Ed Yong has a good good review of a new Neandertal introgression/admixture paper in PNAS. It's not live on the web yet, so let me quote Ed:
Even if the odds of successful interbreeding were just 5 percent, Neanderthal genes would make up the majority of the human genome today. As it is, a lack of viable sex explains why none of the Neanderthals’ mitochondrial DNA made its way into modern humans, and why so little of their main genome did. Currat and Excoffier suggest that either modern humans and Neanderthals didn’t have sex very often, or their hybrids weren’t very fit. They favour the first idea. According to their model, it would only have taken between 197 and 430 liaisons between ancient humans and Neanderthals to fill 1-3 percent of modern Eurasian genomes with Neanderthal DNA. Considering that they two groups probably interacted for 10,000 years or so, it ...
