Whether Denisivans and Neanderthals are different species is mostly just a matter of semantics. We have had a pretty good look at the DNA of each and it's clea

Whether Denisivans and Neanderthals are different species is mostly just a matter of semantics. We have had a pretty good look at the DNA of each and it's clear that the two were evolving in isolation for hundreds of thousands of years resulting in genetically distinct populations. The difference between Denisovans and Neanderthals is about half the difference between humans and either. It's not at all unusual for two different species to be able to interbreed. Lions and tigers for instance or horses and donkeys. Mules, the offspring of horses and donkeys are sterile, though, so the two populations can't mix. Ligoers, the offspring of a male lion and female tigers, aren't entirely sterile but have very low fertility. With humans and neanderthals there are portions of the Neanderthal genome related to reproduction that were very heavily selected against in a way which seems to indicate that male hybrids at least had moderate fertility problems until those genes were removed. So it's not black and white at all. We have zero evidence for any reduction in fertility between Neanderthals and Denisovans since we have very few examples of hybrid offspring and this sort of analysis needs lots of data - so who knows.