In general, in biology species are not defined by the impossibility of having viable hybrids between them, but by the fact that in natural conditions they do no

In general, in biology species are not defined by the impossibility of having viable hybrids between them, but by the fact that in natural conditions they do not normally interbreed, which allows their genes to drift away, becoming more and more different with the passing of time. For a species to exist, interbreeding must be both possible and frequent, which ensures that the entire population of the species remains uniform genetically for as long as the species exists, even when its genes may become more and more different from those of other species. The most frequent way for a new species to appear is when there is geographic isolation between two populations, which prevents interbreeding. Then, after many generations, the genetic differences may become so great that even when the populations are reunited interbreeding will fail, resulting in hybrids that are either non-viable, or sterile, like mules. With Neanderthals and Denisovans, the geographic separation has started the process of development of new species and they have become clearly differentiated from a genetic point of view. Nevertheless, not enough generations had passed so that the differences would become so great as to make interbreeding impossible. Even if we know that successful interbreeding has existed, we do not know which was the percentage of success for such interbreeding, i.e. how many of the hybrids were both viable and non-sterile. So it is a rather arbitrary choice to decide whether to call Neanderthals and modern humans as different subspecies or different species. In any case, they were easy to distinguish. There are many closely related species between which interbreeding is easily performed in captivity, e.g. in zoos, but it almost never happens in natural conditions, where wild animals may choose partners according to their preferences, so those species are easily distinguishable, due to their genetic differences that are preserved by the lack of interbreeding.