Svante Paabo. Professor Paabo, who is director of genetics at the renowned Max Planck Institution for Evolutionary Anthropology in Leipzig, made the claim at a conference in the Cold Springs Laboratory in New York."