The sons of Seth found the girls sired by Cain’s descendants quite attractive?
There are no Neandertal mitrochondreal examples found in genetic testing, nor Y chromosomes.
The best explanation for this is human women giving birth to the hybrid children, but never the other way around, and that none of the male hybrids lived.
A short story on that concept plus mocking post-human genetic engineering
A Step Back in Time, a short story
http://hubpages.com/literature/When-Time-Should-Have-Stood-Still