Here's an interesting bit that may or may not mean something in the context of this discussion:
Neanderthal virus DNA spotted hiding in modern humans
http://www.sott.net/article/268902-Neanderthal-virus-DNA-spotted-hiding-in-modern-humans
Neanderthal virus DNA spotted hiding in modern humans
http://www.sott.net/article/268902-Neanderthal-virus-DNA-spotted-hiding-in-modern-humans
They examined the genomes of 67 people with cancer, and found they each contained seven of the sequences supposedly unique to the ancient humans. Belshaw suspects that all 14 might still be around, although finding the rest will take more time. The viruses insert themselves into DNA repeats - patterns that occur in multiple locations throughout the genome, only one of which will carry the sequence in question, so tracking them down is time consuming.
The finding suggests that the viruses probably infected our ancestors before we split from the lineage that led to Neanderthals and Denisovans, roughly 400,000 years ago.
...modern humans can differ from one another significantly in the non-coding parts of their genomes. "The results show that we can find individuals today who share loci with Denisovans or Neanderthals, but not with other humans alive today," he says. ...
"We know these viruses cause disease in other animals, including mice," says Belshaw. There is also some evidence that the viruses are in an active state in tissue taken from people with motor neurone disease. "But it's never been proven that they can cause disease in humans," he says.