I am unable to find it now, but there was an article either the scientific journal
Nature or
Science that stated that the Neaderthals became extinct because they were cannibals...
That is the problem with cannibalism. You eat your own, you can get infected with numerous diseases--mainly prion diseases that are incurable. That is the problem with Kuru. That is how they think the Hopi Indians died off in the Southwest and possibly the Incas.
So Neanderthals ate each other. But their intelligience was extremely high. They buried their dead. They had stone-age weaponry. And they were nomadic. They in fact killed off a large portion of the Mammoths, so it is thought.
However, there is no evidence that there homologous recombination of DNA sequences between the Homo erectus and Homo sapiens sapiens... So no, Neanderthals did not breed with modern human beings for some reason. If so, the offspring failed to thrive--although some folks may disagree with others...
And PiKA2001,
I thought you were brighter than that...