One of the ways of distinguishing between two species is that while both of them may cross breed, they do not generally produce fertile descendants. However, this concept is much more complex when extinct species are involved. In fact, the story told by current human DNA blurs the lines of these limits, preserving fragments of hominids from other species , such as the Neanderthals and the Denisovans, who coexisted with modern humans more than 40,000 years ago in Eurasia.